Repetition structures: do-while and for statements

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do-while statement for statement

do-while statement Top

do-while structureStructure

do
  <loop_body>;
while (<condition>);

If the loop body contains more than one statement, it should be enclosed by a pair of braces.

In a do-while loop, the condition is tested after execution of the loop body. If the condition is non-zero (i.e. true), the control goes back and the loop body is executed again. If the condition is zero (i.e. false), the loop is exited.

The C++ do-while structure is equivalent to the following:

Equivalent structure using while statement in C++ <loop_body>;
while (<condition>)
  <loop_body>;
Equivalent structure using repeat-until statement in Pascal repeat
  <loop_body>
until not <condition>

Comparison between do-while loop and while loop in C++

do-while loop while loop
The loop body is executed at least once. It is possible that the loop body is never executed (when the condition is zero as the loop is entered).
The condition is tested after the loop body is executed. The condition is tested before the loop body is executed.

Comparison between do-while loop in C++ repeat-until loop in Pascal

do-while loop in C++ repeat-until loop in Pascal
The loop is exited when the condition is false (i.e. zero). The loop is exited when the condition (after until) is true.

Sample program

The result of this program is identical to Program 18 except that there is only one cin statement in the loop in this program. This is because a do-while loop is used.

Program 19: Using a do-while loop
C++ version (using do-while loop) Pascal version (using repeat-until loop)
#include <iostream.h>

main()
{
  int score, sum = 0;

  do {
    cout << "Enter score (-1 to quit): ";
    cin >> score;
    if (score != -1)
      sum += score;
  } while (score != -1);

  cout << "Sum = " << sum << endl;

  return 0;
}		 program prog_19(input, output);
var
  score, sum: integer;
begin
  sum := 0;

  repeat
    write('Enter score (-1 to quit): ');
    readln(score);
    if score <> -1 then
      sum := sum + score
  until score = -1;

  writeln('Sum = ', sum);
end.

for statement Top

for (<expression_1>; <expression_2>; <expression_3>)
  <loop_body>;

If the loop body contains more than one statement, it should be enclosed by a pair of braces.

The above C++ for structure is equivalent to the following:

<expression_1>;
while (<expression_2>) {
  <loop_body>;
  <expression_3>;
}

Therefore Program 16 can be re-written as follows:

Program 20: Using for statement
#include <iostream.h>

main()
{
  int n, product;

  cout << "n? ";
  cin >> n;
  for (product = n; product <= 1000; product *= n)
    cout << product << endl;

  return 0;
}

Usually, for statement is used as a counter-controlled loop. In such case, <expression_1> is an intialisation statement, <expression_2> is an expression testing whether the final value is reached, and <expression_3> indicates how the counter variable is incremented (or decremented) in the loop.

Counter-controlled loop in C++ (using for) for (<counter> = <initial>; <counter> <= <final>; <counter>++)
  <loop_body>;
Equivalent counter-controlled loop in C++ (using while) <counter> = <initial>;
while (<counter> <= <final>) {
  <loop_body>;
  <counter>++;
}
Equivalent Pascal control structure for <counter> := <initial> to <final> do
  <loop_body>

The following shows how Program 17 (containing a counter-controlled loop) can be re-written using for statement:

Program 21: Using a for statement as a counter-controlled loop
C++ version Pascal version
#include <iostream.h>
main()
{
  int score, sum, i;

  sum = 0;
  for (i = 1; i <=5; i++) {
    cout << "Score No. " << i << ": ";
    cin >> score;
    sum += score;
  }
  cout << "Sum = " << sum << endl;

  return 0;
}		 program prog_21(input, output);
var
  score, sum, i: integer;

begin
  sum := 0;
  for i := 1 to 5 do begin
    write('Score No. ', i, ': ');
    readln(score);
    sum := sum + score;
  end;
  writeln('Sum = ', sum)
end.

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