Assigning Values to Variables

Assignment and order of evaluation #

The first two difficulties that most people face when learning to program involve the assignment operation and the order of evaluation in an expression.

The assignment operation #

You will see lines similar to the following in almost every program in almost every programming language:

a = 10;

The meaning of this line is “make a hold the value 10.” Similarly, the following line means “make b hold the value 20”:

b = 20;

Based on that information, what can be said about the following line?

a = b;

Unfortunately, that line is not about the equality concept of mathematics that we all know. The expression above does not mean "a is equal to b!" When we apply the same logic from the earlier two lines, the expression above must mean “make a hold the same value as b.”

The well-known equals-to (=) symbol of mathematics has a completely different meaning in programming: make the left side’s value the same as the right side’s value.

Order of evaluation #

In general, the operations of a program are applied step by step in the order that they appear in the program. There are exceptions to this rule, which we will see in later chapters. We may see the previous three expressions in a program in the following order:

a = 10;  
b = 20;   
a = b;

The meaning of those three lines altogether is this: “make a's value become 10, then make b's value become 20, then make a's value become the same as b's value.”

Accordingly, after those three operations are performed, the value of both a and b should be 20. The value of a variable is the last value that has been assigned to that variable.

Variables #

Concrete, real-world concepts are represented in a program as variables. A value like air, temperature, and more complicated objects, such as a car engine, can be made variables in a program.

Why is a variable needed? #

The main purpose of a variable is to represent a value in the program. Since every value is of a certain type, every variable is of a certain type as well. Most variables have names, but some variables are anonymous.

Example #

As an example of a variable, we can think of the number of students at a school. Since the number of students is a whole number, int is a suitable type, and studentCount would be a sufficiently descriptive name.
According to D’s syntax rules, a variable is introduced by its type, followed by its name. The introduction of a variable to the program is called its definition. Once a variable is defined, its name can be used to access the value that it holds.