Page 231 - Learn To Program With Scratch

a + c = ad + cb a × c = ac
b d bd b d bd

a − c = ad cb a c ad
−
b d bd b ÷ d = bc

Figure 8-35: The Add, Subtract, Multiply, and Divide procedures of the Teacher sprite

After finding the GCD, the script divides ansNum and ansDen by that
value w and calls GiveFeedback x to display whether or not the user’s
answer was correct.
Now let’s look more closely at the details of these procedures, starting
with the four operation procedures shown in Figure 8-35.
These procedures compute the result of performing an operation of
the form

num1 [ +− num2 = ansNum
,, ,×÷]
den1 den2 ansDen

and store the result in two variables (ansNum and ansDen) corresponding to
the answer’s numerator and denominator, respectively.
Let’s now move on to the FindGCD procedure, shown in Figure 8-36.

Set gcd to the lesser of abs(num1)
and abs(num2). For example, if
num1=–10 and num2=6, gcd will
be set to 6.

Start a loop to
check the numbers
6, 5, 4, 3, ....
Stop when num1
and num2 divide
evenly by the
checked number.

Figure 8-36: The FindGCD procedure of the Teacher sprite

String Processing 209

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