Loops

• The for-Statement
• The while-Statement
• The do-while-Statement
• break, continue

The For-Statement

The for-statement is one of the most used statements in C++ (and in other languages ;-)). It is used, when a loop has a well known number of repetitions.

The general form is:

for (expression1; condition; expression2) {
  statement1;
  statement2;
  ...
  }

Example:

#include <iostream.h>
#define NUMBER 10

float values[NUMBER];

int input_numbers();
float output_average();

int main() {
  input_numbers();
  cout << endl << "Average: " << output_average() << endl;
}

int input_numbers() {
  for (int i=0; i<NUMBER; i++) {
    cout << "(" << i << ") number: ";
    cin >> values[i];
  }
  return 0;
}

float output_average() {
  float sum = 0;
  for (int i=0; i<NUMBER; i++) {
    sum += values[i];
    cout << ".";
  }
  return sum/NUMBER;
}

The While-Statement

The last example enables you to put in 10 numbers - exactly 10 numbers. When the number of inputs is unknown you would prefer to use a while-statement...

The general form ist:

while (condition) {
  statement1;
  statement2;
  ...
}

The following example shows how to calculate the average of input numbers. The "while"-loop terminates when the input is 0 or when the array is full (see "break"-statement).

#include <iostream.h>
#define NUMBER 100

float values[NUMBER];

int input_numbers();
float output_average();

int main() {
  input_numbers();
  cout << endl << "Average: " << output_average() << endl;
}

int input_numbers() {
  int i=0;
  int n=1;
  cout << "0 ... Exit" << endl;
    while (n!=0) {
        cout << "(" << i << ") value: ";
        cin >> n;        
        values[i]=n;          
        i++;
        if (i > NUMBER) break;
    }
  return 0;
}

float output_average() {
  float sum = 0;
  int i=0;
  while (values[i]!=0) {
    sum+=values[i];
    cout << ".";
    i++;
  }
  return sum/i;
}

Here is a sample run:

0 ... Exit
(0) value: 3
(1) value: 3
(2) value: 2
(3) value: 1
(4) value: 0
....
Average: 2.25

The Do-While-Statement

The statements will be executed while the condition is true - the condition evaluates after all statements have been executed (compare that with the "while"-statement).

The general form is:

do {
  statement1;
  statement2;
  ...
  } while (condition);

Break and Continue

The "break"-statement is used to interrupt a "for"-, "while"- or "do-while"-loop (for example that would be necessary if an error inside the loop has been detected).

The general form is:

if (condition) break;

The "continue"-statement forces the loop to start the next iteration.

if (condition) continue;