You must write a program to evaluate a string that would be entered into a calculator.

The program must accept input and output the correct answer. For languages that do not have standard input/output functions, you may assume the functions readLine and print.


  • Does not use any kind of "eval" functions
  • Can handle floating point and negative numbers
  • Supports at least the +, -, *, and / operators
  • Can handle input containing one or more spaces between the operators and numbers
  • Evaluates the expression from left to right

The program that is the shortest wins. In the event of a tie, the program that was submitted first wins.

You can assume that the input is valid and follows the correct format

Test Cases


-4 + 5




-7.5 / 2.5




-2 + 6 / 2 * 8 - 1 / 2.5 - 18


  • \$\begingroup\$ My calculator uses postfix. See also Evaluating Mathematical Expressions on Stack Overflow for competition (though I haven't checked if the rules are identical). \$\endgroup\$ – dmckee Feb 5 '11 at 19:04
  • 2
    \$\begingroup\$ Third test case is wrong - whether you follow standard order of operations or perform all operations left to right. Looking at the second test case, does your calculator round the result of each operation? \$\endgroup\$ – PleaseStand Feb 5 '11 at 19:04
  • \$\begingroup\$ Fixed the second and third test case, the result is not rounded. \$\endgroup\$ – Kevin Brown Feb 5 '11 at 19:21
  • \$\begingroup\$ The third test case does not follow the standard order of operations. Are our answers supposed to? \$\endgroup\$ – John Feb 5 '11 at 21:35
  • 1
    \$\begingroup\$ What about using command line arguments ARGV? because the shell auto-splits and lists the arguments. \$\endgroup\$ – Ming-Tang Feb 6 '11 at 3:01

19 Answers 19


Ruby - 74 69 67 65 characters

("+ "+$<.read).split.each_slice 2{|b,c|a=a.send b,c.to_f}
p a
  • 1
    \$\begingroup\$ Instead of using b[0],b[1].to_f you can replace |b| with |b,c| and use b,c.to_f \$\endgroup\$ – Nemo157 Feb 5 '11 at 23:30
  • \$\begingroup\$ \o/ thanks ! :-) \$\endgroup\$ – Arnaud Le Blanc Feb 5 '11 at 23:36
  • 1
    \$\begingroup\$ Instead of a.send(b,c.to_f), use a.send b,c.to_f. It saves a char \$\endgroup\$ – anonymous coward Feb 6 '11 at 5:22
  • 1
    \$\begingroup\$ You can use $< instead of ARGF \$\endgroup\$ – Dogbert Feb 7 '11 at 20:15

Befunge - 37 x 5 = 185 38 x 3 = 114 characters

This is limited to integer numbers as Befunge has no floating point support.

&v      /& _ #`&# "-"$# -#<          v
 >~:0`!#v_:" "`! #v_:","`#^_"*"`#v_&*>
 ^      ># $ .# @#<              >&+ 


The biggest distinguishing feature of Befunge is that instead of being a linear set of instructions like most languages; it is a 2d grid of single character instructions, where control can flow in any direction.

The first & simply inputs the first number. The v and > then redirect control to the main path on the second row.


This inputs a character (~), duplicates it (:), pushes zero onto the stack (0), pops the top two elements and determines if the second is greater than the first (` I'm surprised you can't use ``` to get code backticks.), inverts the truthiness of the top element (!), then goes right if it is zero, down otherwise (#v_).

Basically it's checking whether the input is -1 representing no more input.

># $ .# @

If the input was -1 then the duplicated input value is discarded ($), the top of the stack is output as an integer (.) and the program is halted (@).

:" "`! #v_

Otherwise a similar process is repeated to determine if the input is less than or equal to a space. If it is a space then control goes down, otherwise control heads right.

^      ># $ .# @#<

If it is a space then it's redirected left (<); the program halt (@), output (.) and right redirection (>) are all skipped using #; but the discard is executed to remove the space from the stack. Finally it's redirected up to begin the next execution (^).


If it wasn't a space the same process is used to split on if it is in [+, *] or in [-, \] going right and up respectively.

 >~                         "*"`#v_&*>
 ^                               >&+

For [+, *] it is again split to determine whether it is a + or a *. If + it is directed down then the next number is input (&) and they are added (+), the control then wraps around and is redirected up to the main path for the next character. If * then it inputs (&) and multiplies (*) then directly wraps around.

/& _ #`&# "-"$# -#<

For [-, \] it starts on the right heading left. The #'s skip the character after them so the initial path is "-"`_ which simply determines if it is - or /. If it is / then it continues left to input (&) and divide (/). If it is - then it heads right, again skipping characters so that it executes &"-"$- resulting in the number being input (&) the - character being pushed onto the stack then discarded ("-"$) and then the subtraction being calculated (-). The control is then redirected back to the main path.


Python 3, 105 bytes

Manages the four basic operations, but it only costs 5 characters each to add ^ or %.

while l:o,y,*l=l;x,y=f(x),f(y);x=[x+y,x-y,x*y,x/y]['+-*/'.find(o)]

Precedence of operations is left to right.


Python (156)

from operator import*
while 1:
 while len(l)>2:l[:3]=({'*':mul,'/':div,'+':add,'-':sub}[l[1]](f(l[0]),f(l[2])),)
 print l[0]
  • \$\begingroup\$ It's prob just easier to use Python 3 \$\endgroup\$ – jamylak Apr 20 '13 at 3:29

C - 168 126 characters

main(c){float a,b;scanf("%f",&a);while(scanf("%s%f",&c,&b)!=-1)c=='+'?a+=b:c=='-'?(a-=b):c=='*'?(a*=b):(a/=b);printf("%f",a);}

Tcl 8.6, 57 48 chars.

  • Input from arguments:

    lm o\ b [las $argv a] {set a [exp $a$o$b]};pu $a
  • From Stdin (64 53)

    lm o\ b [las [ge stdin] a] {set a [exp $a$o$b]};pu $a

You have to use the interactive shell for both solutions.

I treat the input as list (Tcl uses spaces as delimiter) take the first element and assign it to a, then I walk over the rest, taking 2 elements each time, the operator and a second number, apply the operator on $a and $b and assign the result to a. At the end the result is in a.

  • \$\begingroup\$ Ideone supports at least stdin. \$\endgroup\$ – Johannes Kuhn Apr 19 '13 at 20:39
  • \$\begingroup\$ Finally, I beat Ruby. Unfortunately Idone does not support Tcl 8.6, but I don't need the result of lmap so foreach is a good replacement. \$\endgroup\$ – Johannes Kuhn Apr 23 '13 at 11:56

C: 111 108 characters

main(c){float a,b;for(scanf("%f ",&a);~scanf("%c%f ",&c,&b);a=c^43?c%5?c%2?a/b:a*b:a-b:a+b);printf("%f",a);}

It fulfills all the requirements, usage:

> ./calc <<< "-43 - 56 + 14.123 / -13.22"
  • 1
    \$\begingroup\$ ~scanf can replace +1. Also, c^45->c%5 and c^42->c%2 should work. \$\endgroup\$ – ugoren Apr 21 '13 at 7:31
  • \$\begingroup\$ @MDXF not on my machine, it passes every test case here. I'm compiling with Clang on a fairly modern Intel Macbook, and it works smashingly well (I tested it again just now, I copypasted the code from here and just compiled it without any flags). What compiler, processor architecture and OS are you using? \$\endgroup\$ – Fors Oct 8 '17 at 1:21
  • \$\begingroup\$ @Fors I believe I had some odd flags that were inducing strange behavior; my mistake, it's working for me now. Sorry to bother you. \$\endgroup\$ – MD XF Oct 8 '17 at 1:24

C++0x 205 203 198 194 chars

#define P [](F l,F r){return l
int main(){typedef float F;F r,v,(*a[])(F,F)={P*r;},P+r;},0,P-r;},0,P/r;}};std::cin>>r;for(char o;std::cin>>o>>v;)r=a[o-42](r,v);std::cout<<r;}

