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label : statement

Expression statements

Compound statements

{ statement... }

Selection statements

if

Iteration statements

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Conditionally executes code.

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

Contents

[edit] 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) an 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

[edit] 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)

[edit] 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.

[edit] Keywords

[edit] Example

Run this code

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

i > 2 is false
i == 3
i != 3 is false

[edit] References

C23 standard (ISO/IEC 9899:2024):

6.8.5.2 The if statement (p: 154)

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

[edit] See also

C++ documentation for if statement

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