Page 185 - Learn To Program With Scratch

To demonstrate, let’s say that you are developing a game with two levels
and you want to let the user select a level to play. The only valid entries in
this case are the numbers 1 and 2. If the user enters a number other than
these two numbers, you’d like to offer another chance to enter an accept-
able value. One way to implement this check is shown in Figure 7-10.

Yes answer= No
1 or 2?

stop script ask again

Figure 7-10: Input validation using the forever block

The GetLevel procedure asks the user to enter a choice and checks
the answer inside a forever loop. If the user’s answer is invalid, the loop
prompts the user to reenter the level. If the user enters a valid number,
the procedure calls stop this script to terminate the loop and end the
procedure. When this happens, the main script, which has been patiently
waiting for the GetLevel procedure to return, moves on to execute the say
command. Figure 7-11 shows how to achieve the same task using the repeat
until block.

answer=
1 or 2? Yes
No
Loop ends. Procedure
returns to caller.
Figure 7-11: Input validation using the repeat until block

The procedure in Figure 7-11 asks the user to enter a choice and waits
for the answer. If the user enters 1 or 2, the condition in the header of the
repeat until block evaluates to true, which naturally terminates the loop
and ends the procedure. On the other hand, if the user enters anything
other than 1 or 2, the loop’s condition evaluates to false, and the ask com-
mand inside the loop executes. This command waits for the user’s input
again, and the repeat until block will continue asking for input until the
user enters a valid choice. Once again, note that this implementation
doesn’t require a stop block.

Repetition: A Deeper Exploration of Loops 163

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