# Check if point lies inside triangle

Your goal is to determine whether a given 2D point X lies within the area of the triangle with given vertices A,B,C.

Write a function that takes in the coordinates of the test point X and the three triangle vertices (so that's 8 coordinates total) and returns True if the point lies inside that triangle, and False if it lies outside.

Don't worry about edge cases. If the point lies on the boundary of the triangle (edge or vertex) or the triangle is actually a line segment, your code can do whatever, including crashing. Also don't worry about numerical stability or floating-point precision.

Your code must be a named function. Code snippets will not be accepted.

Fewest characters wins.

Input:

Eight real numbers representing coordinates. The numbers will lie in the range (-1,1).

The exact input format is flexible. You could, for example, take in eight numbers, a list of eight numbers, a list of four points each given by a tuple, a 2*4 matrix, four complex numbers, two lists of the x-coordinates and y-coordinates, and so on.

The input needs to just be the numbers in some container, with no additional data. You can't use the input to do any preprocessing, nor may you require any constraints on the input, such as requiring the points be given in ascending y coordinate. Your input must allow any eight coordinates (though your code can behave arbitrarily in the edge cases mentioned earlier).

Output:

Either the corresponding Boolean True/False, the corresponding number 1/0, or the analogues in your language.

Test cases

The inputs are given a list [X,A,B,C] of four tuples, the test point first, then the three triangle vertices. I've grouped them into those whose outputs should be True and those that should be False.

True instances:

[(-0.31961, -0.12646), (0.38478, 0.37419), (-0.30613, -0.59754), (-0.85548, 0.6633)]
[(-0.87427, -0.00831), (0.78829, 0.60409), (-0.90904, -0.13856), (-0.80685, 0.48468)]
[(0.28997, -0.03668), (-0.28362, 0.42831), (0.39332, -0.07474), (-0.48694, -0.10497)]
[(-0.07783, 0.04415), (-0.34355, -0.07161), (0.59105, -0.93145), (0.29402, 0.90334)]
[(0.36107, 0.05389), (0.27103, 0.47754), (-0.00341, -0.79472), (0.82549, -0.29028)]
[(-0.01655, -0.20437), (-0.36194, -0.90281), (-0.26515, -0.4172), (0.36181, 0.51683)]
[(-0.12198, -0.45897), (-0.35128, -0.85405), (0.84566, 0.99364), (0.13767, 0.78618)]
[(-0.03847, -0.81531), (-0.18704, -0.33282), (-0.95717, -0.6337), (0.10976, -0.88374)]
[(0.07904, -0.06245), (0.95181, -0.84223), (-0.75583, -0.34406), (0.16785, 0.87519)]
[(-0.33485, 0.53875), (-0.25173, 0.51317), (-0.62441, -0.90698), (-0.47925, 0.74832)]


False instances:

[(-0.99103, 0.43842), (0.78128, -0.10985), (-0.84714, -0.20558), (-0.08925, -0.78608)]
[(0.15087, -0.56212), (-0.87374, -0.3787), (0.86403, 0.60374), (0.01392, 0.84362)]
[(0.1114, 0.66496), (-0.92633, 0.27408), (0.92439, 0.43692), (0.8298, -0.29647)]
[(0.87786, -0.8594), (-0.42283, -0.97999), (0.58659, -0.327), (-0.22656, 0.80896)]
[(0.43525, -0.8923), (0.86119, 0.78278), (-0.01348, 0.98093), (-0.56244, -0.75129)]
[(-0.73365, 0.28332), (0.63263, 0.17177), (-0.38398, -0.43497), (-0.31123, 0.73168)]
[(-0.57694, -0.87713), (-0.93622, 0.89397), (0.93117, 0.40775), (0.2323, -0.30718)]
[(0.91059, 0.75966), (0.60118, 0.73186), (0.32178, 0.88296), (-0.90087, -0.26367)]
[(0.3463, -0.89397), (0.99108, 0.13557), (0.50122, -0.8724), (0.43385, 0.00167)]
[(0.88121, 0.36469), (-0.29829, 0.21429), (0.31395, 0.2734), (0.43267, -0.78192)]

• What is your definition of a character? Ascii? Encodable in 7 bits? In a byte? Any Unicode? – isaacg Jul 5 '14 at 19:07
• What do you suggest? There's already solutions that use compressed code. – xnor Jul 5 '14 at 19:46
• Typically, I believe bytes are used for non-Ascii characters, because otherwise the Utf-32 advantage is insurmountable. – isaacg Jul 6 '14 at 19:51
• Well, I can't go back now; any Unicode character is a character. Compress if you want. – xnor Jul 6 '14 at 21:55

# Javascript / ECMAScript 6, 161159 158 / 152

Javascript:

function $(t,r,i,a,n,g,l,e){b=(-g*l+a*(-n+l)+i*(g-e)+n*e)/2;c=b<0?-1:1;d=(a*l-i*e+(e-a)*t+(i-l)*r)*c;f=(i*g-a*n+(a-g)*t+(n-i)*r)*c;return d>0&&f>0&&d+f<2*b*c}  ECMAScript 6 version (thanks m.buettner, saves 6 characters) $=(t,r,i,a,n,g,l,e)=>{b=(-g*l+a*(-n+l)+i*(g-e)+n*e)/2;c=b<0?-1:1;d=(a*l-i*e+(e-a)*t+(i-l)*r)*c;f=(i*g-a*n+(a-g)*t+(n-i)*r)*c;return d>0&&f>0&&d+f<2*b*c}


Call it as such (returns true or false):

$(pointX, pointY, v1X, v1Y, v2X, v2Y, v3X, v3Y);  Uses some fancy barycentric coordinate math based on code from this answer. An ungolfed version for your reading enjoyment follows: function$ (pointX, pointY, v1X, v1Y, v2X, v2Y, v3X, v3Y) {
var A =  (-v2Y * v3X + v1Y * (-v2X + v3X) + v1X * (v2Y - v3Y) + v2X * v3Y) / 2;
var sign = A < 0 ? -1 : 1;
var s = (v1Y * v3X - v1X * v3Y + (v3Y - v1Y) * pointX + (v1X - v3X) * pointY) * sign;
var t = (v1X * v2Y - v1Y * v2X + (v1Y - v2Y) * pointX + (v2X - v1X) * pointY) * sign;
return s > 0 && t > 0 && s + t < 2 * A * sign;
}

• +1, if only for the parameter names! – Matt Jul 4 '14 at 0:18
• Why do you have to break my character-counting UserScript??? – kitcar2000 Jul 4 '14 at 17:21
• @kitcar2000 what do you mean? – Abraham Jul 4 '14 at 17:40
• The rules says that characters are counted, not bytes. So you can use this: xem.github.io/obfuscatweet to fit in 122 chars – xem Jul 4 '14 at 19:53
• Am I mistaken, or could you have used (a*(l-n)+i*(g-e)+n*e-g*l) instead of (-g*l+a*(-n+l)+i*(g-e)+n*e)? – Adalynn Nov 29 '16 at 22:48

## Python 2.7 12812711711010910399959491 90

My first code-golf attempt!

## Code

f=lambda x,y,t:sum(a*y+c*b+d*x<d*a+c*y+b*x for i in(0,1,2)for a,b,c,d in[t[i-1]+t[i]])%3<1


Takes as input (x,y,t) where (x,y) is the point we're checking and t is a triangle t = ((x1,y1),(x2,y2),(x3,y3)).

## Explanation

I'm calculating the determinants of the matrices

| 1 x1 y1 |      | 1 x2 y2 |      | 1 x3 y3 |
| 1 x2 y2 | ,    | 1 x3 y3 | ,    | 1 x1 y1 | .
| 1 x  y  |      | 1 x  y  |      | 1 x  y  |


These determinants represent the signed distances from the sides of the triangle to the point (x,y). If they all have the same sign, then the point is on the same side of every line and is thus contained in the triangle.

In the code above, a*y+c*b+d*x-d*a-c*y-b*x is a determinant of one of these matrices.

