/C

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.

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

 C++ documentation for `if` statement