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The goal of this code golf is to create a program or function that calculates and outputs the cube root of a number that's given as input.
The rules:

  • No external resources
  • No use of built-in cube root functions.
  • No use of methods/operators that can raise a number to a power.
  • Your function/program must be able to accept floating-point numbers and negative numbers as input.
  • If the cube root is a floating-point number, then round it to 4 numbers after the decimal point.
  • This is a code golf, the shortest code in bytes wins.

Test cases:

27 --> 3
64 --> 4
1  --> 1
18.609625 --> 2.65
3652264 --> 154
0.001 --> 0.1
7  --> 1.9129

You can use all test cases above to test negative numbers (-27 --> -3, -64 --> -4 ...)

share|improve this question
    
What should I output for a cube root of 7? –  yo' Feb 22 at 12:46
    
@tohecz: I updated my question. –  ProgramFOX Feb 22 at 12:48
    
damn, if you allowed only numbers with precise cube, I would have a nice golf –  yo' Feb 22 at 12:51
    
Judging from your test cases I assume the program only needs to deal with real numbers? –  ace Feb 22 at 12:57
    
@ace add complex and I change 2 letters in my code ;) –  yo' Feb 22 at 13:05

20 Answers 20

up vote 6 down vote accepted

J: 16 characters

Loose translation of the Haskell answer:

-:@((%*~)+])^:_~

Test cases:

   -:@((%*~)+])^:_~27
3
   -:@((%*~)+])^:_~64
4
   -:@((%*~)+])^:_~1
1
   -:@((%*~)+])^:_~18.609625
2.65
   -:@((%*~)+])^:_~3652264
154
   -:@((%*~)+])^:_~0.001
0.1
   -:@((%*~)+])^:_~7
1.91293

It works like this:

     (-:@((% *~) + ])^:_)~ 27
↔ 27 (-:@((% *~) + ])^:_) 27
↔ 27 (-:@((% *~) + ])^:_) 27 (-:@((% *~) + ])) 27
↔ 27 (-:@((% *~) + ])^:_) -: ((27 % 27 * 27) + 27)
↔ 27 (-:@((% *~) + ])^:_) 13.5185
↔ 27 (-:@((% *~) + ])^:_) 27 (-:@((% *~) + ])) 13.5185
↔ 27 (-:@((% *~) + ])^:_) -: ((27 % 13.5185 * 13.5185) + 13.5185)
↔ 27 (-:@((% *~) + ])^:_) 6.83313
...

In words:

half =. -:
of =. @
divideBy =. %
times =. *
add =. +
right =. ]
iterate =. ^:
infinite =. _
fixpoint =. iterate infinite
by_self =. ~

-:@((%*~)+])^:_~ ↔ half of ((divideBy times by_self) add right) fixpoint by_self

Not one of the best wordy translations, since there's a dyadic fork and a ~ right at the end.

share|improve this answer

Haskell - 35

c n=(iterate(\x->(x+n/x/x)/2)n)!!99

Example runs:

c 27  =>  3.0
c 64  =>  4.0
c 1  =>  1.0
c 18.609625  =>  2.6500000000000004  # only first 4 digits are important, right?
c 3652264  =>  154.0
c 0.001  =>  0.1
c 7  =>  1.9129311827723892
c (-27)  =>  -3.0
c (-64)  =>  -4.0

Moreover, if you import Data.Complex, it even works on complex numbers, it returns one of the roots of the number (there are 3):

c (18:+26)  =>  3.0 :+ 1.0

The :+ operator should be read as 'plus i times'

share|improve this answer
1  
This deserves a +1. I've been refactoring generalized nth root algs for the last hour, and I just now arrived at the same result. Bravo. –  primo Feb 22 at 14:47
    
@primo I instantly recalled all n'th root approximation algorithms, and after giving up on Taylor/Maclaurin series in APL I used this. –  mniip Feb 22 at 15:28
    
Using Newton method I got x=(2*x+n/x/x)/3, can you explain why you can use x=(x+n/x/x)/2 ? It converges slower but I can't explain why it converges... –  Mig Feb 22 at 15:42
    
