Do-while Loop in C++

June 12, 2020

Do while Loop in C++

In Computer Science do-while loop executes the loop body at least once irrespective of the condition and then executes body based on the condition
Definition: In a do-while first the loop body is executed and then the test condition is checked as a result one time execution of loop body is assured even if the condition is false for the first time
These type of loops are called Exit control or post test loop

Example Program to Print numbers from 1-10 using a do-while loop

main()
{
int i=0;//initialisation expression
do//loop_body
{
cout<<"\t"<<i;
i++;//update expression
}
while(i<=10);//test condition
cout<<"\n control reached here";
}

O/P

0 1 2 3 4 5 6 7 8 9 10
control reached here

loop executed for 11 times, 1 time without checking the condition and 10 times (true)with checking condition
the test condition is evaluated 11 times, it is true for the first 10 times (0<1 to 11<=11) and to becomes false and break the loop at 12 times i.e 12<11(false)

Execution Flow

Loop body i.e the do block is executed first without checking any condition then the test condition is checked
If it is true: control goes to do block and again executes the loop body
Again check the test condition and execute do block
Repeat above 2 steps until the condition becomes false
If it is false: control goes to the next line after the test condition

While V/s Do while

Do-while-Loop

Post Test Loop: Execute the loop body and then check the condition
In a do- while loop even if the test condition is false for the first time itself then loop body will execute at least once

main()
{

do//loop_body
{
cout<<"trishaank";
}
while(false);//test condition false at first time
}

O/P

trishaank

While Loop

Pretest Loop: Check the condition first then execute the loop body
In a while loop if the test condition is false for the first time itself then loop body will not execute at least once

main()
{

while(false)//control doesn't  go inside 
{
cout<<"trishaank";
}

}

O/P

Empty screen

C++ program to find the sum of digits of a given number using a do-while loop

My intention here is to write a sum of digits program within a single line

main()
{
int n,rem=0,sum=0;
cout<<"enter a num:";
cin>>n;

do//loop body
{
sum=sum+(rem=n%10);//sum of digtits program in a single line
}

while(n=n/10);//any number other than 0 is considered as true
cout<<"\nsum is:"<<sum;
}

O/P

enter a number
123
sum is 6

C++ program to print alphabets from A-Z with their ASCII codes

main()
{

char i='A';//inialisation

do
{
cout<<i<<":"<<(int)i<<"\n";
i++;//ascii codes are incremented
}

while(i<='Z');//internally ascicodes are compared 
}

O/P

A:65
B:66
C:67
D:68
E:69
F:70
G:71
H:72
I:73
J:74
K:75
L:76
M:77
N:78
O:79
P:80
Q:81
R:82
S:83
T:84
U:85
V:86
W:87
X:88
Y:89
Z:90

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