C - Variable Declaration in If Condition Available in Else?

Question

If I declare a variable in an if condition in C, is that variable also available to the else branch? For example:

if((int x = 0)){
  foo();
} else{
  x++;
  bar(x);
}

Couldn't find the answer, at least not the way that I worded it. Please help.

Maybe consider doing an experiment, and post an answer to your own question? — user12986714, May 01 '20 at 02:45
Experiment result: one can't declare variables in `if` conditions. Won't compile — user12986714, May 01 '20 at 02:52
@rsjaffe it doesn't appear to be a duplicate; the one you cited is about variables declared in `if` block, but this one is about those in `if` conditions. — user12986714, May 01 '20 at 02:54
No in C. Yes in C++, see [this](https://en.cppreference.com/w/cpp/language/if) for example. — dxiv, May 01 '20 at 02:55
@rsjaffe Not a duplicate, the other question is about variables declared *inside* `if` statements in *C++*. Voted to reopen. — dxiv, May 01 '20 at 03:06
Declarations in `if` conditions in C++ only became legal in C++17. — Keith Thompson, May 01 '20 at 03:08
@user12986714 an answer referring to the Standard would be much better, since the results of experiments might only reveal some peculiarity of a particular compiler — M.M, May 01 '20 at 03:15
reopened - the question is about C but the "duplicate" was C++-only — M.M, May 01 '20 at 03:16
@M.M yes, standard should always be better than mere experiments. However where can I find those standards? ISO wants to charge me quite a few dollars to get a copy :-( — user12986714, May 01 '20 at 03:24
@user12986714 https://stackoverflow.com/questions/81656/where-do-i-find-the-current-c-or-c-standard-documents — M.M, May 01 '20 at 03:26

shu · Answer 1 · 2020-05-01T03:01:43.323

4

You can't declare a variable in an if condition in C...

Declare variable in if statement (ANSI C)

If you declare inside the if scope, for example:

if(something){
  int x = 0;
} else{
  x++; // will cause a compilation error
  bar(x);
}

x in 'else' is undeclared because in C a local variable can only be used by statements contained within the code block where they are declared.

edited May 01 '20 at 03:01

answered May 01 '20 at 02:49

shu

I did not know that, what about C++? – Stephen Alex Wallen May 01 '20 at 02:53
@aYogii experiment result: no you can't, either – user12986714 May 01 '20 at 02:55
Since you say this show me the standard. – user13947194 Feb 07 '23 at 03:19

score 1 · Accepted Answer · answered May 01 '20 at 03:19

You can't declare a variable like this

 if((int a = 0))

The compiler does not allow the code to run and you get an error

and if you try this

if(something_that_is_false){
        int a = 12;
    }
    else{
        do_something;
    }

again error because they are on the same level and they do not have access to their local variables.

Warning: you can use this code and runs without error

int a;
    if(a=0){
         printf("True");
    }
    else{
        printf("False");
    }

and you will see 'False' in screen because It's like writing

if(0) // and its false!

and for the last

int a;
    if(a=0){
         printf("True");
    }
    else{
        printf("False");
    }

you will see 'True' in screen because It's like writing

if(5) // any number other than zero is true!

score 0 · Answer 3 · answered May 01 '20 at 02:55

0

Experiment result: one can't declare variables in if conditions. Won't compile.

answered May 01 '20 at 02:55

user12986714

3 Answers3