CSCI 1110 - C++ Lecture Notes
Chapter 6

The While Statement

Count-Controlled Loops

Event-Controlled Loops

Loop Design

The While Statement

• A Loop is a control structure that causes a sequence of statements to be executed repeatedly.
• The While Statement tests a condition and repeats a block of one or more statments as long as the condidition is true.
• While statement syntax diagram:

•  Examples:
while (inputVal != 25)
cin >> inputVal;
 
 
x = 0;
while (x < 10)
{
cout << x << endl;
x++;
}
 
• While loop flow chart
 

• Phases of While Loop Execution
• Initialization occurs prior to the while statement.
• Loop entry occurs the moment that the flow of control reaches the first statement inside the loop body.
• Each time the body of the loop is executed, a pass is made through the loop. This pass is called an iteration.
• Before each iteration, the loop test is made at the beginning of the loop.
• When the last iteration is complete and flow of control has passed to the first statement after the loop, the program has exited the loop.
• The condition that causes the loop to exit is called the exit condition.

Count-Controlled Loops

• A count-controlled loop executes a specified number of times.
• Examples:
// Loop 10 times (1, 2, ... 10)
loopCount = 1;
while (loopCount <= 10)
{
.
.
.

loopCount++;

}

// Loop 10 times (0, 1, ... 9)
loopCount = 0;
while (loopCount < 10)
{

.
.
.

loopCount++;

}

// Loop 10 times (10, 9, ... 1)
loopCount = 10;
while (loopCount >= 1)
{

.
.
.

loopCount--;

}

Event-Controlled Loops

• An event-controlled loop is a loop that terminates when something happens inside the loop body that signals that the the loop should be exited.
• A sentinel is a special data value that is used to signal that there are no more data values to be input.
• A sentinel-controlled loop reads in and processes lists of data. Each time the loop body is executed, a new piece of data is read. If the data is not the sentinel, the data is processed. If the sentinel is read, the loop is terminated.
• Examples:
// Example 1. data variable initialized
//            to non-sentinel value.
int data = 0;
long sum = 0;

while (data != -1)    // -1 is sentinel
{

cin >> data;
if (data != -1)
sum += data;
}
 
 

// Example 2. Priming read.
int data;
long sum = 0;

cin >> data;
while (data != -1)    // -1 is sentinel
{

sum += data;
cin >> data;
}
 
• An end-of-file-controlled loop is used to read values until the end of file is encountered.
• A file variable can be tested as if it were a Boolean value. If the previous I/O operation was successful, it will test TRUE. If the operation was not successful (i.e. attempt to read beyond the end of file), it will test FALSE.
• Example:

#include <fstream.h>
    . . .
ifstream inFile;
int data;
 
inFile.open("dataset.dat");
if (!inFile)
return 1;
inFile >> data;
while (inFile)    // Read until EOF
{
cout << data;
inFile >> data;
}
 
• A more complex loop example. The following program reads characters from a  file and determines the number alphabetic characters in the file.

// file alphacount.cc

#include <iostream.h>
#include <fstream.h>

int main()
{

ifstream inFile;
char ch;
long totalAlphaChars = 0;
 
inFile.open("dataset.dat");
if (!inFile)
return 1;
inFile.get(ch);
while (inFile)     // Read until EOF
{
cout >> ch;    // echo input

if ((ch >= 'a' && ch <= 'z') ||

        (ch >= 'A' && ch <= 'Z'))
totalAlphaChars++;
 
inFile.get(ch);
}

cout << endl << endl;
cout << "There are " << totalAlphaChars
     << " in the file." << endl;

return 0;

}// end of main()

// end of file alphacount

Loop Design

• Points to consider in designing a loop:
1. What is the condition that ends the loop?
2. How should the condition be initialized?
3. How should the condition be updated?
4. What is the process being repeated?
5. How should the process be initialized?
6. How should the process be updated?
7. What is the state of the program on exiting the loop?