6.4 Examples Using the for Statement

The following examples show techniques for varying the control variable in a for statement. In each case, we write only the appropriate for header. Note the change in the relational operator for loops that decrement the control variable.

  1. Vary the control variable from 1 to 100 in increments of 1.

    
for (int i = 1; i <= 100; ++i)

  2. Vary the control variable from 100 to 1 in decrements of 1.

    
for (int i = 100; i >= 1; --i)

  3. Vary the control variable from 7 to 77 in increments of 7.

    
for (int i = 7; i <= 77; i += 7)

  4. Vary the control variable from 20 to 2 in decrements of 2.

    
for (int i = 20; i >= 2; i -= 2)

  5. Vary the control variable over the sequence 2, 5, 8, 11, 14, 17, 20.

    
for (int i = 2; i <= 20; i += 3)

  6. Vary the control variable over the sequence 99, 88, 77, 66, 55, 44, 33, 22, 11, 0.

    
for (int i = 99; i >= 0; i -= 11)

Common Programming Error 6.3

Using an incorrect relational operator in the loop-continuation condition of a loop that counts downward (e.g., using i <= 1 instead of i >= 1 in a loop counting down to 1) is usually a logic error.

Common Programming Error 6.4

Do not use equality operators (!= or ==) in a loop-continuation condition if the loop’s control variable increments or decrements by more than 1. For example, consider the for statement header for (int counter = 1; counter != 10; counter += 2). The loop-continuation test counter != 10 never becomes false (resulting in an infinite loop) because counter increments by 2 after each iteration (and never becomes 10).

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