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'

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) 

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 

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:

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) 

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); 

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...

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 ); } } 

TI-Basic, 26 24 bytes

Input :1:For(I,1,9:2Ans/3+X/(3AnsAns:End 

Js 57 bytes

f=(x)=>eval('for(w=0;w**3<1e12*x;w++);x<0?-f(-x):w/1e4')

f=(x)=>eval('for(w=0;w**3<1e12*x;w++);x<0?-f(-x):w/1e4') document.getElementById('div').innerHTML += f(-27) + '<br>' document.getElementById('div').innerHTML += f(-64) + '<br>' document.getElementById('div').innerHTML += f(-1) + '<br>' document.getElementById('div').innerHTML += f(-18.609625) + '<br>' document.getElementById('div').innerHTML += f(-3652264) + '<br>' document.getElementById('div').innerHTML += f(-0.001) + '<br>' document.getElementById('div').innerHTML += f(-7) + '<br><hr>' document.getElementById('div').innerHTML += f(27) + '<br>' document.getElementById('div').innerHTML += f(64) + '<br>' document.getElementById('div').innerHTML += f(1) + '<br>' document.getElementById('div').innerHTML += f(18.609625) + '<br>' document.getElementById('div').innerHTML += f(3652264) + '<br>' document.getElementById('div').innerHTML += f(0.001) + '<br>' document.getElementById('div').innerHTML += f(7) + '<br>'

<div id="div"></div>

Javascript: 73/72 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} 

Edit, 4 years later: As suggested by Luis felipe De jesus Munoz, by using ** the code is shorter, but that feature was not available back in 2014 when I wrote this answer. Anyway, by using it, we shave an extra character:

function g(a){if(a<0)return-g(-a);for(h=0;h**3<1e12*a;h++);return h/1e4} 

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 } 

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

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 

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 

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] 

GameMaker Language, 51 bytes

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

MMIX, 44 bytes (11 instrs)

E0013FF0 48000002 E8018000 C1FF0100 1001FFFF 14010001 040101FF E401FFF0 11FF01FF 53FFFFFA F8020000 

Disassembly

cbrt SETH $1,#3FF0 // y = 1. BNN $0,0F // if(x >= 0) goto loop (ensuring sign is correct) ORH $1,#8000 // y = fnabs(y) (bit tricks!) 0H SET $255,$1 // loop: prev = y FMUL $1,$255,$255 // y = prev *. prev FDIV $1,$0,$1 // y = x /. y FADD $1,$1,$255 // y = y +. prev INCH $1,#FFF0 // y = y /. 2. (bit tricks again) FCMPE $255,$1,$255 // prev = comp_eps(y, prev) PBNZ $255,0B // if(prev) goto loop POP 2,0 // return {y,x} 

It is up to the caller to set a suitable value of rE. Much faster convergence may be arranged by essentially setting y up by dividing the exponent by 3, but that costs another four instructions (though in time it pays really quickly, since three FDIVs cost about the same amount as two DIVs):

cbrt SLU $1,$0,1 // 3B010001 shift left to erase sign INCH $1,#8020 // E4018020 unbias exponent DIV $1,$1,3 // 1D010103 divide by 3 as integer; approximate cube root INCH $1,#7FE0 // E4017FE8 rebias exponent SRU $1,$1,1 // 3F010101 shift right again 

and then continue from the second line in the original.

If mispredicted branches are particularly expensive, I would also replace the second and third lines of the original with:

 ZSN $255,$0,1 // 71020001 $255 = ($0 neg? 1 : 0) SLU $255,$255,63 // 3B02023F $255 <<= 63 OR $1,$1,$255 // C0010102 $1 |= $255 

This costs one more instruction, but is branchless.

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 

C, 69 bytes

i;float g(float x){for(float y=x;++i%999;x=x*2/3+y/3/x/x);return x;} 

Just another implementation of Newton's method. Try it online!

Stax, 10 bytes^CP437

╘♀┘A╕äO¶∩' 

Run and debug online!

