The while Loop and Its Three Moving Parts

A while loop has three moving parts, and all three have to work together or the loop either never runs or never stops. First, you set up a variable before the loop starts, often called a counter. Second, the loop tests a condition using that variable, and it checks this condition before every single pass through the body, including the very first one. Third, something inside the body has to update the variable, moving it closer to making the condition false. Leave out the update, or update the wrong variable, and the condition stays true forever, which gives you an infinite loop.

Picture the three parts as a cycle: setup happens once, then test, then body, then update, then back to test. The loop keeps cycling through test-body-update until the test comes back false, at which point it skips the body and moves on. Because the test runs before the body each time, a while loop can run zero times if the condition is already false at the start. This is different from a loop that guarantees at least one pass.

This three-part structure is exactly what you track in a trace table: one row per pass, showing the variable's value, whether the test passes, and what the update does. Understanding while loops here sets up for loops next, which package the same three parts (setup, test, update) into one line, and for comparing while loops to do-while loops, which test after the body instead of before.