Control Flow

Programs execute statements in order, top to bottom. Control flow statements change that order: they let you choose between paths, repeat blocks, or select among many options. This chapter covers the three primary control flow mechanisms -- conditionals, iterations, and multiple selection -- in both C and Auto.

3.1 Conditional Execution

The most basic control flow construct is the conditional: do this, or do that, depending on a condition.

C's if/else

In C, if evaluates an expression. If the result is nonzero, the first branch executes. Otherwise, the else branch (if present) executes:

if (x > 0) {
    printf("positive\n");
} else if (x < 0) {
    printf("negative\n");
} else {
    printf("zero\n");
}

The braces around the body are optional for single-statement branches, but Modern C style always uses them. This prevents a class of bugs where a maintenance edit adds a second statement but forgets to add braces.

Auto's if/else

Auto uses the same logical structure, but without parentheses around the condition:

if x > 0 {
    print("positive")
} else if x < 0 {
    print("negative")
} else {
    print("zero")
}

Auto requires braces for all branches. There is no "optional braces" rule. This eliminates the dangling-else ambiguity and the single-statement brace-forgetting bug entirely.

Truth and Falsehood

In C, truth is simple: zero is false, everything else is true. This applies to all scalar types:

if (42)        // true
if (0)         // false
if (3.14)      // true
if (0.0)       // false
if (ptr)       // true if ptr is not NULL
if (!ptr)      // true if ptr is NULL

Auto inherits the same model. A nonzero value is truthy; zero is falsy. The bool type provides explicit boolean values true and false, but any condition expression works as in C.

C Deep Dive: In C, every scalar value has a truth interpretation: zero is false, nonzero is true. This means if (ptr), if (count), and if (difference != 0) are all valid conditions. Modern C's advice is explicit: "Don't compare to 0, false, or null." But this is a stylistic choice, and many C programmers prefer the explicit comparison for clarity. Auto does not change this -- the same truth model applies.