1.4 Scr0340: arithmetic operators in scratch 2.0  (Page 3/7)

An operator that operates on one operand is called a unary operator. An operator that operates on two operands is called a binary operator. An operator that operates on three operands is called a ternary operator.

Some operators can be either unary or binary

Some operators can behave either as a unary or as a binary operator in languages such as Java and C#. The best-known operator that can behave either way in those languages is the minus character (-).

As a binary operator, the minus character causes its right operand to be subtracted from its left operand. For example, the third statement belowsubtracts the variable y from the variable x and assigns the result of the subtraction to the variable z . After the third statement is executed, the variable z contains the value 1.

• x = 6;
• y = 5;
• z = x - y;

The minus character as a unary operator

As a unary operator, the minus character causes the algebraic sign of the right operand to be changed. For example, the second statement belowcauses a value of -5 to be assigned to (stored in) the variable x .

• y = 5;
• x = -y;

Binary operators use infix notation

To keep you abreast of the current jargon, binary operators in Scratch always use infix notation. This means that the operator appears between its operands.

Some other programming languages have unary operators that use prefix notation and postfix notation. For prefix notation, the operator appears before (to the left of) its operand. For postfix notation, the operator appears after (to the right of) its operand.

General behavior of an operator

As a result of performing the specified action, an operator can be said to return a value (or evaluate to a value) of a given type. The type of value returned depends on the operator and the type of the operands.

To evaluate to a value means that after the action is performed, the operator and its operands are effectively replacedin the expression by the value that is returned.

Operator categories

There are many different kinds of operators. Therefore, the easiest way to study them is to divide them into categories such as the following

• arithmetic
• relational
• logical
• bitwise
• assignment

This module will concentrate on arithmetic operators. Future modules will deal with other kinds of operators.

Preview

In this module, I will present and explain a Scratch program named Arithmetic01 . This program illustrates the use of the following arithmetic operators (see the bottom script in the rightmost panel of Image 4 ):

• + (addition)
• - (subtraction)

Variables with the following names are created and displayed on the stage (see Image 5 ):

• LeftOperand - has a slider
• RightOperand - has a slider
• Sum
• Diff

In addition, a button is displayed on the stage.

The user adjusts the values of the LeftOperand and RightOperand variables with a pair of sliders. When the user clicks the button, the variablenamed Sum displays the sum of the values of the left and right operands. The variable named Diff displays the result of subtracting the right operand from the left operand.

Discussion and sample code

I'm going to walk you through the steps required to develop this program, being brief on those things that you already know about and being more verboseon the new material.