Why put the constant before the variable in a comparison?

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It’s a mechanism to avoid mistakes like this:

if ( var = NULL ) {
  // ...
}

If you write it with the variable name on the right hand side the compiler will be able catch certain mistakes:

if ( NULL = var ) {  // not legal, won't compile
  // ...
}

Of course this won’t work if variable names appear on both sides of the equal sign and some people find this style unappealing.

Edit:

As Evan mentioned in the comments, any decent compiler will warn you about this if you enable warnings, for example, gcc -Wall will give you the following:

warning: suggest parentheses around assignment used as truth value

You should always enable warnings on your compiler, it is the cheapest way to find errors.

Lastly, as Mike B points out, this is a matter of style and doesn’t affect the performance of the program.

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