Control Flow

Every program so far in this domain has run top to bottom, one statement after another, with no decisions. Control flow is how a program chooses which statements to run based on a condition — the difference between a script and something that actually reacts to its input.

Quick reference

What it is: The if/else and switch statements C uses to run different code depending on a condition.
Mental model: A fork in the road — the program evaluates a condition to a true/false answer and takes exactly one branch based on it.
Key numbers: Any nonzero value is "true"; only 0 is "false" — there's no separate boolean type in original C.
Reach for it when: if/else for a handful of conditions or ranges; switch specifically when testing one variable against many exact, known values.

What "true" means in C

Before if makes sense, you need to know what counts as true. C has no dedicated boolean type in its original design — every condition is really just a number, and the rule is simple: 0 is false, and literally every other value is true, including negative numbers.

if / else if / else

int temperature = 85;

if (temperature > 90) {
    printf("Too hot\n");
} else if (temperature < 60) {
    printf("Too cold\n");
} else {
    printf("Comfortable\n");
}
// Comfortable

The moment one condition is true, its branch runs and every later else if/else is skipped entirely — only one branch of the whole chain ever executes.

switch

switch tests one value against a list of exact matches — cleaner than a long else if chain when you're comparing a single variable to several specific values:

int day = 3;

switch (day) {
    case 1:
        printf("Monday\n");
        break;
    case 2:
        printf("Tuesday\n");
        break;
    case 3:
        printf("Wednesday\n");
        break;
    default:
        printf("Some other day\n");
}
// Wednesday

The break at the end of each case is not optional decoration — without it, execution falls through into the next case regardless of whether it matches, which is real, intentional C behavior, not a bug in the language.

switch (day) {
    case 1:
    case 2:
    case 3:
    case 4:
    case 5:
        printf("Weekday\n");   // falls through cases 1-5 into this ONE printf
        break;
    case 6:
    case 7:
        printf("Weekend\n");
        break;
}

Real-world applications

Sensor threshold checks — comparing a reading against safe/warning/critical bands, exactly like the temperature example above.
Command/opcode dispatch — a switch on a received byte value is the standard pattern for parsing a simple command protocol.
State machines — a switch on the "current state" variable, with each case handling one state's behavior, is one of the most common structures in embedded firmware.

Gotchas

Field notes

Forgetting break in a switch is one of the most common real bugs in C — it compiles fine and often "mostly works," failing only on inputs that hit the specific case that was supposed to stop there.
switch only works on integer types (and char) — it cannot switch on a float, double, or string.
A dangling if with no braces only controls the single next statement — adding a second statement "inside" it without braces silently moves that statement outside the condition entirely. Using braces even for one-line bodies avoids this class of bug completely.
Comparing floating-point values with == inside an if is unreliable — tiny rounding errors from floating-point representation mean two values that are mathematically equal can differ by a fraction of a bit. Comparing within a small tolerance (fabs(a - b) < 0.0001) is the standard fix.