Nicely formatted:


int main()
    float r,v;
    float (*a[])(float,float)   ={  [](float l,float r){return l*r;},
                                    [](float l,float r){return l+r;},
                                    [](float l,float r){return l-r;},
                                    [](float l,float r){return l/r;}

    for(char o;std::cin>>o>>v;)


Perl (97)


read from arguments

$b=shift;$b=($r eq'+'?$b+$s:$r eq'-'?$b-$s:$r eq'*'?$b*$s:$b/$s)while($r=shift,$s=shift);print$b;

read from input

@_=split/ /,<>;$b=shift@_;$b=($r eq'+'?$b+$s:$r eq'-'?$b-$s:$r eq'*'?$b*$s:$b/$s)while($r=shift@_,$s=shift@_);print$b

PostScript (145)

Another PostScript entry (thanks to luser droog for digging the golfs interesting for PostScript!):

999 string readline
pop{token not{exit}if
count 4 eq{3 1 roll
4 1 roll
cvx exec}if


[/+{add}/-{sub}/*{mul}/ {div}>>begin
% Read the input
(%stdin)(r)file 999 string readline pop
{                        % .. string
  token not{exit}if      % .. string token
  % If we have 4 objects on the stack, we have two operands, one operator
  % and the input string. This means, we can calculate now.
  count 4 eq{            % a op string b
    % perform operation a op b = c (where op can be +,-,*,/)
    3 1 roll             % a b op string
    4 1 roll             % string a b op 
    cvx exec             % string c
  }if                    % string token (or c)
  exch                   % token string
  • \$\begingroup\$ You keep beating me! +1 This is very exciting. \$\endgroup\$ – luser droog Oct 19 '12 at 7:20
  • \$\begingroup\$ If you can beat my crossword, I'll give you a bounty! N.B. You can only edit 10 times before the post becomes CW and votes don't earn you rep points. \$\endgroup\$ – luser droog Oct 19 '12 at 7:42
  • \$\begingroup\$ I keep beating you because I only chose the ones where I can beat you ;-). I'm not sure whether I can with the crossword grid. I'll maybe try, but only in a few weeks. \$\endgroup\$ – Thomas W. Oct 19 '12 at 9:54
  • 1
    \$\begingroup\$ Community Wiki. It means the post has been edited so many times it now belongs to the community. Any user can edit it (bypassing the moderator approval required for the usual suggested edits), and no more points. So whatever you do, stop on Rev 9. I almost blew it on the guitar tab one. \$\endgroup\$ – luser droog Oct 20 '12 at 6:45
  • 1
    \$\begingroup\$ Ignore all that CW griping. They fixed it! \$\endgroup\$ – luser droog Oct 22 '12 at 6:05

Haskell: 124 114 characters

j(u:m:b:o)=j$show((case m of{"+"->(+);"-"->(-);"*"->(*);"/"->(/)})(read u)(read b)):o

A rather straight-forward answer, using pattern matching and a simple case statement for the heavy lifting. Usage:

> ./calc <<< "123 - 12 + -12 / 12.124 * 9.99 - 1"
  • \$\begingroup\$ Instead of ((case m of{..})(read u)(read b)) you can write ((case m of{..}$read u)$read b), 2 characters less. \$\endgroup\$ – swish Jan 6 '14 at 15:28

Python - 308

import sys;i=sys.argv[1].split();o=[];s=[];a=o.append;b=s.pop;c=s.append
for t in i:
 if t in"+-*/":
  if s!=[]:a(b())
if s!=[]:a(b())
for t in o:
 if t=="+":c(b()+b())
 elif t=="-":m=b();c(b()-m)
 elif t=="*":c(b()*b())
 elif t=="/":m=b();c(b()/m)

Readable version:

# Infix expression calc

import sys

# Shunting-yard algorithm
input = sys.argv[1].split()
output = []
stack = []

for tkn in input:
    if tkn in "+-*/":
        while stack != []:

while stack != []:

# Eval postfix notation
for tkn in output:
    if tkn == "+":
        stack.append(stack.pop() + stack.pop())
    elif tkn == "-":
        tmp = stack.pop()
        stack.append(stack.pop() - tmp)
    elif tkn == "*":
        stack.append(stack.pop() * stack.pop())
    elif tkn == "/":
        tmp = stack.pop()


Takes expression as command-line argument, output on standard output.


