This question is inspired by this question, which features the following code snippet.

int s;
if((s = foo()) == ERROR)
    print_error();

I find this style hard to read and prone to error (as the original question demonstrates -- it was prompted by missing parentheses around the assignment). I would instead write the following, which is actually shorter in terms of characters.

int s = foo();
if(s == ERROR)
    print_error();

This is not the first time I've seen this idiom though, and I'm guessing there are reasons (perhaps historical) for it being so often used. What are those reasons?