do-while loop in C++ with example

as the loop name is – do-while meaning “First do it, then check”.

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C++ do-while loop

a do-while loop is different from the other loops (for loop, while loop), initially, the variable is initialized, then the body of loop is executed, then the condition is checked.

a do-while loop is always executed whether the condition is true or false whereas the other two The first condition is checked in the loop and then the body of loop is executed.

The syntax is given below.

initialization;
 do {
     body of loop;
     updation;
  }
    while (expression);

flow-diagram of a do-while loop will as follows,

do-while loop flow diagram in C++

Example of do-while loop in C++

In the above program, the condition is given that loop-body only execute when user-given value less than 5.

#include<iostream.h>
using namespace std; 
 
int main()
 {

   int num;

   cout<<"Enter number less than 5:";
   cin>>num;
 
   int x=num; //5
   do{
       cout<<x<<".statement executed\n"; // body of loop
       x++;
    }
      while(x<5);
  return 0;
 }

OUTPUT

Here we will execute the above program 2 times, in the 1st execution user given value is num = 1; then, the output will be as

Enter number: 1
1.statement executed
2.statement executed
3.statement executed
4.statement executed

because condition becomes True (1<5) so it will execute 4th times as we can see above, Program will execute as follows,

body of loop	value increased by 1 , x++;	Expression/condition
1.statement executed	2	2 < 5 (True)
2.statement executed	3	3 < 5 (True)
3.statement executed	4	4 < 5 (True)
4.statement executed	5	5 < 5 (False)

but now in the 2nd Execution, this time user given value is num = 6; then,

6.statement executed

this times condition was becoming False (6<5) but body of loop still executed, this will execute follows,

body of loop	increment in initialization value x = 6	Expression/condition
6.statement executed	6	6 < 5 (False)

Why did this happen?

Because in the do-while loop first value, initialized then body of loop executed and in the last condition is checked.

a do-while loop will work as follows,

repetition of the above program will as follows,

Difference between while and do-while loop in c++

In the first while-loop, the first condition is checked, then the body of loop is executed, then the initialize value of the loop is incremented, whereas, in the do-while loop, the first body of loop is executed, then the condition is checked Then there is an increment in the value in the Last.

in the while-loop

   initialization  –    condition – body of loop – increment

in the do-while loop

  initialization –  body of loop – increment   –   condition 

do-while loop with switch statement in C++

as we know in the do-while loop first execution will be done, then after condition will be checked.  creating a menu wizard is a good example of the do-while loop.

In the below Program, First Do, meaning that first display the menu then check the condition if the condition becomes true then execute.

We have conditioned that if the value given by the user matches the serial no[1,2,3,0] of the options so only that option is executed.

#include<iostream>
#include<stdlib>  //exit()
using namespace std; 
int main()
  {
    int select;
    int num1,num2,result;

 do {
     
     cout<<"1.Sum \n";
     cout<<"2.Subtraction\n";
     cout<<"3.Multiply\n";
     cout<<"0.Exit";

     cout<<"\nEnter Your Choice: ";
     cin>>select;

     switch(select)
      {
          case 1:
                  cout<<"\nEnter Two Number: ";
                  cin>>num1>>num2;
                  result = num1+num2;
                  cout<<"\nTotal: "<<result;
                  getch();
                  break;
         case 2:
                  cout<<"\nEnter Two Number: ";
                  cin>>num1>>num2;
                  result = num1-num2;
                  cout<<"\nSubstraction: "<<result;
                  getch();
                  break;
        case 3:
                  cout<<"\nEnter Two Number: ";
                  cin>>num1>>num2;
                  result = num1*num2;
                  cout<<"\nMultiply: "<<result;
                  getch();
                  break;
        case 0:   
                  exit(0);
 
         default: cout<<"\ninvalid selection....";
                  return 0;
      }
   }while(select!=0);
 }

OUTPUT

1.Sum
2.Subtraction 
3.Multiply
0.Exit 
       
Enter Your Choice:

The program will execute until the condition becomes false (select==0), note that the exit() statement will terminate the program while do-while loop will display the menu until the user does not select the Exit option.

what will happen if the do-while loop is not used there, the answer is- Program will execute only a single option so there is no need to exit() statement because, after a single execution, the program will terminate automatically.

here is the output, of without using do-while loop

Note: always avoid to write the same statement multiple times int program, so here in the above program, will not be considered as good programming because of the following statement

cout<<"Enter number:";
cin>>num1>>num2;

which is repeating in every case statement, use function with switch statement in C++ to avoid this.

Related Exercise

• Printing only even number using do-while loop
• a simple menu demonstration of two number operation using switch and do – while loop

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