What is a real life application for a 'do while loop'

Question

I understand how does a do while loop works, it first runs the code once then check the condition.

What is the purpose of a 'do while loop' and is there any real life examples?

Thank you

For one thing, something you want to happen _at least once_. — 001, Aug 24 '17 at 11:50
but basically, to avoid repetition of code that would be needed if you want the instruction to happen at least once. See good example from chepner's answer, for instance. — Pac0, Aug 24 '17 at 11:57
Re: "If we know the condition is false, why do we still want to run it once?" : See https://stackoverflow.com/questions/2314066/do-whilefalse it's a way to write a goto without writing a goto — AndyG, Aug 24 '17 at 12:09

score 3 · Accepted Answer · answered Aug 24 '17 at 12:32

Basically the only difference between while and do-while is that while loops check the loop test condition before entering the loop and do-while loops check the loop test condition after the loop is executed, both effectively having the same result except that do-while guarantees at least one execution of the loop.

Do-while loops are sometimes useful if you want the code to output some sort of menu to a screen so that the menu is guaranteed to show once.

Example:

int data;

do
{
   cout << "Enter 0 to quit: ";
   cin >> data;
   cout << endl << endl;
} while (data != 0);

You can accomplish this same thing using just a while loop also. The only difference is that you must initialize the variable to a value that would not prevent the while loop from being entered.

int data = 1;

while (data != 0)
{
   cout << "Enter 0 to quit: ";
   cin >> data;
   cout << endl << endl;
}

score 2 · Answer 2 · answered Aug 24 '17 at 11:56

You pretty much answer your own question in the first line. However, it's instructive to look at the alternatives in a language like Python, that doesn't have a do-while loop. The main difference is that a while loop checks its condition before executing the body, while a do-while loop checks the condition after the body. Python's general syntax doesn't allow for the second, since there's no way (that I can really think of) to attach a line of code to the end of an indented block.

A typical example is verifying an input value, where you want to read the first value, verify it against some condition, then continue to ask for a new value if it fails the verification. In pseudo-Python, it might look like

do:
    x = input("Enter yes or no")
    while x not in ("yes", "no")

However, Python doesn't have any such construct, so you either repeat the input and use a while loop:

x = input("Yes or no")
while x not in ("yes", "no"):
    x = input("Yes or no")

or you write an "infinite" loop with an explicit if statement to break out:

while True:
    x = input("Yes or no")
    if x in ("yes", "no"):
        break

score 2 · Answer 3 · answered Aug 24 '17 at 11:57

2

Real World Example,

Go to the bath room:

DO { Check_Door_Lock(); } WHILE (WAIT_WHILE_DOOR_IS_LOCKED());

after the loop is done then the WAIT_WHILE_DOOR_IS_LOCKED() has returned a false value, so it isn't locked anymore, thus, the whole loop ends.

answered Aug 24 '17 at 11:57

Sanjay Kumaar

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What is a real life application for a 'do while loop'

3 Answers3