I'm using the fact that True+True+True==3 and False+False+False==0 to determine if these determinants all have the same sign.

I make use of Python's negative list indices by using t[-1] instead of t[(i+1)%3].

Thanks Peter for the idea to use s%3<1 instead of s in(0,3) to check if s is either 0 or 3!

## Sagemath Version

Not really a different solution so I'm including it in this answer, a sagemath solution using 80 characters:

f=lambda p,t,o=[1]:sum([det(Matrix([o+t[i-1],o+t[i],o+p]))<0for i in 0,1,2])%3<1


where p=[x,y], and t=[[x1,y1],[x2,y2],[x3,y3]]

• Could s in (0,3) be shortened to s%3<1? – Peter Taylor Jul 4 '14 at 8:43
• The use of negative indices can be tweaked to save one more: -1,0,1 ... t[i]+t[i+1] is equivalent to 0,1,2 ... t[i-1]+t[i] – Peter Taylor Jul 4 '14 at 9:34
• @PeterTaylor Absolutely right! Too bad I removed the space in in -1,0,1 before reading this. Actually your way is more readable so I'll use it anyway. – Alex L Jul 4 '14 at 9:37
• Welcome to code golf! You can get rid of the square brackets for the list comprehension inside the sum if you encase the 0,1,2 in parentheses, which case a character by replacing a space. The reason is that Python allow unbracketed comprehension to be passed in to functions, but the commas in the naked tuple 1,2,3 confuse it because it tries to parse them as separate arguments. – xnor Jul 6 '14 at 22:07

## Mathematica, 67 bytes

f=Equal@@({#2,-#}&@@(#-#2).(x-#)>0&@@@Partition[x=#;#2,2,1,{1,1}])&


The function takes two arguments, the point X and a list of points {A,B,C}, which are referred to as # and #2 respectively. That is if you call

f[X,{A,B,C}]


then you'll get # as X and #2 as {A,B,C}. (Note that there are two other anonymous functions nested inside the code - # and #2 have a different meaning within those.)

Here is an explanation of the function itself:

                                              x=#;#2            & (* Save X into a variable x, but evaluate to {A,B,C}. *)
Partition[x=#;#2,2,1,{1,1}] & (* Get a cyclic list of pairs {{A,B},{B,C},{C,B}}. *)
(                        &@@@Partition[x=#;#2,2,1,{1,1}])& (* Define an anonymous function and apply it to each
of the above pairs. The two elements are referred
to as # and #2. *)
(          (#-#2)        &@@@Partition[x=#;#2,2,1,{1,1}])& (* Subtract the two points. For a pair of vertices
this yields a vector corresponding to the edge
between them. *)
{#2,-#}&                                                  (* An anonymous function that takes two values,
reverses them, inverts the sign of one of them
and puts them into a list. *)
({#2,-#}&@@(#-#2)        &@@@Partition[x=#;#2,2,1,{1,1}])& (* Applied to the edge, this yields its normal. *)
({#2,-#}&@@(#-#2).(x-#)  &@@@Partition[x=#;#2,2,1,{1,1}])& (* Take the scalar product of that normal with a
vector from a vertex to x. This is projection of
this vector onto that normal and hence the SIGNED
distance of x from the edge. *)
({#2,-#}&@@(#-#2).(x-#)>0&@@@Partition[x=#;#2,2,1,{1,1}])& (* Check the sign of that distance, the exact mapping
between (left, right) and (True, False) is
irrelevant, as long as it's consistent. *)
Equal@@({#2,-#}&@@(#-#2).(x-#)>0&@@@Partition[x=#;#2,2,1,{1,1}])& (* Check if all signs are equal - that is, if point X
lies on the same side of all edges. This is
equivalent to check that the point is inside the
triangle. *)


Note that this function will actually work for any convex n-gon as long as its vertices are given in either clockwise or counter-clockwise order.

• Wouldn't it be more efficient to check if the product of the distances is positive than if all of the signs are equal? I don't Mathematica, but it seems like that should be easier. – isaacg Jul 3 '14 at 20:57
• @isaacg There are three terms, so if they are all negative, their product is negative and if they are all positive, their product is positive. Your approach only works if the signs of two numbers are to be equal. – Martin Ender Jul 3 '14 at 21:01
• Why not use Det? – alephalpha Jul 7 '14 at 5:56
• @alephalpha Well, mostly likely because I didn't think of it. :P ... I'll look into that – Martin Ender Jul 7 '14 at 9:49
• @alephalpha Hm no, I can't find a way right now to build the three required matrices in less characters. – Martin Ender Jul 7 '14 at 10:02

# CJam, 666359524634323130 28 characters

"Ă䒟损崙㩴ァ椟饃꿾藭鑭蘁"2G#b131b:c~


After transforming the Unicode string, the following code (33 bytes) gets evaluated:

{2*2/\f{f{+~@-@@-}~@@*@@*>})-!}:T


Expects X [A B C] as input, where each point is of the form [double double]. Output is 1 or 0.

Try it online.

A big thank you goes to user23013 for saving 6 characters (13 bytes of uncompressed code)!

### Test cases

$cat triangle.cjam "Ă䒟损崙㩴ァ椟饃꿾藭鑭蘁"2G#b131b:c~ [ [-0.31961 -0.12646] [ [0.38478 0.37419] [-0.30613 -0.59754] [-0.85548 0.6633] ] T [-0.87427 -0.00831] [ [0.78829 0.60409] [-0.90904 -0.13856] [-0.80685 0.48468] ] T [0.28997 -0.03668] [ [-0.28362 0.42831] [0.39332 -0.07474] [-0.48694 -0.10497] ] T [-0.07783 0.04415] [ [-0.34355 -0.07161] [0.59105 -0.93145] [0.29402 0.90334] ] T [0.36107 0.05389] [ [0.27103 0.47754] [-0.00341 -0.79472] [0.82549 -0.29028] ] T [-0.01655 -0.20437] [ [-0.36194 -0.90281] [-0.26515 -0.4172] [0.36181 0.51683] ] T [-0.12198 -0.45897] [ [-0.35128 -0.85405] [0.84566 0.99364] [0.13767 0.78618] ] T [-0.03847 -0.81531] [ [-0.18704 -0.33282] [-0.95717 -0.6337] [0.10976 -0.88374] ] T [0.07904 -0.06245] [ [0.95181 -0.84223] [-0.75583 -0.34406] [0.16785 0.87519] ] T [-0.33485 0.53875] [ [-0.25173 0.51317] [-0.62441 -0.90698] [-0.47925 0.74832] ] T [-0.99103 0.43842] [ [0.78128 -0.10985] [-0.84714 -0.20558] [-0.08925 -0.78608] ] T [0.15087 -0.56212] [ [-0.87374 -0.3787] [0.86403 0.60374] [0.01392 0.84362] ] T [0.1114 0.66496] [ [-0.92633 0.27408] [0.92439 0.43692] [0.8298 -0.29647] ] T [0.87786 -0.8594] [ [-0.42283 -0.97999] [0.58659 -0.327] [-0.22656 0.80896] ] T [0.43525 -0.8923] [ [0.86119 0.78278] [-0.01348 0.98093] [-0.56244 -0.75129] ] T [-0.73365 0.28332] [ [0.63263 0.17177] [-0.38398 -0.43497] [-0.31123 0.73168] ] T [-0.57694 -0.87713] [ [-0.93622 0.89397] [0.93117 0.40775] [0.2323 -0.30718] ] T [0.91059 0.75966] [ [0.60118 0.73186] [0.32178 0.88296] [-0.90087 -0.26367] ] T [0.3463 -0.89397] [ [0.99108 0.13557] [0.50122 -0.8724] [0.43385 0.00167] ] T [0.88121 0.36469] [ [-0.29829 0.21429] [0.31395 0.2734] [0.43267 -0.78192] ] T ]p;$ cjam triangle.cjam
[1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0]

