2.7 Control Structures: Assignment Statements

The assignment statement is used in Ada to perform computations. The assignment statement

    SquareYards := MetersToYards * SquareMeters;

in Program 2.6 assigns a value to the variable SquareYards, in this case the result of the multiplication of the constant MetersToYards by the variable SquareMeters. Valid information must be stored in both MetersToYards and SquareMeters before the assignment statement is executed. As shown in Figure 2.2, only the value of SquareYards is affected by the assignment statement; MetersToYards and SquareMeters retain their original values.

Figure 2.2
Effect of SquareYards := MetersToYards * SquareMeters;

Figure 2.2

The symbol := is the assignment symbol in Ada and should be pronounced "becomes" or "takes the value of" rather than "equals." The : and = must be adjacent characters with no intervening space. The general form of the assignment statement is shown in the next display.

SYNTAX DISPLAY
Assignment Statement (Arithmetic)

Form:

result := expression ;

Example:

X := Y + Z + 2.0;

Interpretation:

The variable specified by result is assigned the value of expression. The previous value of result is destroyed. The expression can be a single variable or a single constant, or it can involve variables, constants, and arithmetic operators, some of which are listed in Table 2.2. The variable specified by result must be of the same data type as the expression.

Table 2.2
Some Arithmetic Operators

Operator	Meaning
+		addition
-		subtraction
*		multiplication
/		division
**		exponentiation

It is permissible to write assignment statements of the form

    Sum := Sum + Item;

where the variable Sum is used on both sides of the assignment operator. This is obviously not an algebraic equation, but it illustrates something that is often done in programming. This statement instructs the computer to add the current value of the variable Sum to the value of Item; the result is saved temporarily and then stored back into Sum. The previous value of Sum is destroyed in the process as illustrated in Figure 2.3; however, the value of Item is unchanged.

Figure 2.3
Effect of Sum := Sum + Item;

Figure 2.3

Assignment statements can also be written with an expression part that consists of a single variable or value. The statement

    NewX := X;

instructs the computer to copy the value of X into NewX. The statement

    NewX := -X;

instructs the computer to get the value of X, negate this value, and store the result in NewX (e.g., If X is 3.5, NewX is -3.5; if X is -17.4, NewX is 17.4). Neither of the assignment statements above changes the value of X.

Exercises for Section 2.7

Self-Check

Which of the following are valid Ada assignment statements? Why?

	X = Y;
	A := B - C;
	P + Q := R;
	G := G;
	H := 3 + 4;
	H := 3 + K;
	T := S * T;