1.6 Scr0360: logical operators in scratch 2.0  (Page 3/7)

The range of each slider variable is set to -10 to 10.

Event handlers

Three event handlers, shown as the bottom three scripts in the programming panel in Image A , are written to fire each time the button is clicked.

Program operation

At runtime, the user adjusts the value of each of the slider variables.

When the user clicks the button, each event handler evaluates a relational-logical expression (an expression containing both relational and logical operators) . If the expression evaluates to true, a value of true is displayed in one of the bottom three variables on the stage in Image A . If the expression evaluates to false, a value of false is displayed in thatvariable.

Meaningful variable names

In an earlier module, I told you that you should always strive to use meaningful variable names. At this point, you are probably thinking thatvariables named A , B , C , and D aren't very meaningful, and if so, your thinking is correct. However, there is a practical reason that I didn't use meaningful variable names in this case.

Insufficient space

Really meaningful variable names usually require about five or more characters. However, I have limited display width in this presentation format. If I had used longer names for the variables in this program, theexpressions containing those variable names would have been quite long, and would have made it difficult for me to fit Image A into this presentation format.

You will note, however, that the bottom three variable names on the stage in Image A (also see the variables in the Data toolbox in Image A ) are meaningful. In fact they are probably more meaningful than would be thecase in Java because Java doesn't allow the space character, the left angle bracket character, or the "(" character to be included in a variable name.

A truth table

According to Wikipedia:

"A truth table is a mathematical table used in logic -- specifically in connection withBoolean algebra, boolean functions, and propositional calculus -- to compute the functional values of logical expressions on each of their functionalarguments, that is, on each combination of values taken by their logical variables. In particular, truth tables can be used to tell whether a propositional expression is true for all legitimate input values, that is,logically valid."

What does this mean?

The concept of a truth table is simpler than the above quotation might suggest. Image B is a simple truth table that shows the result of evaluating four logical expressions for all combinations of the values of two variables, A and B , where each variable can only take on the values of true and false .

Figure

Image b. a simple truth table.

The and operation

If both A and B are true, then the expression (A and B) evaluates to true. Otherwise, it evaluates to false. Correspondingly, if (A and B) is true, then not(A and B) is false. Otherwise, it is true. Applying the not operator to a boolean value is often referred to as getting the complement of that value.

The inclusive or operation

The operation that I am getting ready to discuss in known as an inclusive or operation. There is also an exclusive or operation that I won't discuss in this module. That will be a topic for a much more advanced module.