Going in loops?

The next set of control statements is loop statements. C family has 3 loop statements viz for, while and do .. while

for statement 

for is the most compact loop statement. Often it is used when the loop is to be repeated fixed number of times.

Syntax

for(expr1;expr2;expr3)

   statement;

expr1 is initialization. expr2 is condition and expr3 is increment. 

To start with expr1 is executed and next expr2 is evaluated. If expr2 is true the loop is repeated. If it is false, loop is terminated.

After each iteration, expr3 is executed - which increments or decrements the control variable.   After this, again expr2 is evaluated and loop is repeated as long expr2 is true. 

Now let us understand this with an example 

int a;
cin>>a;
for(int i=1;i<5;i++)
   cout<<a*i<<" ";

The output of the program is
 8 16 24 32
 if input is 8

Here i is defined and initialized with 1. Then is compared with 5. Since condition i<5 is true, the cout statement is executed - which print a*i = 8*1 = 8. Next i++ increments i to 2. i is compared with 5 and as condition i<5 is true, the cout is executed.

This loop is repeated till i becomes 5.

One nice thing about C++ for loop is, the control variable i can be defined within the statement. The life and scope of i is within the loop.

Let us look at another example - this time reading an array
   An array is a group of elements having a common name and identified by an integer index.

#include <iostream>
using namespace std;
int main()
{
  int arr[10];
  for(int i=0;i<10;i++)
  {

     int b = i*i*i;

     arr[i]=b;
   }
   int sum=0;
   for(int i=0;i<10;i++)
     sum+=arr[i];
   cout<<"Sum of array elements is "<<sum;
}

Note the following things

• arr is an array of integers.
• In the for loop array elements are initialized with i*i*i. That is arr[0] is 0, arr[1] is 1, arr[2] is 8, arr[3] is 3*3*3 - 27 and so on.
• In the second for loop, the elements of the array are added together into sum.
• We also notice that, in the first loop, instead of one statement in the body of the loop, we use a block - which is enclosed in braces. We can use a block of statements with the help of braces in all three types of loops or if -else statement. 

  inifinite loop anyone?

Do you want an infinite loop? Just write

for(;;)

   cout<<"hello";

 OK, now we know that the three expression in the for loop are optional. 

Now why don't you write a simple program using for loop? 

Write a program to read 10 elements of an integer array and print the sum of only positive integers.

Write a program to find the second largest element of an array.

while loop  

A while loop is quite straight forward. It repeatedly executes a statement (or a block of statements) while a condition is true.

Syntax 

while(expression)

   statement;

To start with expression is evaluated. Expression can be relational or even arithmetic. Remember that C and C++ use 0 as false and 1 as true. If the expression is true, statement is executed. Then again expression is evaluated. This is repeated as long as expression is true. Once it is false, loop is terminated. 

Let us see an example. 

int arr[10];
int i=0;
while(i<10)
  {  
    arr[i]=i*i*i;
    i++;
  }
int sum=0;
for(int i=0;i<10;i++)
    sum+=arr[i];
cout<<"Sum of array elements is "<<sum;

We have re-written the same example, using while for the first loop. Now do you agree that for loop is more suited for this example? :)

break or continue ?

There are situations where we want to terminate the loop abruptly. Some times we may want to skip the current iteration. For these, we can  use break and continue.

break : 

break will terminate the loop. By loop we mean any of the three - for, while or do -while. 

#include <iostream>
using namespace std;
int main()
{ 
int i;
while(true)
  {  
    int m;
    cout<<"Enter a number (-1 to terminate)";
    cin>>m;
    if(m==-1)
      break;
    if(m%2) 
       cout<<"Even"<<endl;
    else
       cout<<"Odd"<<endl; 
    i++;
  }  
}

Here we are reading numbers from console infinitely - we use while(true) . When -1 is entered, we break the loop.

Output of this program will be

Enter a number (-1 to terminate)22 

Even
Enter a number (-1 to terminate)7

Odd
Enter a number (-1 to terminate)678

Even
Enter a number (-1 to terminate)99

Odd
Enter a number (-1 to terminate)-1

do .. while loop

A do while loop is similar to a while loop, except for the fact that condition is tested at the end of loop.  

A for and while loops are entry controlled, but a do ..while loop is exit controlled. So this loop executes its body at least once.

Syntax
do
   statement;
while(expression);

Here the statement is executed. Then the expression is tested. If the expression is true, the loop is repeated.

Let us write an example.  

#include <iostream>
using namespace std;
int main()
{ 
do
  {  
    int m;
    cout<<"Enter a number (-1 to terminate)";
    cin>>m;     
    if(m!=-1 && m%2) 
       cout<<"Even"<<endl;
    else
       cout<<"Odd"<<endl; 
  } while(m!=-1);
}

Here we read numbers and print if the number is even or odd as long as number entered is -1. As while(expression) is executed after the block, we are sure that the body of the loop gets executed at least once.

do while loop is called repeat until in some languages.