@Michael because if you take x=cbrt(n), then x=(x+n/x/x)/2 is true. So is it true for your expression –  mniip Feb 22 at 15:50
    
@Michael I got there this way: codepad.org/gwMWniZB –  primo Feb 22 at 16:03

SageMath, (69) 62 bytes

However, don't ever believe it will give you the result, it's very difficult to go randomly through all the numbers:

def r(x):
 y=0
 while y*y*y-x:y=RR.random_element()
 return "%.4f"%y

if you didn't insist on truncating:

def r(x):
 y=0
 while y*y*y-x:y=RR.random_element()
 return y

SageMath, 12 bytes, if exp is allowed

Works for all stuff: positive, negative, zero, complex, ...

exp(ln(x)/3)
share|improve this answer
    
I believe you are using an operator that can raise a number to a power. –  ace Feb 22 at 13:01
    
ah ok, right, edited –  yo' Feb 22 at 13:02
4  
+1 for a monumentally stupid algorithm that still satisfies the requirements. –  Mechanical snail Feb 22 at 21:30
    
@Mechanicalsnail Thanks. I hope it's obvious that what I do is a sort of recession :D However, if exp is allowed, I'm down to 12 and not being stupid at all :) –  yo' Feb 22 at 21:59

Python - 62 bytes

x=v=input()
exec"x*=(2.*v+x*x*x)/(v+2*x*x*x or 1);"*99;print x

Evaluates to full floating point precision. The method used is Halley's method. As each iteration produces 3 times as many correct digits as the last, 99 iterations is a bit of overkill.

Input/output:

27 -> 3.0
64 -> 4.0
1 -> 1.0
18.609625 -> 2.65
3652264 -> 154.0
0.001 -> 0.1
7 -> 1.91293118277
0 -> 1.57772181044e-30
-2 -> -1.25992104989
share|improve this answer
    
How does this work? –  justhalf Feb 22 at 13:09
1  
@justhalf I think this is the Newton's method of approximation basically. –  yo' Feb 22 at 13:10
    
Btw, fails on 0 –  yo' Feb 22 at 13:11
    
Fails on -2, sorry for that. –  yo' Feb 22 at 13:13
1  
@plg The problem description forbids the use of any exponential function, otherwise v**(1/.3) would be a sure winner. –  primo Feb 25 at 19:31

PHP - 81 bytes

Iterative solution:

$i=0;while(($y=abs($x=$argv[1]))-$i*$i*$i>1e-4)$i+=1e-5;@print $y/$x*round($i,4);
share|improve this answer
    
What happens if it tries to calculate the cube root of zero? –  Victor Feb 22 at 13:42
    
It will just output "0" (thanks to the error suppression operator - "@"). –  Razvan Feb 22 at 13:44
1  
0.0001 can be replaced by 1e-4 and 0.00001 by 1e.5. –  ComFreek Feb 22 at 13:56
    
Thanks! I updated the script. –  Razvan Feb 22 at 14:04

Javascript (55)

function f(n){for(i=x=99;i--;)x=(2*x+n/x/x)/3;return x}

BONUS, General formulation for all roots
function f(n,p){for(i=x=99;i--;)x=x-(x-n/Math.pow(x,p-1))/p;return x}

For cube root, just use f(n,3), square root f(n,2), etc... Example : f(1024,10) returns 2.

Explanation
Based on Newton method :

Find : f(x) = x^3 - n = 0, the solution is n = x^3
The derivation : f'(x) = 3*x^2

Iterate :
x(i+1) = x(i) - f(x(i))/f'(x(i)) = x(i) + (2/3)*x + (1/3)*n/x^2

Tests

[27,64,1,18.609625,3652264,0.001,7].forEach(function(n){console.log(n + ' (' + -n + ') => ' + f(n) + ' ('+ f(-n) +')')})

27 (-27) => 3 (-3)
64 (-64) => 4 (-4)
1 (-1) => 1 (-1)
18.609625 (-18.609625) => 2.65 (-2.65)
3652264 (-3652264) => 154 (-154)
0.001 (-0.001) => 0.09999999999999999 (-0.09999999999999999)
7 (-7) => 1.912931182772389 (-1.912931182772389) 
share|improve this answer
    
One character shorter: function f(n){for(i=x=99;i--;)x-=(x-n/x/x)/3;return x} –  copy Feb 25 at 19:01

APL - 31

(×X)×+/1,(×\99⍴(⍟|X←⎕)÷3)÷×\⍳99

Uses the fact that cbrt(x)=e^(ln(x)/3), but instead of doing naive exponentiation, it computes e^x using Taylor/Maclaurin series.

