1.6 Scr0360: logical operators in scratch 2.0  (Page 5/7)

Compare Image g with image c

If you compare the relational-logical expressions defined by the statements in the green areas of Image G with the expressions in the column headers in Image C , you should see a strong resemblance.

Parentheses are used to eliminate ambiguity and to clearly identify the operands of each operator in Image C . A 3D optical illusion is used for the same purpose in Image G . For example, it appears that everything to the right of the not operator in Image G is a little closer to the viewer than the green platform on which the word not is printed.

No detailed explanation of the construction is required

In the past several modules, I have walked you through the detailed drag, drop, insert, and typing steps required to produce scripts similar to thoseshown in Image G . Hopefully, you understand the mechanics of that process by now and it should no longer be necessary for me to walk you through the process.

Program behavior at runtime

The user begins by clicking the green flag in the upper right corner of the stage toinitialize all of the variables shown in Image D to 0. Then the user can change the value in each of the variables A , B , C , and D by moving the sliders shown in Image D .

Order of execution of scripts

Following that, when the user clicks the button at the bottom of Image D , each of the bottom three scripts in Image G is executed. Note that they are not all executed simultaneously due to practical limitations in the construction ofmost computers. However, I can't tell you the exact order in which they are executed because, as far as I know, that is not public information. It might belogical to assume that the individual scripts are executed from top to bottom, but such an assumption may not be correct.

Not important in this program

The order in which the three scripts are executed doesn't really matter in this program because the end result will be the same regardless of the order ofexecution. However, if two different scripts were to make modifications to the value of the same variable, the order in which those modifications are madecould matter a lot. I mention this simply as a word to the wise. Be careful and make certain that you don't write different event handlers that respond to thesame event and modify the same data in different ways even if that seems to appeal to your organizational sensibilities. In that case, you should combinethe different event handlers into a single event handler because the order of execution of the code within a script is well defined.

Evaluate the relational-logical expressions and take appropriate action

For each of the bottom three scripts in Image G , if the relational-logical expression given by the green blocks with the embedded orange blocks evaluatesto true, a value of true is set into and displayed by the variable identified bythe statement immediately below the word if . Otherwise, a value of false is set into and displayed by that variable as shown in Image D through Image F .

An online version of this program is available

A copy of this program has been posted online for your review (see Resources for the URL) . If you don't find the program using that URL, search the Scratch site for the usernamed dbal.