C# (234) (231) (229) (223)

class A{static void Main (string[]s){var n=true;var o="";var r=0F;foreach(var t in s){if(n){var v=float.Parse(t);if(o=="")r=v;if(o=="+")r+=v;if(o=="-")r-=v;if(o=="*")r*=v;if(o=="/")r/=v;}o=t;n=!n;}System.Console.Write(r);}}

class A{
    static void Main (string[]s){
        var n=true;
        var o="";
        var r=0F;

        foreach(var t in s)
                var v=float.Parse(t);
  • \$\begingroup\$ I'm getting '0' for '1 + 1'. IDEONE \$\endgroup\$ – Rob Oct 6 '12 at 18:55
  • \$\begingroup\$ @Mike Input as arguments, not stdin. \$\endgroup\$ – Johannes Kuhn Apr 20 '13 at 14:55

Postscript (340)

/D<</+{add}/-{sub}/*{mul}/ {div}>>def/eval{/P null def{token not{exit}if exch/rem exch def
dup D exch known{/P load null ne{D/P load get exch/P exch def exec}{/P exch def}ifelse}if
rem}loop/P load null ne{D/P load get exec}if}def {(> )print flush{(%lineedit)(r)file
dup bytesavailable string readline pop eval == flush}stopped{quit}if}loop

And a little more readable:

/oper<</+{add}/-{sub}/*{mul}/ {div}>>def

    /op null def
        token not {exit} if
        exch /rem exch def
        dup oper exch known {
            /op load null ne {
                oper /op load get
                exch /op exch def
                /op exch def
            } ifelse
        } if
    } loop
    /op load null ne { oper /op load get exec } if
} def

    (> )print flush
    dup bytesavailable string readline pop
    eval == flush
    } stopped { quit } if
} loop

JavaScript (208 characters compacted)

For clarity, this is the code before I compacted it down (JS-Fiddle of it):

function math(match, leftDigit, operator, rightDigit, offset, string) {
    var L = parseFloat(leftDigit)
    var R = parseFloat(rightDigit)
    switch (operator)
        case '*': return L*R;
        case '/': return L/R;
        case '+': return L+R;
        case '-': return L-R;

str = prompt("Enter some math:", "-2 + 6 / 2 * 8 - 1 / 2.5 - 18").replace(/ /g, "");
var mathRegex = /(\-?\d+\.?\d*)([\*\/\+\-])(\-?\d+\.?\d*)/;
while(mathRegex.test(str)) {
    str = str.replace(mathRegex, math);

Here it is compacted down to 208 characters (JS-Fiddle of it):

function m(x,l,o,r){
    return o=='*'?L*R:o=='/'?L/R:o=='+'?L+R:L-R;

for(s=prompt().replace(/ /g, "");M.test(s);s=s.replace(M,m)){};

Since I'm ending lines with semi-colons, all removable whitespace was ignored for character counting, but left in for clarity.


Haskell - 124

let p=let f[x]=Just$read x;f(x:o:r)=lookup o[("-",(-)),("+",(+)),("*",(*)),("/",(/))]<*>f r<*>Just(read x)in f.reverse.words

The result will be wrapped in Maybe monad

λ: p"-2 + 6 / 2 * 8 - 1 / 2.5 - 18"
Just (-12.0)

Also it requires importing <*> from Control.Applicative, but imports can be done outside the code, so I hope it's allowed.


