if statement - cppreference.com (original) (raw)

Conditionally executes code.

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

[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

if,else

[edit] Example

#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

[edit] See also