The handshake problem

Question

The handshake problem is the classic problem that for n people in a room, if they all shake hands, what's the total number of handshakes that occur.

You code should take an input of any number and output the number of handshakes, both in base ten.

Also the code should not let non-integers through. You should output a falsey in this case. The only expected inputs are positive whole integers and positive real numbers - you don't need to worry about complex numbers or strings as inputs. This should mean that you should expect n >= 0

Test cases

(N:handshakes)
0:0
10:45
20:190
10.5:0
1337:893116

Normal golfing rules apply and shortest code wins. If your code isn't self explanatory, please explain.

Jelly and 05AB1E are the winners, both at 2 bytes

Answer 1

JavaScript ES6, 16

n=>n%1?0:~-n*n/2

Test

f=n=>n%1?0:~-n*n/2

function test()
{
  var i=I.value
  O.textContent=f(i)
}  

<input id=I oninput=test()><span id=O></span>

Answer 2

05AB1E, 2 bytes

Code:

2c

Try it online!.

Answer 3

MATL, 3 bytes

2Xn

Try it online!

The code computes the binomial coefficient "n choose 2", where n is the input. For non-integer input no output is produced.

       % Take input implicitly
2      % Push 2
Xn     % n-choose-k
       % Display implicitly

Answer 4

Brainfuck, 22 21 bytes

,[-[->+>+<<]>[-<+>]<]

Try it online! Note, that brainfuck takes input and output through ASCII, so to enter a "49", you enter ascii value 49, e.g. 1. Brainfuck has no concept of non-integers, so it is impossible to enter one.

Answer 5

Jelly, 2 bytes

c2

Try it online!

Answer 6

Matlab, 61 23 bytes

@(x)~mod(x,1)*x*(x-1)/2

Improvements courtesy Luis Mendo.

Old answer:

@(x)eval('if isnumeric(x) disp((mod(x,1)==0)*x*(x-1)/2);end')

An anonymous function that calls eval. Matlab considers all numbers entered to be doubles unless you explicitly tell it otherwise, so testing for integerness via built-in isn't really an option unless you want to condition your input heavily-- thus the isnumeric and the mod1.

As well, Matlab doesn't allow conditional statements in anonymous functions, so you have to kludge around that by way of an eval statement that evaluates a string. So fun.

The "falsey" output is that it doesn't print anything at all for nonnumerics and prints 0 for a non-integer numeric. For a valid input, it will print the result.

Answer 7

Python, 37 50, 46, 32, 26 bytes

lambda a:a%1==0and a*~-a/2

Making use of the fact that the solution of the handshake problem is (N-1) + (N-2) ... 2 + 1 for N people. The most problematic is the falsey value when a non-int is given.

Big thanks to Leaky Nun and Martin Ender!

Answer 8

CJam, 7 bytes

ri_(*2/

Computes n*(n−1), where n is the input. For non-integer input it produces no output.

Try it online!

r     e# Read input
i     e# Convert to integer
_     e# Duplicate
(     e# Subtract 1
*     e# Multiply
2/    e# Divide by 2.
      e# Implicit display

Answer 9

Mathematica, 19 bytes

#(#-1)/2/._Real->0&

This first computes the result using the explicit formula n(n-1)/2 but then replaces it with 0 if the that gives a floating-point result.

Just for fun, some alternatives to computing the result (minus the input validation) are:

#~Binomial~2
Tr@Range@#-#
n~Sum~{n,#-1}
PolygonalNumber@#-#

Answer 10

R, 28 bytes

Returns 0 if the input, x, is not an integer

x=scan();(x%%1==0)*x*(x-1)/2

Answer 11

ArnoldC, 266 bytes

ArnoldC has no concept of non-integers, so it is impossible to enter one.

LISTEN TO ME VERY CAREFULLY f
I NEED YOUR CLOTHES YOUR BOOTS AND YOUR MOTORCYCLE n
GIVE THESE PEOPLE AIR
HEY CHRISTMAS TREE r
YOU SET US UP 0
GET TO THE CHOPPER r
HERE IS MY INVITATION n
GET DOWN 1
YOU'RE FIRED n
HE HAD TO SPLIT 2
ENOUGH TALK
I'LL BE BACK r
HASTA LA VISTA, BABY

Explanations:

DeclareMethod f
        MethodArguments n
        NonVoidMethod
        DeclareInt r
        SetInitialValue 0
            AssignVariable r
                   SetValue n
                   MinusOperator 1
                   MultiplicationOperator n
                   DivisionOperator 2
            EndAssignVariable
        Return r
EndMethodDeclaration

Answer 12

CJam, 5 bytes

ri,1b

Prints nothing (except to STDERR) for non-integer input.

Try it online!

Explanation

ri  e# Read input and try to convert to integer N.
,   e# Turn into range [0 1 2 ... n-1].
1b  e# Sum the list by treating it as base-1 digits. We do this instead of
    e# a simple reduction (:+) because the latter fails for empty lists.

