The for Statement and Repetition Structures

Tom Kelliher, CS17

• Pretest repetition: while, for
• Posttest repetition: do while.
• Testing in the middle of a loop: break, continue.
• Counter-, sentinel-controlled loops.

The for Statement

A common situation:

i = 0;          // Initialization.
while (i < n)   // Predicate.
{
   // Do something based on i's value.

   ++i;         // Post-loop processing.
}

Initialization, predicate and post-loop processing are combined in the for statement:

for (i = 0; i < n; ++i)
   ; // Do something based on i's value

General form of a for statement:

for (Initialization; Predicate; PostLoopProcessing)
   LoopBody

Semantics of a for statement:

1. Execute the initialization expression.
2. The predicate is evaluated; if false, the loop is exited.
3. The body of the loop is executed.
4. Post-loop processing is executed.
5. Go to step 2.

Factorial Example

long fact(int n)
{
   long prod = 1;
   int i;

   for (i = 2; i <= n; ++i)
      prod *= i;

   return prod;
}

Simulate for fact(5).

Summing n Inputs Example

• n is a parameter to this function:

  int sum(int n);   // Prototype.
  

• Read n values from the keyboard, return their sum.

Write the function yourself.

Fine Points

• ``Missing'' parts of a for statement; use of a break statement within a loop.
• for and while are ``equivalent.''
• When to use a for, when to use a while.

Loop Structures

Counter-Controlled Loops

See previous examples

Sentinel-Controlled Loops

Entering a list of ages and using a negative age to indicate ``end of input'':

cout << "Enter an age: "
cin >> inputAge;

while (inputAge >= 0)
{
   // Do something with the age.

   cout << "Enter an age: "
   cin >> inputAge;
}

Note the redundancy. Better expressed through a posttest structure:

do
{
   cout << "Enter an age: "
   cin >> inputAge;

   // Do something with the age.
} while (inputAge >= 0);

Suppose the first age is itself negative? (Unlikely in this case.) The ``best'' solution:

while (1)
{
   cout << "Enter an age: "
   cin >> inputAge;

   if (inputAge < 0)
      break;

   // Do something with the age.
}

Continue Statement

Not used very often.

Example: rejecting invalid inputs.

#include <iostream.h>
#include <iomanip.h>

double computeAverage(int number);

int main()
{
   int n;

   do
   {
      cout << "Enter the number of scores: ";
      cin >> n;
   } while (n < 1);

   cout << "\nThe average score is " << setprecision(1)
        << setiosflags(ios::fixed) << computeAverage(n) << ".\n";
   cout << "\nThe bill is in the mail.\n";

   return 0;
}

double computeAverage(int number)   // Why return double?
{
   int sum = 0;
   int score;
   int i;

   // Note comma operator in loop initializer.
   // The form of this differs from a similar example in the text.

   for (sum = 0, i = 0; i < number; ++i)
   {
      cout << "Enter score " << i << ": ";
      cin >> score;

      if (score < 0)
      {
         cout << "Score must be non-negative!  Score discarded.\n";

         // Why is this decrement necessary?
         --i;
         continue;  // Skip to next iteration.
      }

      sum += score;
   }

   return sum / number;   // Will this return a double?
}

Nested Loops

What is printed by:

int i, j;

for (i = 0; i < 3; ++i)
   for (j = 0; j < 4; ++j)
      cout << j << endl;

What is printed by:

int i, j;

for (i = 0; i < 3; ++i)
   for (j = 0; j < 4; ++j)
      cout << setw(8) << i << setw(8) << j << endl;

Exercises

1. Problems 2 and 3, pg. 420.
2. Problems 12 and 15, pg. 422.