2.2 Scr0120: memory, variables, and literals  (Page 9/10)

In addition, I used the pull-down list on each of six blocks to select the name of the variable to which each block applies. (Note that each of those six blocks refers either to Counter or counter .)

Figure

Image 5. the finished program.

Behavior of the program

Click the green flag

The program contains three scripts. Each script remains silent until a specific event occurs. As the label on the uppermostblock in the top programming script in Image 5 indicates, the code in the top script in Image 5 is executed once each time the user clicks the green flag shown in the upper right of Image 1 . This code causes the values stored in each of the two variables to be set to zero. This, in turn causes thevariable displays in the Stage area to each show a value of 0.

Press the space bar

Because of the top block in the middle script reads "when space key pressed" , the code in the middle script is executed once each time the user presses the space bar.

The pull-down list on this block allows you to select among the keys on the keyboard with the space bar being the default.

One of the orange programming blocks in the middle script causes the value of the variable named Counter to change by +1 when the space bar is pressed. The other orangeprogramming block causes the value of the variable named counter to change by +5 when the space bar is pressed.

In other words, repetitively pressing the space bar causes the Counter variable to count up in increments of one and causes the counter variable to count up in increments of five.

Click the mouse on the Stage

Because of the top block in the bottom script reads "when stage clicked" , the code in the bottom script in Image 5 is executed once each time the user clicks the mouse in the large white Stagearea shown in Image 1 .

The bottom two blocks in this script are the same as the bottom two blocks in the middle script except that the algebraic signs on the two literal values are minus instead of plus.

The absence of a "-" character causes a literal value to be positive. A "+" character is notrequired for positive literal values.

Repetitively clicking the mouse in the Stage area causes the Counter variable to count down in increments of oneand causes the counter variable to count down in increments of five.

Event-driven programming

The ability to write programs that cause certain operations to occur in response to events (such as pressing the space bar and clicking the mouse on the Stage) is often referred to as event-driven programming.

It is easy to write event-driven programs in Scratch. On the other hand, writing event-driven programs is not easy in languages such asJava, C#, and C++. You need quite a lot of programming knowledge to write event-driven programs in those languages.

It is useful to give programming students a taste of event-driven programming early, if for no other reason than the factthat it tends to make programming more interesting.

Example blocks with pointed ends

The blocks shown in Image 6 are not used in this program. They are shown in this module to illustrate blocks with pointed endsreferred to earlier .