• Is that a named function? – Martin Ender Jul 3 '14 at 23:02
• @m.buettner: Sort of. The official wiki says the following: Block - a program section delimited by { and } and treated as a single unit. Similar to code blocks in C/java, except blocks are first-class objects and can be assigned to variables (thus defining functions). – Dennis Jul 3 '14 at 23:05
• @xnor 1m<@m* prepares 3 pairs of X and the next (i+1th) vertex of the triangle. @-@@- moves the current (ith) vertex to the origin (and mirrored if it was not @-\@-, but it doesn't matter). @@*@@*> calculates the z-axis of the cross product, aka determinant, and returns 1 if it is negative. :+3%! returns whether they are all the same, that is, all 3 are negative or non-negative, which means positive except for the edge cases. I think it is more challenging to read CJam than golfing. – jimmy23013 Jul 7 '14 at 10:43
• 37 bytes: {[_1m<\]z\f{f{+~@-@@-}~@@*@@*>})-!}:T. Use 2m> or Wm< for Unicode safety. – jimmy23013 Jul 7 '14 at 19:31
• 33 bytes: {2*2/\f{f{+~@-@@-}~@@*@@*>})-!}:T – jimmy23013 Jul 13 '14 at 12:39

# C – 156 bytes

Input are array of 3 floats in X, 3 floats in Y and separate x and y for the test point. Bonus: handles all edge cases!

int f(float*X,float*Y,float x,float y){int i,j,c=0;for(i=0,j=2;i<3;j=i++)if(((Y[i]>y)!=(Y[j]>y))&&(x<(X[j]-X[i])*(y-Y[i])/(Y[j]-Y[i])+X[i]))c=!c;return c;}


• i;j;c;f(float*X,float*Y,float x,float y){for(c=i=0,j=2;i<3;)c^=(Y[i]>y)-(Y[j]>y)&(x<(X[j]-X[i])*(y-Y[i])/(Y[j]-Y[i])+X[j=i++]);return c;} 137 - tested in javascript – bebe Jul 4 '14 at 11:20
• @bebe - That causes a syntax error. – Derek 朕會功夫 Jul 6 '14 at 0:00
• That does not cause a syntax error. – bebe Jul 6 '14 at 15:01

# Pyth 1.0.5, 5754 51

DgYb=Z0J'bWbK;bDiHNR*-'H'K-@N1@K1~Z>iYJiJY=JK)R!%Z3


Defines the function g, which takes two inputs: the test point, and then the list of the vertices of the triangle. Outputs True and False. Note: Destroys the input, specifically b, the list of the vertices of the triangle.

Try it here. The last few characters, gvwvw call the function with a test case on the next two lines.

Based on this algorithm

Explanation:

DgYb                  Define g(Y,b):
=Z0                     Z=0
J'b                     J=b[0]              (No = is needed because j is special).
Wb                      While len(b)>0:     (While b:)
K;b                       K=b.pop()
DiHN                      Define i(H,N):
R*-'H'K-@N1@K1              Return half of the linked equation.
~ZiYJiJY                  Z+=i(Y,J)>i(J,Y)
=JK                       J=K
)                       Wend
R!%Z3                   return not Z%3==0   (True iff Z == 0 or 3)


The CJam - Pyth war rages on!

• This should be a named function. Is w taking STDIN input? – xnor Jul 3 '14 at 22:50
• @xnor Oops, I missed that piece of the description. Will edit. – isaacg Jul 3 '14 at 22:57
• @xnor Are functions that print out the answer allowed, or must they return the answer? Currently, this prints out the answer, but I could have it return for one more character. – isaacg Jul 3 '14 at 23:09
• Return the answer. – xnor Jul 3 '14 at 23:11
• Can you perhaps save characters by replacing the counter Z with an empty set that you accumulate with Z|=, then test its length to see if only 0's or 1 were seen? The strategy turned out longer in Python, but perhaps it's worth it using Pyth primitives. – xnor Jul 6 '14 at 22:11

# J 64 45 (42 without assignment)

c=:*./@(>:&0)@({.(,(1-+/))@%.|:@}.)@(}:-"1{:)


The assignment is not necessary for the thing to be a function, so unsure whether to count it or not. Taking advantage of the flexible input: I'd like to have an array of (1 + number of vertices) x (dimensionality of the space).

Hoping to score some extra points here ... : This thing works for any dimension of simplex, not just triangles in a plane, but also a 3 sided pyramid in 3d space and so on. It also works when the number of vertices of the simplex is smaller than (n+1), then it computes whether the projection of the point onto the simplex is inside or not.

It converts to barycentric coordinates, then checks for negative ones, indicating the point is outside. Do mind J uses _ for negative

