Decisions with if, else and switch

if and else#

int temperature = 31;

if (temperature > 30) {
    printf("It's hot\n");
} else if (temperature > 20) {
    printf("It's warm\n");
} else {
    printf("It's cool\n");
}

The condition goes in brackets, the body in braces. C checks each condition in turn and runs the first branch that is true, then skips the rest.

Any number can be a condition: 0 is false and everything else is true. So if (count) means "if count is not zero".

Always use braces#

Braces are optional when a branch is one statement, but leaving them out invites a well-known bug:

if (x > 0)
    printf("positive\n");
    count++;              // looks like part of the if, but always runs

Indentation means nothing to the compiler. With braces, what you see is what runs.

Combining conditions#

if (age >= 18 && has_ticket) { ... }
if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') { ... }
if (!found) { ... }

The ternary operator#

A compact choice between two values:

int larger = (a > b) ? a : b;
printf("%s\n", (n % 2 == 0) ? "even" : "odd");

Read it as "if a is greater than b, a, otherwise b". Keep it for short choices; nested ternaries are hard to read.

switch#

When one value is compared against many fixed alternatives, switch is tidier than a chain of else if:

switch (day) {
    case 1:
        printf("Monday\n");
        break;
    case 6:
    case 7:
        printf("Weekend\n");
        break;
    default:
        printf("Weekday\n");
}

Two things to know:

• break ends each case. Without it, execution falls through into the next case. That is occasionally useful, as with 6 and 7 above sharing one body, but usually a forgotten break is a bug.
• switch works on whole numbers and characters only, not strings or doubles. For strings, use if with strcmp.

Comparing doubles#

Floating-point results are approximate, so never test them with ==:

double x = 0.1 + 0.2;
if (x == 0.3) { ... }                 // false
if (fabs(x - 0.3) < 1e-9) { ... }     // true; fabs is in <math.h>

Handling the odd cases first#

Checking special cases at the top and leaving early keeps the main logic flat:

int digits(int n) {
    if (n == 0) {
        return 1;
    }
    int count = 0;
    while (n != 0) {
        n /= 10;
        count++;
    }
    return count;
}

Key points#

• if, else if, else: only the first true branch runs.
• Zero is false; anything else is true.
• Always write braces, even around one line.
• cond ? a : b chooses between two values.
• switch needs break in each case, and only works on integers and characters.

Next: repeating work with loops.