Page 232 - Learn To Program With Scratch

Let’s trace the operation of FindGCD when num1 = –10 and num2 = 6.
We need to find the largest positive integer that divides num1 and num2 with-
out a remainder. The procedure starts by setting gcd, the result, to the lesser
absolute value of the two numbers, −6 in our example. A loop then tests the
numbers 6, 5, 4, and so on, until both num1 and num2 divide evenly by the
checked number. This is the result we are after. In this example, gcd will be
set to 2 since both numbers (–10 and 6) divide by 2 without a remainder.
The last procedure to examine is the GiveFeedback procedure, which
compares the user’s answer with the correct answer and displays an appropri-
ate message, as shown in Figure 8-37. The figure also shows some examples
that demonstrate the different cases of the if/else structure.

Examples
Correct User
Answer Answer
ansNum num3
 ansDen den3
 3 3
4 4
  3 6
4 8
  3 2
4 3

Figure 8-37: The GiveFeedback procedure of the Teacher sprite

try it out 8-8
Modify the fraction tutor program to keep track of the number of correct and incor-
rect answers . Devise a scheme for calculating a score and showing it to the user .

Summary

String processing is an important programming skill. In this chapter, you
learned how to access individual characters of a string to combine them,
compare them, remove them, and shuffle them around.
We started with a detailed look at the string data type and how strings
are stored as sequences of characters. We then wrote several procedures that
demonstrated basic string manipulation techniques. After that, we used these
techniques to write several interesting and practical applications. The con-
cepts developed in these projects can be applied in many other areas, and
I certainly hope they’ll help you think of your own projects.
In the next chapter, you’ll learn about lists and how to use them to store
and manipulate a bunch of values. Equipped with this new data structure,
you’ll have all the tools you need to write professional programs in Scratch.

210 Chapter 8
www.it-ebooks.info