NB. example in triangle
D =: 4 2 $1 1 0 0 3 0 0 2 NB. 4 rows , x first, then the vertices of the triangle NB. subtract last vertex coordinates from the rest and drop reference node n=: (}:-"1{:) NB. preprocessed to barycentric coordinates bar=: {. (, 1 - +/)@%. |:@}. NB. all positive ap =: *./@(>:&0) insided =: ap@bar@n inside D 1  A run on the given examples:  true =: 0 : 0 [(-0.31961, -0.12646), (0.38478, 0.37419), (-0.30613, -0.59754), (-0.85548, 0.6633)] [(-0.87427, -0.00831), (0.78829, 0.60409), (-0.90904, -0.13856), (-0.80685, 0.48468)] [(0.28997, -0.03668), (-0.28362, 0.42831), (0.39332, -0.07474), (-0.48694, -0.10497)] [(-0.07783, 0.04415), (-0.34355, -0.07161), (0.59105, -0.93145), (0.29402, 0.90334)] [(0.36107, 0.05389), (0.27103, 0.47754), (-0.00341, -0.79472), (0.82549, -0.29028)] [(-0.01655, -0.20437), (-0.36194, -0.90281), (-0.26515, -0.4172), (0.36181, 0.51683)] [(-0.12198, -0.45897), (-0.35128, -0.85405), (0.84566, 0.99364), (0.13767, 0.78618)] [(-0.03847, -0.81531), (-0.18704, -0.33282), (-0.95717, -0.6337), (0.10976, -0.88374)] [(0.07904, -0.06245), (0.95181, -0.84223), (-0.75583, -0.34406), (0.16785, 0.87519)] [(-0.33485, 0.53875), (-0.25173, 0.51317), (-0.62441, -0.90698), (-0.47925, 0.74832)] ) false =: 0 : 0 [(-0.99103, 0.43842), (0.78128, -0.10985), (-0.84714, -0.20558), (-0.08925, -0.78608)] [(0.15087, -0.56212), (-0.87374, -0.3787), (0.86403, 0.60374), (0.01392, 0.84362)] [(0.1114, 0.66496), (-0.92633, 0.27408), (0.92439, 0.43692), (0.8298, -0.29647)] [(0.87786, -0.8594), (-0.42283, -0.97999), (0.58659, -0.327), (-0.22656, 0.80896)] [(0.43525, -0.8923), (0.86119, 0.78278), (-0.01348, 0.98093), (-0.56244, -0.75129)] [(-0.73365, 0.28332), (0.63263, 0.17177), (-0.38398, -0.43497), (-0.31123, 0.73168)] [(-0.57694, -0.87713), (-0.93622, 0.89397), (0.93117, 0.40775), (0.2323, -0.30718)] [(0.91059, 0.75966), (0.60118, 0.73186), (0.32178, 0.88296), (-0.90087, -0.26367)] [(0.3463, -0.89397), (0.99108, 0.13557), (0.50122, -0.8724), (0.43385, 0.00167)] [(0.88121, 0.36469), (-0.29829, 0.21429), (0.31395, 0.2734), (0.43267, -0.78192)] ) NB. replace - by _ to avoid problems NB. cut up per row, drop the [ ] and convert to numbers$dat_t =: ((4 2 $".)@}:@}.;._2) (true='-')} true ,: '_' 10 4 2$dat_f =: ((4 2 $".)@}:@}.;._2) (false='-')}false,: '_' 10 4 2 NB. this results in arrays with shape 10 4 2 NB. for each 4 x 2 array (rank 2), do c for all true instances c=:*./@(>:&0)@({.(,(1-+/))@%.|:@}.)@(}:-"1{:) c"2 dat_t 1 1 1 1 1 1 1 1 1 1 NB. the same for the false ones, demonstrating anonymous usage NB. still a function though (or verb in J parlance) *./@(>:&0)@({.(,(1-+/))@%.|:@}.)@(}:-"1{:)"2 dat_f 0 0 0 0 0 0 0 0 0 0  • I asked for a named function, so the assignment characters count. Here are some points for generalizing to polygons! ······ – xnor Jul 6 '14 at 22:01 • Well, actually, I don't quite generalize to polygons, but to N-dimensional simplexes, with maximum N+1 vertices. For instance a 4 vertex pyramid in 3-D space, or a 5 vertex simplex in 4-D space. The number of vertices can be lower than N+1, in which case the algorithm looks whether the orthogonal projection onto the hyperplane the simplex resides in lies inside the simplex or not (eg a 2 point simplex in 2-D will be projected on the line and checked whether this projection lies between the end points) – jpjacobs Jul 7 '14 at 7:34 ## HTML5 + JS, 13b + 146b / 141b / 114 chars HTML: <canvas id=C>  JS (146b): // @params: t1x, t1y, t2x, t2y, t3x, t3y, pointx, pointy function T(a,b,c,d,e,f,g,h){with(C.getContext("2d"))return beginPath(),moveTo(a,b),lineTo(c,d),lineTo(e,f),fill(),!!getImageData(g,h,1,1).data[3]}  or ES6 (141b): T=(a,b,c,d,e,f,g,h)=>{with(C.getContext("2d"))return beginPath(),moveTo(a,b),lineTo(c,d),lineTo(e,f),fill(),!!getImageData(g,h,1,1).data[3]}  or ES6 unicode-obfuscated (114 chars): eval(unescape(escape('𥀽𚁡𛁢𛁣𛁤𛁥𛁦𛁧𛁨𚐽🡻𭱩𭁨𚁃𛡧𩑴𠱯𫡴𩑸𭀨𘠲𩀢𚐩𬡥𭁵𬡮𘁢𩑧𪑮𤁡𭁨𚀩𛁭𫱶𩑔𫰨𨐬𨠩𛁬𪑮𩑔𫰨𨰬𩀩𛁬𪑮𩑔𫰨𩐬𩠩𛁦𪑬𫀨𚐬𘐡𩱥𭁉𫑡𩱥𡁡𭁡𚁧𛁨𛀱𛀱𚐮𩁡𭁡𦰳𧑽').replace(/uD./g,'')))  Unicode obfuscation made with: http://xem.github.io/obfuscatweet/ • It doesn't seem to yield the correct result when the point is close to the side: jsfiddle.net/L2B2A I believe this is because all inputs are between (-1,1), and your code is only testing the 4 pixels around the origin. – Derek 朕會功夫 Jul 6 '14 at 0:48 • that's right, to fit the examples, I should change the origin and the scale of my canvas to handle triangles inside [-1,1]. But why are those triangles so small anyways? – xem Jul 6 '14 at 6:56 • the problem says that all x y are between -1 and 1. Don't really know why, but I believe you can just multiply every input by 1e7 (to maintain precision) can get the correct result :D – Derek 朕會功夫 Jul 6 '14 at 16:21 • A graphical solution, very clever! – xnor Jul 6 '14 at 21:57 # Python (65) People seem to be done submitting, so I'll post my own solution to my question. f=lambda X,L:sum(((L[i-1]-X)/(L[i]-X)).imag>0for i in(0,1,2))%3<1  X is the complex number representing the test points, and L is a list of three points, each a complex number. First, I'll explain a less golfed version of the code; def f(X,A,B,C):A-=X;B-=X;C-=X;return((A/B).imag>0)==((B/C).imag>0)==((C/A).imag>0)  We shift the points A,B,C,X so that X is at the origin, taking advantage of Python's built-in complex arithmetic. We need to check if the origin is contained in the convex hull of A,B,C. This is equivalent to the origin always lying on the same side (left or right) of the line segments AB, BC, and AC. A segment AB has the origin on the left if one travel counterclockwise less than 180 degrees to get from A to B, and on the right otherwise. If we consider the angles a, b, and c corresponding to these points, this means b-a < 180 degrees (taken angles in the range 0 to 360 degrees). As complex numbers, angle(B/A)=angle(B)/angle(A). Also, angle(x) < 180 degrees exactly for point in he upper half-plane, which we check via imag(x)>0. So whether the origin lies to the left of AB is expressed as (A/B).imag>0. Checking whether these are all equal for each cyclic pair in A,B,C tells us whether triangle ABC contains the origin. Now, let's return to the fully golfed code f=lambda X,L:sum(((L[i-1]-X)/(L[i]-X)).imag>0for i in(0,1,2))%3<1  We generate each cyclic pair in (A-X,B-X,C-X)=(L[0]-X,L[1]-X,L[2]-X), taking advantage of negative Python list indices wrapping around (L[-1] = L[2]). To check that the Bools are all True (1) or all False (0), we add them and check divisibility by 3, as many solutions did. # Fortran - 232218195 174 Bloody awful. The function is horrendous because of the requirement that the data is passed to it and we cannot preprocess it. logical function L(x);real::x(8);p=x(1)-x(3);q=x(2)-x(4);r=x(5)-x(3);s=x(6)-x(4);t=x(7)-x(3);u=x(8)-x(4);L=ALL([p*(s-u)+q*(t-r)+r*u-t*s,p*u-q*t,q*r-p*s]>=r*u-t*s);endfunction  The decrease of 14 characters is because I forgot to golf the function name from my test runs. The further decrease is due to implicit typing and forgetting to change the function name. The next 20 characters came off due to reading in the points as a single array. The full program is program inTriagle real, dimension(2) :: a,b,c,x do print*,"Enter coordinates as x,a,b,c" read*,x,a,b,c if(all(x==0.0).and.all(a==0.0).and.all(b==0.0).and.all(c==0.0)) exit print*,"Is point in triangle: ",T(x,a,b,c) enddo contains! logical function L(x) real::x(8) p=x(1)-x(3);q=x(2)-x(4);r=x(5)-x(3) s=x(6)-x(4);t=x(7)-x(3);u=x(8)-x(4) L=ALL([p*(s-u)+q*(t-r)+r*u-t*s,p*u-q*t,q*r-p*s]>=r*u-t*s) endfunction end program inTriagle  • You can make this a bit shorter by relying on Fortran's implicit typing and using a single input array containing all 8 numbers: logical function T(x);real x(8);p=x(1)-x(3);q=x(2)-x(4);r=x(5)-x(3);s=x(6)-x(4);u=x(7)-x(3);v=x(8)-x(4);o=r*v-u*s;T=ALL([p*(s-v)+q*(u-r)+o,p*v-q*u,q*r-p*s]>=o);end I've tried shortening this further by using list operations, but unfortunately that didn't work out very well. – Ventero Jul 3 '14 at 21:47 • Even shorter by eliminating more common subexpressions: logical function T(x);real