Page 55 - Learn To Program With Scratch

the Power of repeat
Our programs have been relatively simple so far, but as you start writing
longer scripts, you’ll often need to replicate the same stack of blocks several
times in a row. Duplicating scripts can make your program longer, harder
to understand, and tougher to experiment with. If you need to change one
number, for example, you’ll have to make the same change in each copy of
the block. The repeat command from the Control palette can help you avoid
this problem.
DrawSquare .sb2 For example, let’s say that you want to draw the square shown in Fig-
ure 2-11 (left). You could command the sprite to follow these repetitive
instructions:
1. Move some distance and turn 90° counterclockwise.
2. Move the same distance and turn 90° counterclockwise.
3. Move the same distance and turn 90° counterclockwise.
4. Move the same distance and turn 90° counterclockwise.

Command 3

Command 4 Command 2

START

Command 1
Figure 2-11: A square (left) and a script to draw it (right) using a sequence of
move and turn commands

Figure 2-11 also shows a script that implements these instructions.
Notice that it repeats the commands move 100 steps and turn 90 degrees
four times. In contrast, we can
avoid using the same two blocks Run the commands
inside the repeat
over and over with the repeat block four times.
block, which runs the commands
inside it as many times as you tell it
to, as shown in Figure 2-12. Using
a repeat block can also make Figure 2-12: Using the repeat block to
the instructions much easier to draw a square
understand.
The square you draw with the script in Figure 2-11 depends on the
direction your sprite faces when you start. This concept is illustrated in
Figure 2-13. Note that after drawing the square, the sprite will return to its
starting point and face the same direction it did before it began to move.

Motion and Drawing 33

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