1.2 Scr0320: memory, variables, and literals in scratch 2.0  (Page 9/11)

Figure

Image 3. programming blocks in the events category.

The finished program

The rightmost panel in Image 1 shows the finished program. I created the program by dragging blocks fromthe Data toolbox and the Events toolbox and snapping them together to form three separatescripts.

(Actually the program that you see in Image 1 is the result of an automatic conversion from v1.4 to v2.0 that was performed by the folks at MIT during the transition period fromv1.4 to v2.0. I originally wrote that program in v1.4. Some of the toolboxes had different names in v1.4.)

Note that I also entered literal values of 0, 1, 5, -1, and -5 into the white text fields in the programming blocks for the variables after dragging them intothe programming area.

In addition, I used the pull-down list on each of six orange Data blocks in Image 1 to select the name of the variable to which that block applies. (Note that each of those six blocks refers either to Counter or counter .)

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 1 indicates, the code in the top script in Image 1 is executed once each time the user clicks the green flag shown in the upper right ofthe stage. 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 this spriteclicked" , the code in the bottom script in Image 1 is executed once each time the user clicks the mouse in the large white Stagearea shown in Image 1 .

(In this case, there are no sprites so this event is triggered by clicking the stage. I believe this isa new label on this block in v2.0. This label, which refers to a sprite, seems to be a little too specific.)

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.