switch, case, and fall-through

The shape

switch (choice) {
    case 1:
        printf("deposit\n");
        break;
    case 2:
        printf("withdraw\n");
        break;
    default:
        printf("unknown\n");
        break;
}

• The controlling value must be an integer type (int, char, enum — module 15).
• case labels are entry points, not sections: without break, execution falls through into the next case. Occasionally useful (grouping cases), usually a bug.
• default runs when nothing matched — handle it explicitly.

switch vs if

Use switch when one integral value selects among fixed options (menus, state codes). Use if for ranges (x > 100) and mixed conditions.