2.2 Scr0120: memory, variables, and literals  (Page 7/10)

Specifying the values

After dragging a programming block from the Variables area into the programming area, you can:

• Identify the variable to which that block applies by selecting the variable name in the pull-down list built into the block.
• Enter a literal numeric or string value from the keyboard into the white or gray area at the right end ofthe block, or
• Drag some other block and drop it into the white or gray area.

In some cases, the area looks white and in other cases it looks gray. I will refer to it as the white area in both cases.

There are at least two different shapes that are eligible for being dropped into such areas:

1. A horizontal rectangle with rounded corners such as the orange block labeled Counter in Image 2 . (This shape seems to always be associated with a numeric or string value.)
2. A horizontal rectangle with pointed ends as shown by several of the blocks in Image 6 . (This shape seems to always be associated with a boolean result of true or false.)

Blocks having the rounded corners are eligible for being dropped into the white areas with rounded corners of the programming blocks in Image 2 .

Blocks having the pointed ends are eligible for being dropped into the darker orange areas having a similar shape on the blocks near the bottom of Image 4 .

Either shape of block is eligible for being dropped into the white areas with square corners shown in Image 2 . Thus the block in Image 2 that is used to set a variable to a value can set the value to a numeric value,a string value, or a boolean value.

(Note that the two variable blocks shown in Image 2 labeled Counter and counter are eligible for being dropped into the white areas of the two programming blocks shown in Image 2 . This makes it possible to cause the value of one variable to depend on the valueof another variable.)

If you click the Operators button shown in Image 2 (see Image 6 ) you will expose several different blocks with rounded corners that are eligible for being dropped into thewhite areas in Image 2 according to the rules described above.

Cause the variables to be displayed

Checking the boxes to the left of the variable names in Image 2 causes them to be displayed in the upper-left corner of the large white Stage area shown in Image 1 . (Clicking the show and hide blocks will also check and uncheck the checkboxes, which in turn will show or hide the variables on the stage.)

Create a new variable

You create a new variable by clicking the button labeled Make a variable shown in Image 2 . When you click that button, the dialog shown in Image 3 appears on the screen asking you to provide the name for the new variable.

Figure

Image 3. dialog for entering a variable name.

At that point, you simply type in the name for the new variable and click the OK button.

The radio buttons on that dialog allow you to specify which sprites have access to the variable.

Allowable variable names and lengths

Most programming languages have very specific requirements regarding the allowable characters for variable names. I suspect that this is also true ofScratch, but I haven't found that specification anywhere. You will probably be okay as long as you stick with letters, numbers, the underscore character"_", and the hyphen or minus sign "-".