1.5 Scr0350: relational operators in scratch 2.0  (Page 4/6)

Where are the blocks with the correct shape?

There are two buttons (possibly more) that we can click in the upper left in Image 5 that will expose blocks with shapes matching the pocket immediately to the right of the word if in Image 5 :

• Sensing
• Operators

Will use blocks from the Operators group

In this module, we will use three of the blocks that are exposed by clicking the green Operators button as shown in Image 6 .

Figure

Image 6. green blocks exposed by clicking the operators button.

Six blocks have the correct shape

As you can see in Image 6 , there are six blocks of the correct shape exposed by clicking the Operators button. Those blocks are identified as follows:

We will refer to the first three blocks above as relational operators . (They are used to evaluate the relationship between two values.) We will refer to the other three blocks in the above list as logical operators , and will deal with them in a future module.

Relational operators in other languages

Many other modern programming languages also include the following three relational operators:

Although these operators are a great convenience, they are not essential. We can get by with only the three that are provided by Scratch.

Also, in many other programming languages, a pair of equal characters (==) is used for the "equal to" operator instead of the single equal character (=) used in Scratch.

Three steps at once

Image 7 shows the result of three more steps in the development of the program:

1. Dragging the green less than (indicated by a left angle bracket) block and dropping it into the pocket immediately to the right of the word if .
2. Dragging the orange block labeled set...to for the variable named LessThan and inserting it immediately below the word if . I also set its literal value to "Less than is true".
3. Dragging another copy of the orange block labeled set...to for the variable named LessThan and inserting it immediately below the word else . I also set its literal value to "Less than is false".

Figure

Image 7. intermediate stage in construction of if-else script.

Still need to complete the conditional clause

The script in Image 7 includes the action to be taken if the relational expression following the word if evaluates to true. It also includes the action to be taken if therelational expression evaluates to false. (This relational expression is often referred to as a conditional clause.) .

However, we still haven't completed the conditional clause in Image 7 . We don't want to test for the relationship between the literal values blank and blank as shown in Image 7 . Instead we want to test the relationship between the values of the variablesnamed LeftOperand and RightOperand as shown in the corresponding script in Image 4 .

Complete the relational expression

The next and final step for constructing this script is to: