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Do You Know What ++i And i++ Really Do?

Thu, 2008-10-30 21:44 — jason

An interviewer who thinks he is being clever might present you with a code sample like the following and ask you what the output would be:

//C
#include <stdio.h>
 
void dosomething(int i, int j, int k, int l)
{
  printf("%d, %d, %d, %d\n", i,j,k,l);
}
 
int main(int argc, char **argv)
{
  int i =1;
  dosomething(i++, ++i, i++, ++i);
 
  i = 1;
  printf("%d\n", i++ + ++i + i++ + ++i);
 
  i = 1;
  printf("%d\n", (i++) + (++i) + (i++) + (++i));
}

You understand what the difference is between the pre and post incrementers, so you say:

1, increment, increment, 3, 3, increment, increment, 5

Then you say, "well, the next two should just be 1 + 3 + 3 + 5 = 12."

The answer seems to depend on which language you ask, ultimately.

The output from the above C program (with GCC on Linux) is:

4, 5, 2, 5
9
9

It's Undefined

The fact is, most languages specify that using a variable more than once in a statement with an increment or decrement operator has undefined results.

C and Perl show very unexpected output, while PHP, Javascript and Java all show the output that a human might come up with.

The bottom line is: don't do it, it's a Bad Idea.

Additional Language Outputs

Javascript

//Javascript
function dosomething( $i, $j, $k, $l ) {
         print($i, $j, $k, $l );
}
 
$i = 1;
dosomething( $i++, ++$i, $i++, ++$i );
 
$i = 1;
print($i++ + ++$i + $i++ + ++$i);
 
$i = 1;
print(($i++) + (++$i) + ($i++) + (++$i));

1 3 3 5
12
12

Java

//Java
class PrePostIncrement
{
  public static void doSomething(int i, int j, int k, int l)
  {
    System.out.println(i + ", " + j + ", " + k + ", " + l);
  }
 
  public static void main(String[] args)
  {
    int i = 1;
    doSomething(i++, ++i, i++, ++i);
 
    i = 1;
    System.out.println(i++ + ++i + i++ + ++i);
 
    i = 1;
    System.out.println((i++) + (++i) + (i++) + (++i));
  }
}

1, 3, 3, 5
12
12

Perl

#!/usr/bin/perl
use strict;
 
sub dosomething {
         print join( ', ', @_ ), "\n";
}
 
my $i = 1;
dosomething( $i++, ++$i, $i++, ++$i );
 
$i = 1;
print ($i++ + ++$i + $i++ + ++$i);
print "\n";
 
$i = 1;
print ($i++) + (++$i) + ($i++) + (++$i);
print "\n";

1, 5, 3, 5
12
1

PHP

<?php
function dosomething( $i, $j, $k, $l ) {
         printf( "%d, %d, %d, %d\n", $i, $j, $k, $l );
}
 
$i = 1;
dosomething( $i++, ++$i, $i++, ++$i );
 
$i = 1;
printf("%d\n", $i++ + ++$i + $i++ + ++$i);
 
$i = 1;
printf("%d\n", ($i++) + (++$i) + ($i++) + (++$i));
?>

1, 3, 3, 5
12
12

Comments

Fri, 2008-10-31 11:25 — Shantanu Goel (not verified)

Very well said Jason. A

Very well said Jason. A related reading material would be sequence points (http://c-faq.com/expr/seqpoints.html)