15 Loops

Sometimes the steps of our algorithm need to be repeated until a condition is met.

For example, when we look for primes, we’ll keep on checking different numbers for primeness, but the process each time is the same.  There are essentially three types of loops we might introduce into an algorithm:

• Repeat a process a given number of times
• Repeat a process indefinitely (or until the process is stopped manually)
• Repeat a process until a condition is met

For example, previously we created a program that determined whether or not the scratch cat was on the right.  This only worked when we placed the cat and then executed the code, however if we want the program to keep on checking as we move the cat around, we can put in a loop that will keep on checking until we end the program.

Image: screenshots from Scratch

Let’s have a look at some other examples in Scratch

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Loops involving if-then statement

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