Page 187 - Learn To Program With Scratch

granted and changes the laptop’s costume to the on image. Otherwise, the
script displays Access denied! and the sprite continues to show its initial off
costume.
The GetPassword procedure sets the gotPass flag to 0, to indicate that it
hasn’t received a valid password yet, and initializes the failCount variable (our
loop counter) to 0. It then executes a repeat loop with a maximum repeat
count of three. During each iteration of the loop, the user is prompted to
enter a password. If the user enters the correct password (Pass123 in this
example), the gotPass flag is set to 1, the procedure stops itself by invok-
ing the stop this script command, and execution returns to the caller.
Otherwise, if the user hasn’t used up all three attempts, an error message
is displayed, and the user is given another chance. If the user fails three
consecutive times, the repeat loop automatically terminates, and the pro-
cedure returns to the caller with the value of the gotPass flag still set to 0.

try it out 7-4
Open this application and run it . What happens if you enter paSS123 (instead
of Pass123) for the password? What does this tell you about string comparison
in Scratch? Try to implement the GetPassword procedure using a repeat until
block .

Counting by a Constant Amount

CountingBy Of course, you don’t always have to increase your counters by 1 each time
ConstAmount through a loop. The script in Figure 7-13 at u, for example, has a sprite
.sb2 count from 5 to 55 in increments of 5. The script at v causes the sprite
to count down from 99 to 0 in decrements of 11—in other words, 99, 88,
77, ... , 11, 0.

 

Figure 7-13: You can increment and decrement counters by amounts
other than 1 .

To see this counting technique in a practical application, let’s say that
we want to find the sum of all even integers from 2 to 20 (inclusive). (That
is, we want the sum 2 + 4 + 6 + 8 + ... + 20.) The script of Figure 7-14 does
exactly that.

Repetition: A Deeper Exploration of Loops 165

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