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Section 1.29 Worked Example: Swap

Subgoals for evaluating an assignment statement.

  1. Determine resultant data type of expression

  2. Update variable for pre-increment or pre-decrement operators (side effect)

  3. Evaluate arithmetic expression according to operator precedence

  4. If an assignment statement (=), is Left Hand Side (LHS) a variable? Check data type of value against data type of variable.

  5. Update variable for post-increment or post-decrement operators (side effect)

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Consider these declarations.

int x,y,z;
x = 7;
y = 13;

Now consider this code for swapping the values of x and y. Note the need for a third variable to hold the value of x so that it is not lost when assigning the value of y into x.

z = x;
x = y;
y = z;

The diagram below shows the changes in memory after executing each of the six lines of code.

Figure 1.29.1.

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