Sample runs:

⎕: 27
3
⎕: 64
4
⎕: 1
1
⎕: 18.609625
2.65
⎕: 3652264
154
⎕: 0.001
0.1
⎕: 7
1.912931183
⎕: ¯27
¯3
⎕: ¯7
¯1.912931183

Seeing as there's a J answer in 16 characters, I must be really terrible at APL...

share|improve this answer

Java, 207 182 181

Sometimes when I play golf I have two many beers and play really really bad

class n{public static void main(String[]a){double d=Double.valueOf(a[0]);double i=d;for(int j=0;j<99;j++)i=(d/(i*i)+(2.0*i))/3.0;System.out.println((double)Math.round(i*1e4)/1e4);}}

Iterative Newton's Method of Approximation, runs 99 iterations.

Here is the unGolfed:

class n{
    public static void main(String a[]){
        //assuming the input value is the first parameter of the input
        //arguments as a String, get the Double value of it
        double d=Double.valueOf(a[0]);
        //Newton's method needs a guess at a starting point for the 
        //iterative approximation, there are much better ways at 
        //going about this, but this is by far the simplest. Given
        //the nature of the problem, it should suffice fine with 99 iterations
        double i=d;

        //make successive better approximations, do it 99 times
        for(int j=0;j<99;j++){
            i=( (d/(i*i)) + (2.0*i) ) / 3.0;
        }
        //print out the answer to standard out
        //also need to round off the double to meet the requirements
        //of the problem.  Short and sweet method of rounding:
        System.out.println( (double)Math.round(i*10000.0) / 10000.0 );
    }
}
share|improve this answer
1  
You may rename the args variable to something like z, reducing 6 chars. You may remove the space and the curly braces in the body of the for-loop, reducing 3 chars. You may replace 10000.0 by 1e4, reducing 6 chars. The class does not needs to be public, so you can reduce more 7 chars. This way it will be reduced to 185 characters. –  Victor Feb 22 at 22:07
    
Is the cast at the end really necessary? It does not for me. –  Victor Feb 22 at 22:13
    
@Victor Thanks for the good eye, the use of E notation for the 10000.0 double was a spectacularly good idea. By the design of the question, I think it is legit to make this a method instead of a functioning cli class, which would reduce the size considerably. With Java, I didn't think I had a chance, so I erred on the side of functional. –  md_rasler Feb 25 at 3:28
    
Welcome to CodeGolf! Don't forget to add an in-answer explanation of how this works! –  Quincunx Feb 25 at 6:06
    
@Quincunx, Thanks, made recommended change. –  md_rasler Feb 25 at 15:42

Javascript - 157 characters

This function:

  • Handle negative numbers.
  • Handle floating-pointing numbers.
  • Execute quickly for any input number.
  • Has the maximum precision allowed for javascript floating-point numbers.
function f(a){if(p=q=a<=1)return a<0?-f(-a):a==0|a==1?a:1/f(1/a);for(v=u=1;v*v*v<a;v*=2);while(u!=p|v!=q){p=u;q=v;k=(u+v)/2;if(k*k*k>a)v=k;else u=k}return u}

Ungolfed explained version:

function f(a) {
  if (p = q = a <= 1) return a < 0 ? -f(-a)      // if a < 0, it is the negative of the positive cube root.
                           : a == 0 | a == 1 ? a // if a is 0 or 1, its cube root is too.
                           : 1 / f (1 / a);      // if a < 1 (and a > 0) invert the number and return the inverse of the result.

  // Now, we only need to handle positive numbers > 1.