x(8);p=x(1)-x(3);q=x(2)-x(4);r=x(5)-x(3);s=x(6)-x(4);u=x(7)-x(3);v=x(8)-x(4);a=r*v-u*s;b=p*v-q*u;d=q*r-p*s;T=ALL([a-b-d,b,d]>=a);end I hope I didn't make any mistakes in the transformations! Though it looks like your original code doesn't pass all testcases. – Ventero Jul 3 '14 at 21:57 • @Ventero: I can't believe I forgot to abuse implicit typing :(. Thanks for your help! – Kyle Kanos Jul 3 '14 at 22:39 • @Ventero: Also, it seems my answer depends on the orientation of the triangle. The first True example in OP gives False if I swap B and C's values while giving True for the original orientation. – Kyle Kanos Jul 4 '14 at 0:54 • Ah, indeed, the problem is caused when (re-using the notation from my previous comment) a < 0, which effectively inverts the condition you have to test. Unfortunately this can't just be fixed by wrapping everything in an abs, as then the implied condition of b and d having the same sign as a gets lost. This can be fixed by using something like (again, re-using the notation and pre-defined variables from my last comment) e=a-b-d;T=ALL([a*a-b*b,a*a-d*d,a*a-e*e,a*b,a*d,a*e]>=0) - which probably can be golfed more. – Ventero Jul 4 '14 at 14:56 # MATLAB: 9! Not a whole lot of me to write here inpolygon  Can be called like so: inpolygon(2/3, 2/3, [0 1 1], [0 0 1])  Output is assigned to a variable named ans If I actually had to write a function, it may be something like so, could probably be optimized: function y=f(a,b,c,d) inpolygon(a,b,c,d)  • can be shorter using a function handle: f=@(a,b,c,d)inpolygon(a,b,c,d) – jpjacobs Jul 4 '14 at 12:43 # C# 218 (149?) using P=System.Drawing.PointF; bool F(P[]p){for(int i=0;i<4;i++){p[i].X*=1e7f;p[i].Y*=1e7f;}P[]a=new P[3];Array.Copy(p,1,a,0,3);var g=new System.Drawing.Drawing2D.GraphicsPath();g.AddLines(a);return g.IsVisible(p[0]);}  Probably not as character-efficient as a mathematical method, but it's a fun use of libraries. Incidentally, also rather slow. Also taking advantage of "Also don't worry about numerical stability or floating-point precision." - unfortunately, GraphicsPath uses ints internally, so a value in the range -1 < f < 1 can only have three possible values. Since floats only have 7 digits of precision, I just multiply by 1e7 to turn them into whole numbers. Hm, I guess it's not really losing any precision. It's also exploitable in another way: I probably could have taken advantage of ignoring precision and just given the "wrong" answer. If I'm allowed to ignore the character cost of importing libraries, 149 (at the very least, System.Linq and System.Drawing are pretty standard on most WinForms projects, but System.Drawing.Drawing2D might be a bit of a stretch): bool G(PointF[]p){for(int i=0;i<4;i++){p[i].X*=1e7f;p[i].Y*=1e7f;}var g=new GraphicsPath();g.AddLines(p.Skip(1).ToArray());return g.IsVisible(p[0]);}  Test program (yea, it's ugly): using System; using System.Drawing; using System.Drawing.Drawing2D; using P=System.Drawing.PointF; using System.Linq; class Program { static void Main(string[] args) { Program prog = new Program(); foreach (string test in @"[(-0.31961, -0.12646), (0.38478, 0.37419), (-0.30613, -0.59754), (-0.85548, 0.6633)] [(-0.87427, -0.00831), (0.78829, 0.60409), (-0.90904, -0.13856), (-0.80685, 0.48468)] [(0.28997, -0.03668), (-0.28362, 0.42831), (0.39332, -0.07474), (-0.48694, -0.10497)] [(-0.07783, 0.04415), (-0.34355, -0.07161), (0.59105, -0.93145), (0.29402, 0.90334)] [(0.36107, 0.05389), (0.27103, 0.47754), (-0.00341, -0.79472), (0.82549, -0.29028)] [(-0.01655, -0.20437), (-0.36194, -0.90281), (-0.26515, -0.4172), (0.36181, 0.51683)] [(-0.12198, -0.45897), (-0.35128, -0.85405), (0.84566, 0.99364), (0.13767, 0.78618)] [(-0.03847, -0.81531), (-0.18704, -0.33282), (-0.95717, -0.6337), (0.10976, -0.88374)] [(0.07904, -0.06245), (0.95181, -0.84223), (-0.75583, -0.34406), (0.16785, 0.87519)] [(-0.33485, 0.53875), (-0.25173, 0.51317), (-0.62441, -0.90698), (-0.47925, 0.74832)] [(-0.99103, 0.43842), (0.78128, -0.10985), (-0.84714, -0.20558), (-0.08925, -0.78608)] [(0.15087, -0.56212), (-0.87374, -0.3787), (0.86403, 0.60374), (0.01392, 0.84362)] [(0.1114, 0.66496), (-0.92633, 0.27408), (0.92439, 0.43692), (0.8298, -0.29647)] [(0.87786, -0.8594), (-0.42283, -0.97999), (0.58659, -0.327), (-0.22656, 0.80896)] [(0.43525, -0.8923), (0.86119, 0.78278), (-0.01348, 0.98093), (-0.56244, -0.75129)] [(-0.73365, 0.28332), (0.63263, 0.17177), (-0.38398, -0.43497), (-0.31123, 0.73168)] [(-0.57694, -0.87713), (-0.93622, 0.89397), (0.93117, 0.40775), (0.2323, -0.30718)] [(0.91059, 0.75966), (0.60118, 0.73186), (0.32178, 0.88296), (-0.90087, -0.26367)] [(0.3463, -0.89397), (0.99108, 0.13557), (0.50122, -0.8724), (0.43385, 0.00167)] [(0.88121, 0.36469), (-0.29829, 0.21429), (0.31395, 0.2734), (0.43267, -0.78192)]".Split('\n')) { string t = test.Replace("[(", "").Replace(")]", ""); string[] points = t.Split(new string[] { "), (" }, StringSplitOptions.None); string[] p = points[0].Split(','); P[] xabc = new P[4]; for (int i = 0; i < 4; i++) { p = points[i].Split(','); xabc[i] = new F(float.Parse(p[0]), float.Parse(p[1])); } Console.WriteLine(test + "=>" + prog.F(xabc)); } Console.ReadKey(); } bool G(PointF[]p) { for(int i=0;i<4;i++){p[i].X*=1e7f;p[i].Y*=1e7f;} var g=new GraphicsPath(); g.AddLines(p.Skip(1).ToArray()); return g.IsVisible(p[0]); } bool F(P[]p) { for(int i=0;i<4;i++){p[i].X*=1e7f;p[i].Y*=1e7f;} var g=new System.Drawing.Drawing2D.GraphicsPath(); g.AddLines(p.Skip(1).ToArray()); return g.IsVisible(p[0]); } }  • Cute, getting the drawing engine to do the work. – xnor Jul 6 '14 at 21:58 ### Haskell — 233 127 Using cross products as described here: h(a,b)(p,q)(r,s)(t,u)=z a b p q r s==z a b r s t u&&z a b r s t u==z a b t u p q where z j k l m n o =(o-m)*(j-l)+(l-n)*(k-m)>0  Previous solution implemented using barycentric coordinates and the formulae described in this Stack Exchange answer: g(p,q)(r,s)(t,u)(v,w)= let (j,k)=(p+(-r),q+(-s)) (l,m)=(t+(-r),u+(-s)) (n,o)=(v+(-r),w+(-s)) d=l*o-n*m a=(j*(m-o)+k*(n-l)+l*o-n*m)/d b=(j*o-k*n)/d c=(k*l-j*m)/d in (0<=a&&a<1)&&(0<=b&&b<1)&&(0<=c&&c<1)  Both functions g and h take four pairs, the first of which is the point to be tested for inclusion and the rest being the coordinates of the vertices of the triangle. To test with the sample input: let trueTestCases = [((-0.31961, -0.12646), (0.38478, 0.37419), (-0.30613, -0.59754), (-0.85548, 0.6633)), ((-0.87427, -0.00831), (0.78829, 0.60409), (-0.90904, -0.13856), (-0.80685, 0.48468)), ((0.28997, -0.03668), (-0.28362, 0.42831), (0.39332, -0.07474), (-0.48694, -0.10497)), ((-0.07783, 0.04415), (-0.34355, -0.07161), (0.59105, -0.93145), (0.29402, 0.90334)), ((0.36107, 0.05389), (0.27103, 0.47754), (-0.00341, -0.79472), (0.82549, -0.29028)), ((-0.01655, -0.20437), (-0.36194, -0.90281), (-0.26515, -0.4172), (0.36181, 0.51683)), ((-0.12198, -0.45897), (-0.35128, -0.85405), (0.84566, 0.99364), (0.13767, 0.78618)), ((-0.03847, -0.81531), (-0.18704, -0.33282), (-0.95717, -0.6337), (0.10976, -0.88374)), ((0.07904, -0.06245), (0.95181, -0.84223), (-0.75583, -0.34406), (0.16785, 0.87519)), ((-0.33485, 0.53875), (-0.25173, 0.51317), (-0.62441, -0.90698), (-0.47925, 0.74832))] let falseTestCases = [((-0.99103, 0.43842), (0.78128, -0.10985), (-0.84714, -0.20558), (-0.08925, -0.78608)), ((0.15087, -0.56212), (-0.87374, -0.3787), (0.86403, 0.60374), (0.01392, 0.84362)), ((0.1114, 0.66496), (-0.92633, 0.27408), (0.92439, 0.43692), (0.8298, -0.29647)), ((0.87786, -0.8594), (-0.42283, -0.97999), (0.58659, -0.327), (-0.22656, 0.80896)), ((0.43525, -0.8923), (0.86119, 0.78278), (-0.01348, 0.98093), (-0.56244, -0.75129)), ((-0.73365, 0.28332), (0.63263, 0.17177), (-0.38398, -0.43497), (-0.31123, 0.73168)), ((-0.57694, -0.87713), (-0.93622, 0.89397), (0.93117, 0.40775), (0.2323, -0.30718)), ((0.91059, 0.75966), (0.60118, 0.73186), (0.32178, 0.88296), (-0.90087, -0.26367)), ((0.3463, -0.89397), (0.99108, 0.13557), (0.50122, -0.8724), (0.43385, 0.00167)), ((0.88121, 0.36469), (-0.29829, 0.21429), (0.31395, 0.2734), (0.43267, -0.78192))] type Point = (Double, Double) test :: [(Point, Point, Point, Point)] -> [Bool] test testCases = map (\((px,py),(ax,ay),(bx,by),(cx,cy)) -> h (px,py) (ax,ay) (bx,by) (cx,cy)) testCases test trueTestCases --> [True,True,True,True,True,True,True,True,True,True] test falseTestCases --> [False,False,False,False,False,False,False,False,False,False]  Ungolfed solutions: type Point = (Double, Double) -- using cross products triangulate' (a, b) (p, q) (r, s) (t, u) = (side a b p q r s == side a b r s t u) && (side a b r s t u == side a b t u p q) where side j k l m n o = (o - m) * (j - l) + (-n + l) * (k - m) >= 0 -- using barycentric coordinates triangulate :: (Point, Point, Point, Point) -> Bool triangulate ((px, py), (ax, ay), (bx, by), (cx, cy)) = let (p'x, p'y) = (px + (-ax), py + (-ay)) (b'x, b'y) = (bx + (-ax), by + (-ay)) (c'x, c'y) = (cx + (-ax), cy + (-ay)) d = b'x * c'y - c'x * b'y a = (p'x * (b'y - c'y) + p'y * (c'x - b'x) + b'x * c'y - c'x * b'y) / d b = (p'x * c'y - p'y * c'x) / d c = (p'y * b'x - p'x * b'y) / d in (0 <= a && a < 1) && (0 <= b && b < 1) && (0 <= c && c < 1)  # JavaScript (ES6) 120 C=(p,q,i,j,k,l,m,n, z=j*(m-k)+i*(l-n)+k*n-l*m, s=(j*m-i*n+(n-j)*p+(i-m)*q)/z, t=(i*l-j*k+(j-l)*p+(k-i)*q)/z )=>s>0&t>0&s+t<1  Directly copied from my answer to This other question Test In FireFox/FireBug console Output all 1s ;[ C(-0.31961, -0.12646, 0.38478, 0.37419, -0.30613, -0.59754, -0.85548, 0.6633), C(-0.87427, -0.00831, 0.78829, 0.60409, -0.90904, -0.13856, -0.80685, 0.48468), C(0.28997, -0.03668, -0.28362, 0.42831, 0.39332, -0.07474, -0.48694, -0.10497), C(-0.07783, 0.04415, -0.34355, -0.07161, 0.59105, -0.93145, 0.29402, 0.90334), C(0.36107, 0.05389, 0.27103, 0.47754, -0.00341, -0.79472, 0.82549, -0.29028), C(-0.01655, -0.20437, -0.36194, -0.90281, -0.26515, -0.4172, 0.36181, 0.51683), C(-0.12198, -0.45897, -0.35128, -0.85405, 0.84566, 0.99364, 0.13767, 0.78618), C(-0.03847, -0.81531, -0.18704, -0.33282, -0.95717, -0.6337, 0.10976, -0.88374), C(0.07904, -0.06245, 0.95181, -0.84223, -0.75583, -0.34406, 0.16785, 0.87519), C(-0.33485, 0.53875, -0.25173, 0.51317, -0.62441, -0.90698, -0.47925, 0.74832) ]  Output all 0s ;[ C(-0.99103, 0.43842,0.78128, -0.10985,-0.84714, -0.20558,-0.08925, -0.78608), C(0.15087, -0.56212,-0.87374, -0.3787,0.86403, 0.60374,0.01392, 0.84362), C(0.1114, 0.66496,-0.92633, 0.27408,0.92439, 0.43692,0.8298, -0.29647), C(0.87786, -0.8594,-0.42283, -0.97999,0.58659, -0.327,-0.22656, 0.80896), C(0.43525, -0.8923,0.86119, 0.78278,-0.01348, 0.98093,-0.56244, -0.75129), C(-0.73365, 0.28332,0.63263, 0.17177,-0.38398, -0.43497,-0.31123, 0.73168), C(-0.57694, -0.87713,-0.93622, 0.89397,0.93117, 0.40775,0.2323, -0.30718), C(0.91059, 0.75966,0.60118, 0.73186,0.32178, 0.88296,-0.90087, -0.26367), C(0.3463, -0.89397,0.99108, 0.13557,0.50122, -0.8724,0.43385, 0.00167), C(0.88121, 0.36469,-0.29829, 0.21429,0.31395, 0.2734,0.43267, -0.78192) ]  # SmileBASIC, 111 100 characters DEF T X,Y,A,B,C,D,E,F Q=9e5GCLS GTRI(A-X)*Q,Q*(B-Y),Q*(C-X),Q*(D-Y),Q*(E-X),Q*(F-Y)?!!GSPOIT(0,0)END  Draws a triangle and checks the color of the pixel at the point. The triangle is scaled up 99999x and shifted so that the point to check will be at (0,0) before being drawn, to minimize loss in precision. # Intel 8087 FPU assembly, 222 220 bytes Uses only the 8087 FPU hardware to calculate. Here is the unassembled (ungolfed in this case too) version as a MACRO (will spare you the 220 hex byte codes): ; calculate the area of of a triangle ABC using determinate ; input: coordinates (float), Ax,Ay,Bx,By,Cx,Cy ; output: area in ST TAREA MACRO A1,A2,B1,B2,C1,C2 FLD A1 FLD B2 FLD C2 FSUB ; ST = By - Cy FMUL ; ST = Ax * ( By - Cy ) FLD B1 FLD C2 FLD A2 FSUB ; ST = Cy - Ay FMUL ; ST = Bx * ( Cy - Ay ) FLD C1 FLD A2 FLD B2 FSUB ; Ay - By FMUL ; Cx * ( Ay - By ) FADD ; Cx * ( Ay - By ) + Bx * ( Cy - Ay ) FADD ; Cx * ( Ay - By ) + Bx * ( Cy - Ay ) + Ax * ( By - Cy ) FLD1 ; make a value of 2 FADD ST,ST ; ST = 2 FDIV ; divide by 2 FABS ; take abs value ENDM ; determine if point X is in triangle ABC ; input: points X, A, B, C ; output: ZF=1 if X in triangle, ZF=0 if X not in triangle TXINABC MACRO X1,X2,A1,A2,B1,B2,C1,C2 TAREA A1,A2,B1,B2,C1,C2 ; ST(3) = area of triangle ABC TAREA X1,X2,B1,B2,C1,C2 ; ST(2) = area of triangle XBC TAREA A1,A2,X1,X2,C1,C2 ; ST(1) = area of triangle AXC TAREA A1,A2,B1,B2,X1,X2 ; ST(0) = area of triangle ABX FADD ; add areas of triangles with point FADD ; ST = ST + ST(1) + ST(2) FCOMPP ; compare ST to ST(1) and pop results FWAIT ; sync CPU/FPU FSTSW R ; store result flags to R MOV AX, R ; move result to AX SAHF ; store result into CPU flags for conditional check ENDM  ## Explanation Uses the determinate to calculate the area of the ABC triangle, and then the triangle formed with the X point and two other points of the ABC triangle. If the area of triangle ABC equals the sum of areas of triangles XBC + AXC + ABX, then the point is within the triangle. The result is returned as ZF. ### What's neat about this All of the math and floating point operations are done in hardware with 80-bit extended precision. The final floating point comparison is also done in hardware so will be very accurate. This also uses all eight of the 8087's stack registers at once. ### What's not quite as neat about this As the points of the triangle must be plugged back into the formulas several times during the calculation, it requires each variable in memory to be loaded into the FPU's stack registers one at a time in the correct order. While this can be fairly easily modeled like a function as a MACRO, it means that the code is expanded each time at assembly, creating redundant code. 41 bytes were saved by moving some of the same repeated code segments into PROCs. However it makes the code less readable, so the above listing is without it (which is why it's labeled as "ungolfed"). ## Tests Here is a test program using IBM DOS showing output: TTEST MACRO T LOCAL IS_IN_TRI TXINABC T,T+4*1,T+4*2,T+4*3,T+4*4,T+4*5,T+4*6,T+4*7 MOV DX, OFFSET TEQ ; load true string by default JZ IS_IN_TRI ; if ZF=1, it is in triangle, skip to display MOV DX, OFFSET FEQ ; otherwise ZF=0 means not in triangle, so load false string IS_IN_TRI: MOV AH, 9 ; DOS write string function INT 21H ENDM START: FINIT ; reset 8087 TTEST T0 ; true tests TTEST T1 TTEST T2 TTEST T3 TTEST T4 TTEST T5 TTEST T6 TTEST T7 TTEST T8 TTEST T9 TTEST F0 ; false tests TTEST F1 TTEST F2 TTEST F3 TTEST F4 TTEST F5 TTEST F6 TTEST F7 TTEST F8 TTEST F9 RET ; return to DOS T0 DD -0.31961, -0.12646, 0.38478, 0.37419, -0.30613, -0.59754, -0.85548, 0.6633 T1 DD -0.87427, -0.00831, 0.78829, 0.60409, -0.90904, -0.13856, -0.80685, 0.48468 T2 DD 0.28997, -0.03668, -0.28362, 0.42831, 0.39332, -0.07474, -0.48694, -0.10497 T3 DD -0.07783, 0.04415, -0.34355, -0.07161, 0.59105, -0.93145, 0.29402, 0.90334 T4 DD 0.36107, 0.05389, 0.27103, 0.47754, -0.00341, -0.79472, 0.82549, -0.29028 T5 DD -0.01655, -0.20437, -0.36194, -0.90281, -0.26515, -0.4172, 0.36181, 0.51683 T6 DD -0.12198, -0.45897, -0.35128, -0.85405, 0.84566, 0.99364, 0.13767, 0.78618 T7 DD -0.03847, -0.81531, -0.18704, -0.33282, -0.95717, -0.6337, 0.10976, -0.88374 T8 DD 0.07904, -0.06245, 0.95181, -0.84223, -0.75583, -0.34406, 0.16785, 0.87519 T9 DD -0.33485, 0.53875, -0.25173, 0.51317, -0.62441, -0.90698, -0.47925, 0.74832 F0 DD -0.99103, 0.43842, 0.78128, -0.10985, -0.84714, -0.20558, -0.08925, -0.78608 F1 DD 0.15087, -0.56212, -0.87374, -0.3787, 0.86403, 0.60374, 0.01392, 0.84362 F2 DD 0.1114, 0.66496, -0.92633, 0.27408, 0.92439, 0.43692, 0.8298, -0.29647 F3 DD 0.87786, -0.8594, -0.42283, -0.97999, 0.58659, -0.327, -0.22656, 0.80896 F4 DD 0.43525, -0.8923, 0.86119, 0.78278, -0.01348, 0.98093, -0.56244, -0.75129 F5 DD -0.73365, 0.28332, 0.63263, 0.17177, -0.38398, -0.43497, -0.31123, 0.73168 F6 DD -0.57694, -0.87713, -0.93622, 0.89397, 0.93117, 0.40775, 0.2323, -0.30718 F7 DD 0.91059, 0.75966, 0.60118, 0.73186, 0.32178, 0.88296, -0.90087, -0.26367 F8 DD 0.3463, -0.89397, 0.99108, 0.13557, 0.50122, -0.8724, 0.43385, 0.00167 F9 DD 0.88121, 0.36469, -0.29829, 0.21429, 0.31395, 0.2734, 0.43267, -0.78192 TEQ DB 'In Triangle',0DH,0AH,'$'
FEQ DB 'Not In Triangle',0DH,0AH,'\$'


