A computer takes input, processes the input, and produces output.

• input: input statements
• processing: assignment statements
• output: output statements

Here we look at processing in terms of assignment statements.

An assignment is a task that must be done.

An assignment statement is used to move data within memory.

The assignment statement has the following form.

variable = expression

An assignment statement has a variable on the LHS (Left Hand Side), an expression on the RHS (Right Hand Side), and is executed in two steps.

• 1. Evaluate the expression on the RHS to a (literal) value.



• 2. Store the value into the memory location represented by variable on the LHS, destroying whatever value was previously in that location.

This operation is known as a destructive update as the value is gone forever. The assignment statement has the following form.

variable = expression

The assignment statement can be read in any of the following ways.

• Store (the value of) expression into variable
• variable becomes (the value of) expression
• variable gets (the value of) expression
• variable is assigned (the value of) expression

You should not say (or think) the following.

• variable equals expression

Consider the following assignment statement.

The value of x can never equal x+1 so saying "equals" does not make sense.

The operator symbol for "equals" is "==", not "=".

What is the difference between

x = x + 1

and

x == x + 1

The former is an assignment statement that assigns variable x the value of x+1. The side effect is that the value of x is changed. The latter is a relational expression that evaluates to false (x can never be equal to x+1).

A crucial distinction is that

• statements are executed for their side-effects, and
• expressions are evaluated for their values.

Warning: A variable can only have one value at any one time. Updating the value destroys the previous value. This destructive update of a memory storage location is called a side-effect.

A destructive update is sometimes called a mutation. A changed variable is said to be mutated.

Note that the LHS must be a variable, while the RHS is an expression. A variable is an expression, but an expression is not a variable. For now, the LHS and RHS should be of the same type.

Explain the destructive update principle in terms of an assignment statement.

The following are valid assignment statements that initialize the variables on the LHS to the literal values on the RHS (using the previous declarations).

Here are some invalid assignment statements.

Here are some of the many shortcut ways that Lua has to write assignment statements.

An increment is the adding of 1 to a value.

A decrement is the subtraction of 1 from a value.

Here are some of the many shortcut ways that Lua has to write increment and decrement statements.

Note that the "++" and "--" operators, when used in expressions, have a more complicated semantics that we will ignore for now. Explain the primary differences between a statement and an expression. Give a specific example of each.

There are only two ways in which computer memory can be changed.

• input statement
• assignment statement

Of course, there are variations of input statements and assignment statements. What are the two types of statements that can change the contents of memory at run-time? Give a specific example of each.

The assignment statement evaluates the expression on the right side of the assignment operator and places that value into the memory location specified by the variable on the left part of the assignment statement. Whatever value was there before is gone forever.

Consider the following problem.

The hours worked (for a week) is in variable hoursWorked.

The hourly rate is in variable hourlyRate.

How would you compute the amount paid for the week and put in into variable weeklyPay?

Here is just one way to calculate the weeklyPay.

Note that hoursWorked is changed using a destructive update by an assignment statement.