  // Start u and v with 1, and double v until it becomes a power of 2 greater than the given number.
  for (v = u = 1; v * v * v < a; v *= 2);

  // Bisects the u-v interval iteratively while u or v are changing, which means that we still did not reached the precision limit.
  // Use p and q to keep track of the last values of u and v so we are able to detect the change.
  while (u != p | v != q) {
    p = u;
    q = v;
    k = (u + v) / 2;
    if (k * k * k > a)
      v=k;
    else
      u=k
  }

  // At this point u <= cbrt(a) and v >= cbrt(a) and they are the closest that is possible to the true result that is possible using javascript-floating point precision.
  // If u == v then we have an exact cube root.
  // Return u because if u != v, u < cbrt(a), i.e. it is rounded towards zero.
  return u
}
share|improve this answer

Javascript: 73 characters

This algorithm is lame, and exploits the fact that this question is limited to 4 digits after the decimal point. It is a modified version of the algorithm that I suggested in the sandbox for the purpose of reworking the question. It counts from zero to the infinite while h*h*h<a, just with a multiplication and division trick to handle the 4 decimal digits pecision.

function g(a){if(a<0)return-g(-a);for(h=0;h*h*h<1e12*a;h++);return h/1e4}
share|improve this answer

TI-BASIC

Input X:round((X>0)*e^ln(|X|)/3),4
share|improve this answer
    
That directly uses the ^ operator, doesn't it. It is forbidden by the rules –  mniip Feb 22 at 20:48
    
@mniip: Is e^ is a single operator on the TI-83 series? I don't remember. Either way, it's violating the spirit of the rules. –  Mechanical snail Feb 22 at 21:29
    
@Mechanicalsnail It doesn't matter I would say. In most languages you could just do exp(ln(x)/3) or e^(ln(x/3)) if you allowed any of these two. But somehow I understand to exp(ln(x)/a) as too much equivalent to x^(1/a) to be allowed by the rules :-/ –  yo' Feb 22 at 21:56

PHP, 61

Based on Newton's method. Slightly modified version of Michael's answer:

for($i=$x=1;$i++<99;)$x=(2*$x+$n/$x/$x)/3;echo round($x,14);

It works with negative numbers, can handle floating point numbers, and rounds the result to 4 numbers after the decimal point if the result is a floating point number.

Working demo

share|improve this answer

Game Maker Language, 54

for(i=x=1;i++<99;i=i)x=(2*x+argument0/x/x)/3;return x}

Yes, i=i is required.

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Perl, 92 bytes

sub a{$x=1;while($d=($x-$_[0]/$x/$x)/3,abs$d>1e-9){$x-=$d}$_=sprintf'%.4f',$x;s/\.?0*$//;$_}
  • The function a returns a string with the number without an unnecessary fraction part or insignificant zeroes at the right end.

Result:

              27 --> 3
             -27 --> -3
              64 --> 4
             -64 --> -4
               1 --> 1
              -1 --> -1
       18.609625 --> 2.65
      -18.609625 --> -2.65
         3652264 --> 154
        -3652264 --> -154
           0.001 --> 0.1
          -0.001 --> -0.1
               7 --> 1.9129
              -7 --> -1.9129
 0.0000000000002 --> 0.0001
-0.0000000000002 --> -0.0001
               0 --> 0
              -0 --> 0

Generated by

sub test{
    my $a = shift;
    printf "%16s --> %s\n", $a, a($a);
    printf "%16s --> %s\n", "-$a", a(-$a);
}
test 27;
test 64;
test 1;
test 18.609625;
test 3652264;
test 0.001;
test 7;
test "0.0000000000002";
test 0;

The calculation is based on Newton's method:

Calculation

share|improve this answer

Java - 157

  • No use of methods/operators that can raise a number to a power.

pow() can be split up into an exp/log combination, so I'm not sure this applies, but here we go anyway:

public class X{
    public static void main(String[]a){
        double x=Double.valueOf(a[0]);
        double y=Math.exp(Math.log(x<0?-x:x)/3);
        System.out.printf("%.4f",x<0?-y:y);
    }
}
share|improve this answer
    
You don't need the public. Also, use double x=x=Double.valueOf(a[0]),y=Math.exp(Math.log(x<0?-x:x)/3);. This shortens it to 143 bytes. –  nyuszika7h Aug 3 at 15:22

Befunge 98 - Work in progress

This language does not support floating point numbers; this attempts to emulate them. It currently works for positive numbers that do not start with 0 after the decimal point (mostly). However, it only outputs to 2 decimal places.

