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if statement

Conditionally executes code.

Used where code needs to be executed only if some condition is true.

Syntax

attr-spec-seq(optional) `if (` expression `)` statement-true	(1)
attr-spec-seq(optional) `if (` expression `)` statement-true `else` statement-false	(2)

attr-spec-seq	-	(C23)optional list of attributes, applied to the `if` statement
expression	-	an expression of any scalar type
statement-true	-	any statement (often a compound statement), which is executed if expression compares not equal to `0`
statement-false	-	any statement (often a compound statement), which is executed if expression compares equal to `0`

Explanation

expression must be an expression of any scalar type.

If expression compares not equal to the integer zero, statement-true is executed.

In the form (2), if expression compares equal to the integer zero, statement_false is executed.

As with all other selection and iteration statements, the entire if-statement has its own block scope:

enum {a, b};
int different(void)
{
    if (sizeof(enum {b, a}) != sizeof(int))
        return a; // a == 1
    return b; // b == 0 in C89, b == 1 in C99
}

(since C99)

Notes

The else is always associated with the closest preceding if (in other words, if statement-true is also an if statement, then that inner if statement must contain an else part as well):

int j = 1;
if (i > 1)
   if(j > 2)
       printf("%d > 1 and %d > 2\n", i, j);
    else // this else is part of if(j>2), not part of if(i>1) 
       printf("%d > 1 and %d <= 2\n", i, j);

If statement-true is entered through a goto, statement-false is not executed.

Keywords

if, else.

Example

#include <stdio.h>
 
int main(void)
{
    int i = 2;
    if (i > 2) {
        printf("first is true\n");
    } else {
        printf("first is false\n");
    }
 
    i = 3;
    if (i == 3) printf("i == 3\n");
 
    if (i != 3) printf("i != 3 is true\n");
    else        printf("i != 3 is false\n");
}

Output:

first is false
i == 3
i != 3 is false

References

C17 standard (ISO/IEC 9899:2018):

6.8.4.1 The if statement (p: 108-109)

C11 standard (ISO/IEC 9899:2011):

6.8.4.1 The if statement (p: 148-149)

C99 standard (ISO/IEC 9899:1999):

6.8.4.1 The if statement (p: 133-134)

C89/C90 standard (ISO/IEC 9899:1990):

3.6.4.1 The if statement