Learn C++ - C++ while Loop

The while loop is a for loop stripped of the initialization and update parts; it has just a test condition and a body:

while (test-condition) 
   body

If the expression evaluates to true, the program executes the statement(s) in the body.

The following code shows how to use while loop to read user input.


#include <iostream>
int main()/*from   ww w  .  jav a2  s .co m*/
{
    using namespace std;
    char ch;
    int count = 0;      // use basic input
    cout << "Enter characters; enter # to quit:\n";
    cin >> ch;          // get a character
    while (ch != '#')   // test the character
    {
        cout << ch;     // echo the character
        ++count;        // count the character
        cin >> ch;      // get the next character
    }
    cout << endl << count << " characters read\n";
    return 0; 
}

The code above generates the following result.

Example

The following loop cycles through each character in a string and displays the character and its ASCII code.


#include <iostream>
const int ArSize = 20;
int main()/* ww  w  .  j a v  a2 s .co  m*/
{
    using namespace std;
    char name[ArSize];

    cout << "Your first name, please: ";
    cin >> name;
    cout << "Here is your name, verticalized and ASCIIized:\n";
    int i = 0;                  // start at beginning of string
    while (name[i] != '\0')     // process to end of string
    {
        cout << name[i] << ": " << int(name[i]) << endl;
        i++;               
    }
    return 0; 
}

The code above generates the following result.

Example 2

The followng code shows how to use clock() and the ctime header to create a time-delay loop.


#include <iostream>
#include <ctime> // for clock() function, clock_t type
int main()
{/*from   w w  w.j a v a 2s . c  o m*/
    using namespace std;
    cout << "Enter the delay time, in seconds: ";
    float secs;
    cin >> secs;
    clock_t delay = secs * CLOCKS_PER_SEC;  // convert to clock ticks
    cout << "starting\a\n";
    clock_t start = clock();
    while (clock() - start < delay )        // wait until time elapses
        ;                                   // note the semicolon
    cout << "done \a\n";
    return 0; 
}

The code above generates the following result.

Number-Reading Loops

The following code uses a loop that terminates if the array is full or if you enter non-numeric input.


#include <iostream>
using namespace std;
const int Max = 5;
int main()/*from  w w w .j  av  a  2  s  .co  m*/
{
    double fish[Max];
    cout << "You can enter up to " << Max
            << " numbers <q to terminate>.\n";
    cout << "fish #1: ";
    int i = 0;
    while (i < Max && cin >> fish[i]) {
        if (++i < Max)
            cout << "#" << i+1 << ": ";
    }
    double total = 0.0;
    for (int j = 0; j < i; j++)
        total += fish[j];
    if (i == 0)
        cout << "No value\n";
    else
        cout << total / i << " = average of " << i << "\n";
    return 0; 
}

The code above generates the following result.

The do while Loop

The do while loop is different from the other two because it's an exit-condition loop.

If the condition evaluates to false, the loop terminates; otherwise, a new cycle of execution and testing begins.

Such a loop always executes at least once.

Here's the syntax for the do while loop:

do 
  body 
while (test-expression);

The body portion can be a single statement or a brace-delimited statement block.


#include <iostream> 
using namespace std; 
int main() { /*from  ww  w . java  2s  .co  m*/
     int n; 

     cout << "Enter numbers in the range 1-10 to find "; 
     cout << "my favorite number\n"; 
     do 
     { 
          cin >> n;       // execute body 
     } while (n != 7);   // then test 
     cout << "Yes, 7 is my favorite.\n" ; 
     return 0; 
}

The code above generates the following result.