&5ka5k*&+00pv
:::**00g`!jv>1+
/.'.,aa*%.@>1-:aa*

It works by inputting the part before the decimal point, multiplying that by 100000, then inputting the part after the point and adding the two numbers together. The second line does a counter until the cube is greater than the inputted number. Then the third line extracts the decimal number from the integer.

If anyone can tell me why the third line only divides by 100 to get the correct values, please tell me.

IOs:

27.0       3 .0
64.0       4 .0
1.0        1 .0
18.609625  2 .65
0.001      0 .1
7.0        1 .91

0.1        0 .1
share|improve this answer

Smalltalk, 37

credit goes to mniip for the algorithm; Smalltalk version of his code:

input in n; output in x:

1to:(x:=99)do:[:i|x:=2*x+(n/x/x)/3.0]

or, as a block

[:n|1to:(x:=99)do:[:i|x:=2*x+(n/x/x)/3.0].x]
share|improve this answer

Haskell: 99C

Can't beat @mniip in cleverness. I just went with a binary search.

c x=d 0 x x
d l h x
 |abs(x-c)<=t=m
 |c < x=d m h x
 |True=d l m x
 where m=(l+h)/2;c=m*m*m;t=1e-4

Ungolfed:

-- just calls the helper function below
cubeRoot x = cubeRoot' 0 x x

cubeRoot' lo hi x
    | abs(x-c) <= tol = mid           -- if our guess is within the tolerance, accept it
    | c < x = cubeRoot' mid hi x      -- shot too low, narrow our search space to upper end
    | otherwise = cubeRoot' lo mid x  -- shot too high, narrow search space to lower end
    where
        mid = (lo+hi)/2
        cubed = mid*mid*mid
        tol = 0.0001
share|improve this answer

J 28

*@[*(3%~+:@]+(%*~@]))^:_&|&1

Using Newtons method, finding the root of x^3 - X the update step is x - (x^3 - C)/(3*x^2), where x is the current guess, and C the input. Doing the maths on this one yields the ridiculously simple expression of (2*x+C/x^2) /3 . Care has to be taken for negative numbers.

Implemented in J, from right to left:

  1. | Take abs of both arguments, pass them on
  2. ^:_ Do until convergence
  3. (%*~@]) is C / x^2 (*~ y is equivalent to y * y)
  4. +:@] is 2 x
  5. 3%~ divide by three. This yields the positive root
  6. *@[ * positive_root multiplies positive root with the signum of C.

Test run:

   NB. give it a name:
   c=: *@[*(3%~+:@]+(%*~@]))^:_&|&1
   c 27 64 1 18.609625 3652264 0.001 7
3 4 1 2.65 154 0.1 1.91293
share|improve this answer

Polyglot, 24 chars (cheeky)

Depends if you consider EXP to be raising a number to a power. Anyway, EXP is a function, not a "method/operator", so I guess it falls within the spec.

Fails for X=0, but the question says "negative numbers" but doesn't mention zero. Complies with all test cases, positive and negative.

Hey, I could save quite a few chars by making it work ONLY for negative numbers and not positive ones..

SGN(X)*EXP(LN(ABS(X))/3)
share|improve this answer
3  
"funcation" and "method" are used synonymously. –  primo Feb 22 at 15:08
    
@primo According to wikipedia a method is a function associated with an object. What object is EXP associated with? Terminology could vary with language, sure. –  steveverrill Feb 22 at 15:15
1  
Interesting approach, but I believe exp is against the rules –  mniip Feb 22 at 15:22
    
@mniip hence the word "cheeky" in the title. I could have gone for pow(x,1/3) as that's technically not a cube root function, but that would just be trolling. –  steveverrill Feb 22 at 15:24

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