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The else clause is an optional syntactic construct in C that pairs with an if statement to define an alternative execution path. It specifies a statement or a compound statement (block) that the program executes strictly when the controlling scalar expression of the preceding if statement evaluates to zero (logical false).

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Syntax

#include <stdio.h>

int main(void) {
    int condition = 0;

    if (condition) {
        printf("Executed if condition evaluates to non-zero.\n");
    } else {
        printf("Executed if condition evaluates to zero.\n");
    }

    return 0;
}

​
Structural Mechanics

  • Mutual Exclusivity: The control flow guarantees that either the if branch or the else branch will execute, but never both. After the executed branch completes normally, control passes to the next statement following the entire if-else construct. However, if the executed branch contains a jump statement (such as return, break, continue, or goto), control flow will diverge according to that instruction and will not pass to the statement following the construct.
  • Statement Binding: By default, the else keyword binds only to the single statement immediately following it. To execute multiple statements, they must be enclosed in braces {} to form a compound statement.
  • Grammatical Requirement: An else clause cannot exist independently; it must immediately follow the statement or compound statement associated with an if condition.

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The Dangling else Rule

C resolves grammatical ambiguity in nested if statements using the “dangling else” rule. The C standard dictates that an else binds to the lexically nearest preceding unclosed if allowed by the syntax, regardless of indentation or implicit scope boundaries. Visual formatting can deceive the reader if indentation does not match the compiler’s parsing behavior: 
#include <stdio.h>

int main(void) {
    int flag_a = 1;
    int flag_b = 0;

    if (flag_a)
        if (flag_b)
            printf("Both flags are non-zero.\n");
    else /* Binds to 'if (flag_b)', despite visual alignment with 'if (flag_a)' */
        printf("flag_a is non-zero, flag_b is zero.\n");

    /* To force the else to bind to the outer if, the inner if 
       must be explicitly enclosed in a compound statement: */
    if (flag_a) {
        if (flag_b)
            printf("Both flags are non-zero.\n");
    } else { /* Now correctly binds to 'if (flag_a)' */
        printf("flag_a is zero.\n");
    }

    return 0;
}

​
else if Chaining

C does not possess a dedicated elseif or elif keyword. The common else if pattern is structurally just an else clause where the single statement that follows it happens to be another if statement. 
#include <stdio.h>

int main(void) {
    int state = 2;

    if (state == 1) {
        printf("State is 1.\n");
    } else if (state == 2) {
        printf("State is 2.\n");
    } else {
        printf("State is unknown.\n");
    }

    return 0;
}
In the abstract syntax tree, this is parsed as deeply nested if-else constructs, though it is conventionally formatted linearly to prevent excessive indentation.
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