Why do I get "error undeclared identifier" unless I declare my variable at the beginning?

Question

When I have following:

#include "stdafx.h"
#include<stdio.h>
int main()
{

int val1,val2;

printf("Enter the first value");
scanf("%d",val1);
scanf("%d",&val2);
int c;
c=val1 + val2;
printf(" the value is : %d", c);



return 0; // 0 means no error

 }

I get error undeclared identifier c. Also, syntax error. missing ; before type.

However, if I change above to following error disappears. Please help

#include "stdafx.h"
#include<stdio.h>

int main()
{

int val1,val2,c;

printf("Enter the first value");
scanf("%d",&val1);
scanf("%d",&val2);

c=val1 + val2;
printf(" the value is : %d", c);



return 0; // 0 means no error

}

I am running C in VS 2010.

Answer 1

In C, at least back in the old days, variable declarations have to come at the top of the block. C++ is different in that regard.

edit — apparently C99 is different from C90 in this respect (C99 being essentially the same as C++ on this issue).

Answer 2

Objects may only be declared at the top of a statement block in ISO C90. You can therefore do this:

#include<stdio.h>

int main()
{
    int val1,val2;

    printf("Enter the first value");
    scanf("%d",val1);
    scanf("%d",&val2);

    // New statement block
    {
        int c;
        c=val1 + val2;
        printf(" the value is : %d", c);
    }

    return 0; // 0 means no error
 }

Though it would perhaps be unusual to do so. Contrary to somewhat popular belief, the start of a function is not the only place you can declare an automatic variable. It is more common, rather than creating a dummy block, to use existing statement blocks introduced as part of an if or for construct for example.

It is useful to enclose case blocks in { ... }, even though not normally necessary, so that you can introduce temporary case specific variables:

switch( x )
{
    case SOMETHING :
    {
        int case_local = 0 ;
    }
    break ;

    ...
}