CODEWITHSUNDEEP
c
struggling_learner
struggling_learner

Reputation: 1258

why do I need the curly braces here?

Consider this simple program:

fails.c:

#include <stdio.h>                                            

int main(){                                                   
    int i = 10;                                                                                         
    if (i == 10)                                          
        int j = 11;                                                                                         
    return 0;        
}

That fails to compile (gcc fails.c), giving this error:

fails.c: In function ‘main’:
fails.c:7:3: error: expected expression before ‘int’
   int j = 11;
   ^

But this one goes through just fine:

#include <stdio.h>

int main(){
    int i = 10;
    if (i == 10){
        int j = 11;
    }
    return 0;
}

I figured that the work around, is to put those {} in. But I wish to know why this is required.

Why does it behave this way, when something like printf is acceptable?

#include <stdio.h>

int main(){
    int i = 10;
    if (i == 10)
        printf("some text\n");
    return 0;
}

Upvotes: 12

Views: 149

Answers (3)

Yury Schkatula
Yury Schkatula

Reputation: 5369

It seems you compile your ANSI C code with C89 standard. And, unlike his successor C99, it requires all variables to be declared at the beginning of the scope. So, having int j = 11; somewhere in-between other statements just contradicts that C89 rule. If you open a new scope just before that int j = 11;, it's back to OK.

Actual reason of such C89 limitation should be an attempt to simplify memory management for stack-allocable variables. For instance, compare it with Pascal language that requires all variables to be declared in the special var section before the code (so, it's even more strict that C89).

Upvotes: 0

haccks
haccks

Reputation: 105992

There is a difference between declaration and statement. 

int j = 11;

is a declaration. if statement shall be followed by a statement. Putting {} after if statement results in a compound statement. A compound statement can have no other statement in it or can have a declaration.

Upvotes: 1

llllllllll
llllllllll

Reputation: 16404

This is because if must be followed by a statement:

C99/6.8.4

if ( expression ) statement

However, a declaration is not a statement:

C99/6.8

statement:

labeled-statement

compound-statement

expression-statement

selection-statement

iteration-statement

jump-statement

When put inside a {}, it is a compound-statement, thus ok.

Upvotes: 8

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