if statement

 

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expression ;

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if

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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

Run this code

#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