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2.6 Memory Concepts

Variable names such as number1, number2 and sum actually correspond to locations in the computer's memory. Every variable has a name, a type, a size (in bytes) and a value.

In the addition program of Fig. 2.7, when the following statement (line 18) executes:

number1 = input.nextInt(); // read first number from user

the number typed by the user is placed into a memory location corresponding to the name number1. Suppose that the user enters 45. The computer places that integer value into location number1 (Fig. 2.8), replacing the previous value (if any) in that location. The previous value is lost.

Figure 2.8

Fig. 2.8 Memory location showing the name and value of variable number1.

When the statement (line 21)

number2 = input.nextInt(); // read second number from user

executes, suppose that the user enters 72. The computer places that integer value into location number2. The memory now appears as shown in Fig. 2.9.

Figure 2.9

Fig. 2.9 Memory locations after storing values for number1 and number2.

After the program of Fig. 2.7 obtains values for number1 and number2, it adds the values and places the total into variable sum. The statement (line 23)

sum = number1 + number2; // add numbers, then store total in sum

performs the addition, then replaces any previous value in sum. After sum has been calculated, memory appears as shown in Fig. 2.10. The values of number1 and number2 appear exactly as they did before they were used in the calculation of sum. These values were used, but not destroyed, as the computer performed the calculation. When a value is read from a memory location, the process is nondestructive.

Figure 2.10

Fig. 2.10 Memory locations after storing the sum of number1 and number2.

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