Used where code needs to be executed only if some condition is true.
| 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 |
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 }
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 \n", i, j);
If statement-true is entered through a goto, statement-false is not executed.
#include 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"); }
first is false i == 3 i != 3 is false