C error: Expected expression before int

Question

When I tried the following code I get the error mentioned.

if(a==1)
  int b =10;

But the following is syntactically correct

if(a==1)
{
   int b = 10;
}

Why is this?

question from:https://stackoverflow.com/questions/22419790/c-error-expected-expression-before-int

Answer 1

This is actually a fairly interesting question. It's not as simple as it looks at first. For reference, I'm going to be basing this off of the latest C11 language grammar defined in N1570

I guess the counter-intuitive part of the question is: if this is correct C:

if (a == 1) {
  int b = 10;
}

then why is this not also correct C?

if (a == 1)
  int b = 10;

I mean, a one-line conditional if statement should be fine either with or without braces, right?

The answer lies in the grammar of the if statement, as defined by the C standard. The relevant parts of the grammar I've quoted below. Succinctly: the int b = 10 line is a declaration, not a statement, and the grammar for the if statement requires a statement after the conditional that it's testing. But if you enclose the declaration in braces, it becomes a statement and everything's well.

And just for the sake of answering the question completely -- this has nothing to do with scope. The b variable that exists inside that scope will be inaccessible from outside of it, but the program is still syntactically correct. Strictly speaking, the compiler shouldn't throw an error on it. Of course, you should be building with -Wall -Werror anyways ;-)

(6.7) declaration:
            declaration-speci?ers init-declarator-list

_opt (;) static_assert-declaration (6.7) init-declarator-list: init-declarator init-declarator-list (,) init-declarator (6.7) init-declarator: declarator declarator (=) initializer (6.8) statement: labeled-statement compound-statement expression-statement selection-statement iteration-statement jump-statement (6.8.2) compound-statement: ({) block-item-list_opt (}) (6.8.4) selection-statement: (if () expression ()) statement (if () expression ()) statement (else) statement (switch () expression ()) statement