JavaScript (87 characters)

alert(prompt().split(/ +/).reduce((a,b,i)=>i%2?(o=b,a):o+1-0?a-b*-(o+1):o<'/'?a*b:a/b))

Java 11, 151 (as lambda function)

s->{float r=0,t;int o=43,q;for(var x:s.split(" ")){if(x.length()>1|(q=x.charAt(0))>47){t=new Float(x);r=o<43?r*t:o<44?r+t:o<46?r-t:r/t;}o=q;}return r;}

Lambda function taking a String input and outputting a float.

Try it online.

Java 11, 241 bytes (as full program with asked I/O)

interface M{static void main(String[]a){float r=0,t;int o=43,q;for(var x:new java.util.Scanner(System.in).nextLine().split(" ")){if(x.length()>1|(q=x.charAt(0))>47){t=new Float(x);r=o<43?r*t:o<44?r+t:o<46?r-t:r/t;}o=q;}System.out.print(r);}}

Full program taking a String-line through STDIN and outputting to STDOUT.

Try it online.


interface M{                  // Class
  static void main(String[]a){//  Mandatory main-method
    float r=0,                //   Result float, starting at 0
          t;                  //   Temp float
    int o=43,                 //   Operator flag, starting at '+'
        q;                    //   Temp operator flag
    for(var x:new java.util.Scanner(System.in)
                              //   Create an STDIN-reader
               .nextLine()    //   Get the user input
               .split(" ")){  //   Split it on spaces, and loop over it:
      if(x.length()>1         //    If the current String length is larger than 1
                              //    (work-around for negative values)
         |(q=x.charAt(0))>47){//    Or the first character is an operator
                              //    (and set `q` to this first character at the same time)
        t=new Float(x);       //     Convert the String to a float, and set it to `t`
        r=                    //     Change `r` to:
          o<43?               //      If `o` is a '*':
            r*t               //       Multiply `r` by `t`
          :o<44?              //      Else-if `o` is a '+':
            r+t               //       Add `r` and `t` together
          :o<46?              //      Else-if `o` is a '-':
            r-t               //       Subtract `t` from `r`
          :                   //      Else (`o` is a '/'):
            r/t;}             //       Divide `r` by `t`
      o=q;}                   //    And at the end of every iteration: set `o` to `q`
    System.out.print(r);}}    //    Print the result `r` to STDOUT

05AB1E, 30 bytes


Try it online or verify all test cases.


#           # Split the (implicit) input-string by spaces
 ć          # Pop the list, and push the remainder and first item separated to the stack
  s         # Swap so the remainder is at the top of the stack
   2ô       # Split it into parts of size 2 (operator + number pairs)
     í      # Reverse each pair so the numbers are before the operators
v           # Loop over each of the pairs:
 y`         #  Push the number and operator separated to the stack
   …+-*     #  Push a string "+-*"
       sk   #  Get the index of the operator in this string
         ©  #  Store this index in the register (without popping)
   i        #  If the index is 1 (the "-"):
    -       #   Subtract the numbers from each other
   ë®>i     #  Else-if the index is 0 (the "+"):
       +    #   Add the numbers together
   ë®<i     #  Else-if the index is 2 (the "*"):
       *    #   Multiply the numbers with each other
   ë        #  Else (the index is -1, so "/"):
    /       #   Divide the numbers from each other
            # (and output the result implicitly)

If an eval builtin was allowed, this could be an alternative approach (16 bytes):


Try it online or verify all test cases.


#ćs2ô    # Same as above
     J   # Join each operator+number pair together to a single string
v        # Loop over the operator+number strings:
 …(ÿ)    #  Surround the top of the stack in parenthesis
     y«  #  And append the operator+number string
}.E      # After the loop: evaluate the string using a Python-eval

This would change "-2 + 6 / 2 * 8 - 1 / 2.5 - 18" to "((((((-2)+6)/2)*8)-1)/2.5)-18" before using the eval builtin (using .E directly would give operator precedence of */ over +-, hence the conversion with parenthesis first).


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