Answer 13

Desmos, 11 bytes

.5nn-.5n
n=1

Try it here

Ungolfed formula:

Answer 14

Hoon, 30 bytes

|=
n/@
(div (mul n (dec n)) 2)

Just compute (n * (n-1))/2. Hoon is strongly typed, so you can't call this function with a float or negative number.

Answer 15

Pyth, 7 bytes

?sIQ.cQ2Z
.x.cQ2Z

Test suite.

Answer 16

Python 3, 22 bytes

lambda z:sum(range(z))

Makes use of Python's standard library. Raises an error whenever the function is fed with a non-integer.

Answer 17

Python 3, 32 16 bytes

lambda n:n*~-n/2

Throws an error if n is not an integer.

Old Version:

lambda n:(0,n*~-n/2)[n==int(n)]

Answer 18

PowerShell v2+, 33 bytes

param($n)$n*($n-1)/2*($n-is[int])

Full program. 20 bytes to compute n(n-1)/2 and 13 bytes to validate input. Uses the -is operator, so requires V2 or newer.

Answer 19

Dyalog APL, 9 bytes

2∘!×(|=⌊)

The chose-2 times (whether the abs equals the floor).

Answer 20

J, 14 8 bytes

2&!*]=<.

6 bytes saved thanks to miles!

Explaination:

2&!      NB. Binomial coefficient of 2 and the argument
   *     NB. Times...
    ]=<. NB. argument == floor(argument)

Previous solution: -:@(*<:)*(=<.)

Answer 21

dc, 18 bytes

[d/]sqdX0!=qd1-*2/

Explanation

[d/]sq

Store in register q a macro that replaces top of stack with 1 (by dividing by itself).

dX0!=q

If there are any fraction digits, then execute the macro.

d1-*2/

Compute ½n(n-1). If we started with 1 (either as input or because a non-integer value was entered) the result will be 0.

Answer 22

Factor with load-all, 12 bytes

[ iota sum ]

Same as this answer.

Answer 23

Java, 363 bytes

I thought I ought to make sure everybody gets a chance to meet everybody else.

Unfortunately some of the party guests do have to ask if they've met someone before, to avoid shaking someone's hand twice. Hopefully the guests are pretty good at recognizing faces.

Golfed:

import java.util.ArrayList;public class H{static int h = 0;public static void main(String[] a){int b = Integer.parseInt(a[0]);P[] e = new P[b];for(int i=0;i<b;i++){e[i] = new P();}for(P p : e){for(P o : e){p.s(o);}}System.out.print(h);}}class P {ArrayList<P> l = new ArrayList<P>();void s(P o){if(o.equals(this)){return;}if(!o.l.contains(this)){H.h++;l.add(o);}}}

Un-golfed:

import java.util.ArrayList;

public class PartyHost {
    static int shakes = 0;
    public static void main(String[] args){
        int amount = Integer.parseInt(args[0]);
        Person[] people = new Person[amount];
        for(int i=0; i<amount;i++){
            people[i] = new Person();
            // welcome to the party!
        }
        // well, we gotta get this party rockin'. hey, have you met my friend...
        for(Person p : people){
            for(Person other : people){
                p.handshake(other);
            }
        }
        System.out.print(shakes);
    }
}

class Person {
    ArrayList<Person> already_shook = new ArrayList<Person>();
    void handshake(Person other){
        // hmm, have we met already?
        if(other.equals(this)){
            // I mean, I guess I could shake hands with myself... maybe later.
            return;
        }
        if(!other.already_shook.contains(this)){
            // it seems not!
            // *firm grasp. maintain eye contact*
            // uh oh don't hold on too long
            // hope he doesn't grab too early or too late
            PartyHost.shakes++;
            already_shook.add(other);
        }
    }
}

Answer 24

𝔼𝕊𝕄𝕚𝕟, 5 chars / 7 bytes

МƦ⁽ï2

Try it here (ES6 browsers only).

Builtins are pretty cool.

Answer 25

RETURN, 8 bytes

[$1-×2÷]

Try it here.

Anonymous lambda that leaves result as top of stack. Usage:

10[$1-×2÷]!

Explanation

[      ]  lambda
 $1-      n-1
    ×     *n
     2÷   /2

Answer 26

Google Sheets, 17 14 bytes

=((B1^2)-B1)/2

Cell B1 is the input.

Answer 27

Haskell, 15 bytes

f x=sum[1..x-1]

[x..y] ranges are inclusive...

Oh and I don't know how to handle the non-integers...

Answer 28

Golisp, 18 bytes

{(n)~[+range[1n]]}

Python equivalent: lambda n: sum(range(1, n)).

Oh and Golisp don't have floating point number (for now)

Answer 29

Perl 5, 25 bytes

A subroutine:

{($_=pop)*!/\D/*($_-1)/2}

See it in action:

perl -e'print sub{($_=pop)*!/\D/*($_-1)/2}->(3.14)'