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are also called counter-controlled loops or counted loops. Other types of loops
continue to repeat until some condition occurs; these are called condition-
controlled loops or indefinite loops. Another loop, called an infinite loop, repeats
forever.
In this chapter, you’ll learn about the different repetition structures
available in Scratch. I’ll explain both counter-controlled and condition-
controlled loops in detail, and I’ll introduce you to the stop block, which
you can use to end infinite loops. You’ll learn how to use loops to validate
user input, as well.
The chapter also discusses nested loops (loops that contain other loops)
and shows several examples of their use. We’ll also talk about recursion—a
procedure calling itself—as another way to achieve repetition. At last, we’ll
develop a number of interesting applications that use both loops and condi-
tionals, and we’ll look at incorporating loops into practical programs.

more Loop blocks in Scratch

As you learned in Chapter 2, loop blocks allow you to repeat a command
or a set of commands in a program. Scratch supports the three repetition
blocks shown in Figure 7-1.

Figure 7-1: Scratch’s repeat blocks

You’ve already used two of these blocks, the repeat and the forever
blocks, in many examples throughout this book. In this section, we’ll exam-
ine the third kind of loop block—the repeat until block—and explain
some of the technical terms associated with loops in general.
Each repetition of a loop is called an iteration, and the word count
is often used to describe the number of times a loop repeats. The repeat
block, which you are very familiar with, is a counter-controlled loop because
it repeats its commands a specific number of times. We usually use this loop
when we know the number of repetitions the loop will need to execute,
as when we want to draw a polygon with a known number of sides.
On the other hand, the repeat until block is a condition-controlled loop.
The statements inside this block are repeated based on the truth or falsity
of its test expression. We use this block when we don’t know in advance how
many times the loop will need to repeat and want the repetition to continue
until some condition is met. You can say, for example, “Repeat the ask com-
mand until the user enters a positive number.” Or, “Repeat firing missiles
until the player’s energy level drops below a certain value.” The following
sections explain condition-controlled loops in more detail.

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