### Output

In Triangle
In Triangle
In Triangle
In Triangle
In Triangle
In Triangle
In Triangle
In Triangle
In Triangle
In Triangle
Not In Triangle
Not In Triangle
Not In Triangle
Not In Triangle
Not In Triangle
Not In Triangle
Not In Triangle
Not In Triangle
Not In Triangle
Not In Triangle


## C 414 (was 465)

Golfed

#define D double
int F(D ax,D ay,D bx,D by,D cx,D cy,D px,D py){int y=0;double J,K;D m=(ax-bx<0.001)?(by-ay)/(ax-bx):1000;D b=m*ax+ay;J=m*cx-cy+b;K=m*px-py+b;if(J*K>=0)y=1;return y;}D T[8],k;int i,n;void G(){while(i<8){scanf("%lf",&k);T[i++]=k;}n+=F(T[2],T[3],T[4],T[5],T[6],T[7],T[0],T[1]);n+=F(T[4],T[5],T[6],T[7],T[2],T[3],T[0],T[1]);n+=F(T[2],T[3],T[6],T[7],T[4],T[5],T[0],T[1]);printf(n==3?"True":"False");}


Original function declaration added for explanation

/**
* determine if points C & P are on same side of line AB
* return 1 if true, 0 otherwise
*/
int PointsSameSide(D ax,D ay,D bx,D by,D cx, D cy, D px, D py);


Rewritten as a named function: input via stdin one each line or all in one line space-separated.

#define D double
int F(D ax,D ay,D bx,D by,D cx, D cy, D px, D py)
{
int y=0;
double J,K;
D m = (ax-bx<0.001)?(by-ay)/(ax-bx):1000;
D b = m*ax+ay;
J=m*cx-cy+b;
K=m*px-py+b;
if(J*K>=0)y=1;
return y;
}
double T[8],k;
int i,n;
void G()
{
while(i<8){scanf("%lf",&k);T[i++]=k;}
n+=F(T[2],T[3],T[4],T[5],T[6],T[7],T[0],T[1]);
n+=F(T[4],T[5],T[6],T[7],T[2],T[3],T[0],T[1]);
n+=F(T[2],T[3],T[6],T[7],T[4],T[5],T[0],T[1]);
printf(n==3?"True":"False");
}

• You could save some bytes by getting rid of newlines and unnecessary spaces. Also, you have double redefined as D but you still use double in the code. – gronostaj Jul 3 '14 at 22:18

Java, 149 characters

g=Math.atan2(100*(d-y),(a-x));h=Math.atan2(100*(e-y),(b-x));i=Math.atan2(100*(f-y),(c-x));k=Math.round(Math.abs(g-h)+Math.abs(h-i)+Math.abs(i-g))==6;


Horrible considering I have to write "Math." every time. This is the actual program:

package mathPackage;
public class InTriangle {
public static void main(String[] args) {
boolean k;
double a=-1,b=0,c=1,d=0,e=1,f=0,x=0,y=0.4;
double g,h,i;
g=Math.atan2(100*(d-y),(a-x));
h=Math.atan2(100*(e-y),(b-x));
i=Math.atan2(100*(f-y),(c-x));
k=Math.round(Math.abs(g-h)+Math.abs(h-i)+Math.abs(i-g))==6;
System.out.println(k);
System.out.println(g);
System.out.println(h);
System.out.println(i);
System.out.print(Math.abs(g-h)+Math.abs(h-i)+Math.abs(i-g));
}
}


where a is the x of point a,b is the x of point b, c for x of c, d is y of a, e is y of b, f is the y of c, and x and y are the x and y of the point. The boolean k determines whether it is true or not.

