Page 243 - Learn To Program With Scratch

work for our scoreList, and the script shown in Figure 9-11 uses this sentinel
to know when the user is done entering values.

The contents of scoreList
if the user enters
85, 100, 95, –1

Figure 9-11: Using a sentinel to control list growth

In each iteration of the loop, the script prompts the user to enter a
number and compares that value to the sentinel. Note that the script speci-
fies the sentinel (–1 in this case) in its prompt to the user. If the user enters
–1, then the script stops because it knows the user is done entering scores.
Otherwise, the input value is appended to the list, and the user is prompted
for another entry. Figure 9-11 shows how scoreList should look if the user
enters three scores followed by the sentinel.

Creating a Bar Chart
BarChart .sb2 As a practical example of collecting user input with lists, let’s write an appli-
cation that draws a bar chart (also called a histogram) from the user’s num-
bers. For simplicity, we’ll only accept five numbers between 1 and 40. Once
the program has received all five numbers, it will draw five bars with heights
proportional to the entered values. The user interface for our chart maker
is illustrated in Figure 9-12.

Driver sprite
Painter sprite
(invisible)

Variables n1, n2,
n3, n4, and n5
Frame sprite

Figure 9-12: The Bar Chart application

This application contains three sprites. The Driver sprite controls the
flow of the application; it contains scripts that accept user input, populate
the list, and tell the Painter sprite to start drawing. The Painter sprite is an
invisible sprite that draws the bar chart. The Frame sprite is purely cosmetic;
it hides the bottom of each bar to make it look flat; without it, the bottoms

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