Loops and Repeated Computation

Doing It Again

A loop repeats a block of code. The staple forms are the while loop and the for loop.

A while loop runs as long as a condition stays true, checking it before each pass. Correct execution requires three ingredients:

Forget the update, and you get an infinite loop that freezes the program by running forever.

A for loop handles known repetitions or steps through collections. For example, $range(n)$ produces values starting at 0 and stopping before $n$:

$$\text{range}(n): 0, 1, \ldots, n-1$$

Control and Pitfalls

The for loop convention stops before $n$, a frequent source of the off-by-one error where a loop runs one time too many or too few because a boundary like < versus <= is wrong.

Two control statements adjust loops internally:

Loops can also nest, placing one inside another. If an outer loop runs $n$ times and an inner loop runs $m$ times, the inner body executes $n \times m$ times, directly impacting performance.