• What are the 100* for? – xnor Jul 6 '14 at 21:56

# JavaScript 125/198

If points are provided in 8 arguments:

function d(x,y,a,b,c,d,e,f){function z(a,b,c,d){return(y-b)*(c-a)-(x-a)*(d-b)>0}return(z(a,b,c,d)+z(c,d,e,f)+z(e,f,a,b))%3<1}


If points are provided in a 2-dimensional array:

function c(s){return (z(s[1][0],s[1][1],s[2][0],s[2][1])+z(s[2][0],s[2][1],s[3][0],s[3][1])+z(s[3][0],s[3][1],s[1][0],s[1][1]))%3<1;function z(a,b,c,d){return (s[0][1]-b)*(c-a)-(s[0][0]-a)*(d-b)>0}}


This code doesn't use any of those fancy vector math. Instead, it only uses a simple algebra trick to determine if the point is inside the triangle or not. The formula:

(y-b)(c-a) - (x-a)(d-b)


which tells the point is on which side of a line, is derived from rearranging the definition of slope:

            m = (y2-y1)/(x2-x1)
(y2-y1) = m(x2-x1)
(y-y1) = m(x-x1)     ,substituting point we are testing (x,y) to be the 2nd point
(y-y1) = (x-x1)(y2-y1)/(x2-x1)  ,substitute back the original definition of m
(y-y1)(x2-x1) = (x-x1)(y2-y1)    <-- left side will be greater than the right side, if
the point is on the left; otherwise, it's on the right
0 = (y-b)(c-a)-(x-a)(d-b) ,where (a,b)=(x1,y1), (c,d)=(x2,y2)


If we test all 3 sides, all 3 should yield some numbers with the same sign only when the point is inside the triangle since we are testing it around the triangle. If the point is on a side then one of the test should return 0.

jsFiddle test code: http://jsfiddle.net/DerekL/zEzZU/

var l = [[-0.31961, -0.12646, 0.38478, 0.37419, -0.30613, -0.59754, -0.85548, 0.6633],[-0.87427, -0.00831, 0.78829, 0.60409, -0.90904, -0.13856, -0.80685, 0.48468],[0.28997, -0.03668, -0.28362, 0.42831, 0.39332, -0.07474, -0.48694, -0.10497],[-0.07783, 0.04415, -0.34355, -0.07161, 0.59105, -0.93145, 0.29402, 0.90334],[0.36107, 0.05389, 0.27103, 0.47754, -0.00341, -0.79472, 0.82549, -0.29028],[-0.01655, -0.20437, -0.36194, -0.90281, -0.26515, -0.4172, 0.36181, 0.51683],[-0.12198, -0.45897, -0.35128, -0.85405, 0.84566, 0.99364, 0.13767, 0.78618],[-0.03847, -0.81531, -0.18704, -0.33282, -0.95717, -0.6337, 0.10976, -0.88374],[0.07904, -0.06245, 0.95181, -0.84223, -0.75583, -0.34406, 0.16785, 0.87519],[-0.33485, 0.53875, -0.25173, 0.51317, -0.62441, -0.90698, -0.47925, 0.74832],
[-0.99103, 0.43842, 0.78128, -0.10985, -0.84714, -0.20558, -0.08925, -0.78608],[0.15087, -0.56212, -0.87374, -0.3787, 0.86403, 0.60374, 0.01392, 0.84362],[0.1114, 0.66496, -0.92633, 0.27408, 0.92439, 0.43692, 0.8298, -0.29647],[0.87786, -0.8594, -0.42283, -0.97999, 0.58659, -0.327, -0.22656, 0.80896],[0.43525, -0.8923, 0.86119, 0.78278, -0.01348, 0.98093, -0.56244, -0.75129],[-0.73365, 0.28332, 0.63263, 0.17177, -0.38398, -0.43497, -0.31123, 0.73168],[-0.57694, -0.87713, -0.93622, 0.89397, 0.93117, 0.40775, 0.2323, -0.30718],[0.91059, 0.75966, 0.60118, 0.73186, 0.32178, 0.88296, -0.90087, -0.26367],[0.3463, -0.89397, 0.99108, 0.13557, 0.50122, -0.8724, 0.43385, 0.00167],[0.88121, 0.36469, -0.29829, 0.21429, 0.31395, 0.2734, 0.43267, -0.78192]];

function d(x,y,a,b,c,d,e,f){function z(a,b,c,d){return(y-b)*(c-a)-(x-a)*(d-b)>0}return(z(a,b,c,d)+z(c,d,e,f)+z(e,f,a,b))%3<1}

for(var i = 0; i < l.length; i++){
console.log(d.apply(undefined,l[i]));    //10 true, 10 false
}


97 characters (not counting spaces or tabs) count if converted into CoffeeScript:

d=(x,y,a,b,c,d,e,f)->
z=(a,b,c,d)->
(y-b)*(c-a)-(x-a)*(d-b)>0
(z(a,b,c,d)+z(c,d,e,f)+z(e,f,a,b))%3<1


115 characters if converted into ES6:

d=(x,y,a,b,c,d,e,f)=>{z=(a,b,c,d)=>{return (y-b)*(c-a)-(x-a)*(d-b)>0};return(z(a,b,c,d)+z(c,d,e,f)+z(e,f,a,b))%3<1}

• That is the "fancy vector math" I'm using :D (not the fancy barycentric coordinate approach some others took, though). Like the top-voted answer, you can save a few bytes by using ES6 and defining the functions like d=(x,y,...)=>{...}. In your case, you can save even more by using CoffeeScript, which doesn't need return: pastebin.com/RVFk1D5k ... and in any case you can save one byte by using <1 instead of ==0. – Martin Ender Jul 5 '14 at 9:25
• @m.buettner :o I thought the equation I used have nothing to do with vectors (derived from simple algebra) but apparently they both yield the same equation. Math is wonderful. – Derek 朕會功夫 Jul 5 '14 at 15:27

## R, 23

Inspired by MATLAB,

SDMTools::pnt.in.poly()


called like SDMTools::pnt.in.poly(point,triangle) where point is a length-2 vector and triangle is a 3x2 matrix of vertices. SDMTools is available on CRAN.

# Mathematica, 38 characters

RegionMember[Polygon[#[[1]]],#[[2]]] &


Example:

d = {{{0, 0}, {1, 0}, {.5, .7}}, {.5, .6}};

RegionMember[Polygon[#[[1]]], #[[2]]] & @ d


(* True *)

• It's standard to count spaces as characters, but presumably here you can remove them without anything breaking. – xnor Feb 8 '19 at 6:44
• Also, you need to take input and produce output rather than using predefined variables. You can search up some Mathematica answers to see how they do it. – xnor Feb 8 '19 at 6:46

# C (gcc), 108 bytes

i;f(a,b,c,d,e,f,g,h)float a,b,c,d,e,f,g,h;{i=(e-=a)*(h-=b)>(f-=b)*(g-=a);i=(c-=a)*f>(d-=b)*e==i&i==g*d>h*c;}


Try it online!

Takes three cross products and returns 1 if the sign of the k̂ component doesn't change.