Selection and IF Statements

A program that runs every line every time can't make decisions. Selection fixes that by putting a fork in the road: a condition that evaluates to true or false, and a block of statements that only runs when the condition is true. This is how a program reacts to different inputs instead of doing the same thing no matter what. A login check, a game over screen, a grade cutoff — all of these need the program to behave differently depending on circumstances, and IF is the tool that makes that possible.

Mechanically, the IF statement evaluates its Boolean condition first. If the condition is true, the program executes the statements inside the IF block, in order, then moves on. If the condition is false, it skips that whole block entirely and jumps straight to whatever comes after it. Nothing inside the block runs, not even partially — it's all or nothing based on that one true-or-false check. An ELSE, when present, gives the false branch something to do; without one, false just means skip and continue.

This connects directly to what comes next in the unit: loops also run on a Boolean condition, repeating a block instead of running it once. And once you're comfortable with a single fork, you'll combine conditions and nest IF statements inside each other to handle more complex decisions. Selection is also the first place where tracing a program by hand really matters — you have to track the value of a variable, check it against a condition, and predict exactly which path execution takes.