Page 230 - Learn To Program With Scratch

First, the user is asked to enter an answer in fraction form (for example,
3/5 or –7/8). The script then extracts the numerator and the denominator
of the answer string (which are separated by the division sign) and assigns
them to the num3 and den3 variables, respectively. For example, if the user
enters –23/15, num3 will be set to –23 and den3 will be set to 15. After that,
the script broadcasts a GotAnswer message to tell the Digit sprite to show the
user’s answer on the Stage. When the Digit sprite receives this message, it
stamps the digits of num3 and den3 at the correct positions on the Stage
in the same way it displayed the numerators and denominators of the two
operands. You can check the file FractionTutor.sb2 for the details.
After entering an answer, the user can click the Check button to see if
the answer is correct. The script for the Check sprite broadcasts a CheckAnswer
message to inform the other sprites of the user’s request. This message is
trapped and processed by the Teacher sprite, which will execute the script
shown in Figure 8-34.

Compute the correct answer based
on the current operation, which is
determined by the costume number

 of the Operation sprite. The four
procedures (Add, Subtract, Multiply,
and Divide) compute the numerator
(ansNum) and the denominator
(ansDen) of the correct answer.

Find the greatest common divisor
(GCD) of ansNum and ansDen in
 order to put the answer in its
simplest form.

 Reduce ansNum and ansDen by
dividing both of them by the GCD.
 Give feedback to the user.
Figure 8-34: The CheckAnswer script

The current costume of the Operation sprite tells which operation pro-
cedure (Add, Subtract, Multiply, or Divide) to execute u. The operations
take num1, den1, num2, and den2 as inputs and set the values of ansNum and
ansDen, which represent the numerator and denominator of the correct
answer, respectively. The four procedures are shown in Figure 8-35.
After finding the answer, CheckAnswer needs to put it in its simplest
form. For example, 2/4 should be simplified to 1/2. To perform this reduc-
tion, the script first finds the greatest common divisor (GCD), also known
as the greatest common factor, of the numerator and denominator v.
(We’ll look at this procedure in a moment.)

208 Chapter 8
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