c 2/5 25 XP

Control Flow

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Quick Reference

C truth values:

Value	Boolean meaning
`0`	false
Any non-zero	true
`NULL`	false (it’s 0)

Loop patterns:

Pattern	When to use
`for (int i = 0; i < n; i++)`	Known number of iterations
`while (condition)`	Unknown iterations, may run zero times
`do { ... } while (condition);`	Must run at least once (input validation)

The = vs == trap:

if (x = 5)    // ASSIGNMENT — always true (assigns 5 to x)
if (x == 5)   // COMPARISON — true only when x is 5

switch template:

switch (value) {
    case 'A': action_a(); break;
    case 'B': action_b(); break;
    case 'C':
    case 'D': action_cd(); break;   // C and D share code (fall-through)
    default:  action_default(); break;
}

Common Pitfalls

= vs == — Assignment in condition is legal C and almost always a bug. Compile with -Wall.
Dangling else — Without braces, else attaches to the nearest if. Always use braces.
Missing break in switch — Causes fall-through to the next case. Intentional fall-through should be commented.
switch on strings — Not possible in C. Use if/else if with strcmp().
Infinite loops — Forgetting to update the loop variable. while(1) is intentional; while(x > 0) without x-- is a bug.
Off-by-one in loops — <= vs < is the most common loop bug. Draw the first and last iterations to verify.

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