Review of Repetition Control Stuctures while, do/while & Menu-Driven Programs
Repetition Control Structure: for
Nested Loops
Using a Sentinel

Syntax: for statement

for(<data class> <counter> = <initial value>; counter test expression; increment/decrement counter)
{

statement 1;
statement 2;
.
.
.
statement n;

As usual when there is only one statement the curly brackets can be omitted.

To illustrate the use of the for statement, we revisit the problem of computing the numbers from 1 to n, which we solved using a while statement in the Oct. 13 lecture.

//For a given positive integer N, this program uses a for loop to
compute the sum of the numbers from 1 to N.

#include <iostream.h>  // used for cout and cin

int main()
{
   int Sum = 0;
   int N;

   cout << "Please enter a positive integer N: ";
   cin >> N;

   //COMPUTE SUM OF NUMBERS FROM 1 TO N
   for (int Counter = 1; Counter <= N)
        Sum += Counter;
     
   //DISPLAY RESULT
   cout << "The sum of the numbers from 1 to " << N << " is " << Sum
        << '.' << endl;
   return 0;
} //END 

Nested Loops and Using a Sentinel

Suppose we wish to compute the sum of the numbers from 1 to N for many different values of N. This is accomplished in the next program using nested loops. We have an outer while loop that is executed until the user enters a negative integer, and an inner for loop that computes the sum. The negative integer is called a sentinel value, because its purpose is simply to mark the end of the sequence of integer N's being input, and it is not actually a part of the input data, i.e., an N for which we are computing a sum.

#include <iostream.h>  // used for cout and cin

int main()
{
   int Sum = 0;
   int N;

   cout << "Please enter a positive number N, or negative number to terminate: ";
   cin >> N;
   //KEEP LOOPING UNTIL A NEGATIVE NUMBER IS INPUT
   while (N > 0)
   { 
        //COMPUTE SUM OF NUMBERS FROM 1 TO N
        for (int Counter = 1; Counter <= N)
             Sum += Counter;

        //DISPLAY RESULT
        cout << "The sum of the numbers from 1 to " << N << " is " << Sum
             << '.' << endl;
        cout << "Please enter a positive number N, or negative number to terminate: ";X
        cin >> N; //INPUT ANOTHER INTEGER N
        Sum = 0; //INITIALIZE Sum TO 0 FOR NEW PASS AND SUMMATION COMPUTATION
   }//END OUTER while loop 
   return 0;
} //END