Page 192 - Learn To Program With Scratch

Since infinite recursion is generally not a good idea, you must control
the execution of a recursive procedure with conditionals. For example, the
procedure could include an if block that determines whether the recursive
call should be made. To demonstrate this technique, Figure 7-18 shows a
recursive procedure that counts from some initial number (specified by
the parameter count) down to 0.

CountDown(3)
say 3 say 2 say 1 say 0

(3 > 0)? (2 > 0)? (1 > 0)? (0 > 0)?
Yes Yes Yes No
CountDown(2) CountDown(1) CountDown(0) return
return path
... return to return to
CountDown (2) CountDown (1)
Figure 7-18: The if block is used to determine whether (or not) the recursive call should
be made .

Let’s walk through how CountDown works when it is called with an argu-
ment of three. When the procedure starts, the say command shows the num-
ber 3, then checks whether count is greater than 0. Since 3 is greater than 0,
the procedure subtracts 1 from count to call itself with an argument of 2.
In the second call, the procedure shows the number 2 and, because 2 is
greater than 0, calls itself one more time with an argument of 1. This con-
tinues until the call CountDown(0) is made. After showing the number 0 in
a voice bubble, the procedure checks whether count is greater than 0. Since
the expression in the header of the if block evaluates to false, no further
recursive calls will be made, and the procedure returns. Try to follow the
return path shown in Figure 7-18.
Now that we’ve covered the basics of tail recursion, we can apply it to
more interesting applications. Let’s consider, for example, the Blade proce-
dure shown in Figure 7-19.
RecursionBlade
.sb2

...
Start
Figure 7-19: Using a sprite’s direction to stop recursion

170 Chapter 7
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