Explanation

Uses the unpacked version to explain.

gpJux*_+h4je gp Iterate until a fixed point is found, output the fix point Ju Inverse of square x* Multiplied by input _+h Average of the value computed by last command and the value at current iteration 4je Round to 4 decimal digits 

APL(NARS), 46 chars

r←q w;t t←1 →3×⍳1e¯9>∣t-r←t-3÷⍨t-w÷t×t⋄t←r⋄→2 

//8+4+34=46

I followed the formula written in the post: https://codegolf.stackexchange.com/a/21793/120560. It seems is ok with test

 aaa←27 64 1 18.609625 3652264 0.001 7 2E¯13 0 q¨aaa 3 4 1 2.65 154 0.1 1.912931183 0.00005848035476 1.568328545E¯9 q¨-aaa ¯3 ¯4 ¯1 ¯2.65 ¯154 ¯0.1 ¯1.912931183 ¯0.00005848035476 1.568328545E¯9 

It seems work for input big float v but not work for rationals...It could be ok with the restriction of question so ok.

The problem would be find in this case cube root function, the min number e that can find all digits possibles of one float when the sys has float XXXbits=⎕FPC (without generate one never ending loop for some number of input w due the number e is too small)....

I propose this function that would break convension of return the same type of the input, it would return a v number a float according ⎕FPC variable

r←q3 w;t;e t←1v⋄e←÷10*⌊(⎕FPC-{1>∣w:0⋄3÷⍨1+2⍟∣w})÷3.323v →3×⍳e>∣t-r←t-3÷⍨t-w÷t×t⋄t←r⋄→2 

The error in this case would be less than e←÷10*⌊(⎕FPC-{1>∣w:0⋄3÷⍨1+2⍟∣w})÷3.323v where w is the input number, ⎕FPC is the lenght in bit of the float point 1+2⍟w would be the lenght in bit of integer part of w, but I have to consider the 1/3 of that number because the resul will have 1/3 of these digits...log_2(10)=3.32192809 here approximated as 3.323 (I don't know if it is the right number, 3.322 is not right because I remember generate one never ending loop with the input 18.609625v). Test

 q3¨aaa 3 4 1 2.65 154 0.1 1.912931183 0.00005848035476 1.381590388E¯38 q3¨-aaa ¯3 ¯4 ¯1 ¯2.65 ¯154 ¯0.1 ¯1.912931183 ¯0.00005848035476 1.381590388E¯38 q3¨aaa×1v 3 4 1 2.65 154 0.1 1.912931183 0.00005848035476 1.872107764E¯154 q3¨aaa×1x 3 4 1 2.65 154 0.1 1.912931183 0.00005848035476 1.872107764E¯154 

note that it seems ok even with rationals...

 q3 2 1.25992105 q3 2v 1.25992105 q3 2x 1.25992105 q3 18.609625x 2.65 

for big numbers it is possible too

 ⎕fpc←512 512÷3.322 154.1240217 150⍕q 2v 1.259921049894873164779198323884500528076109625698508732184198175469013575303302345933958149409424814616937169595159013873478464862408338554282464069753 150⍕q3 2v 1.259921049894873164767210607278228350570251464701507980081975112155299676513959483729396562436255094154310256035615665259399024040613737228459110304269 

note how q3 wuould be the right result, but q is right until 19th digit.

It is possible that I make some error and there is a number of input that generate one infinite loop because e variable in code is too little (for example I wrong the number 3.323 or if variable t or r have integer part > of input w even if t-r decrease)

I have not consider the case the number of input w is too big for variable ⎕FPC, for example when digits of number are > ⎕FPC÷3.322 (float for me has to be fixed point float because this is too tricly)

AWK, 49 bytes

{for(n=x=$1;x-(x=(2*x+n/x/x)/3););printf"%.4f",x} 

Try it online!

Thanks go to @Mig for the JavaScript solution which this is derived from. It runs surprisingly quickly given that the for loop requires the iteration to stop changing.

Note: This solution has trailing 0s for all results with fewer than 4 digits to the right of the decimal, including integer results. Also, subsequent results are not separated.
Changing the %.4f to %.5g removes trailing 0s, but will lose digits to the right of the decimal for results >= 10.

An alternative solution adds 4 bytes, but it also makes the output prettier:

{for(n=x=$1;x-(x=(2*x+n/x/x)/3););CONVFMT="%.4f"}$0=x 

Try it online!