Control flow

A list of instructions executed top to bottom does the same thing every time, which is not yet useful. Control flow is what makes a program respond to its input. There are only two mechanisms: choosing between alternatives, and repeating.

Choice is the conditional. An if–else evaluates a test and runs exactly one branch — the matching one — then continues after the whole statement. Both branches are written down; only one happens on any given pass. Chaining several tests together is how a program handles more than two cases, and the order matters, since the first matching branch wins.

Repetition is the loop, and it is where beginners meet their first genuinely dangerous bug. A while loop keeps going as long as its condition holds, so something inside the body must eventually make that condition false. If nothing does, the loop never ends — the program hangs and no error is reported. A for loop over a known collection is safer, since the collection supplies the ending. This is also why the number of iterations often cannot be read off the code: when a loop runs until a condition is met, only the data decides how long it takes, and reasoning about that — rather than counting lines — is the skill being trained.