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pythonloopsdo-while
Bahubali Patil
Bahubali Patil

Reputation: 1135

Why there is no do while loop in python

Why doesn't Python have a 'do while' loop like many other programming language, such as C?

Example : In the C we have do while loop as below : 

do {
   statement(s);
} while( condition );

Upvotes: 49

Views: 52498

Answers (2)

Caleb Fuller
Caleb Fuller

Reputation: 229

Because everyone is looking at it wrong. You don't want DO ... WHILE you want DO ... UNTIL.

If the intitial condition is true, a WHILE loop is probably what you want. The alternative is not a REPEAT ... WHILE loop, it's a REPEAT ... UNTIL loop. The initial condition starts out false, and then the loop repeats until it's true.

The obvious syntax would be

repeat until (false condition):
  code
  code

But for some reason this flies over everyone's heads.

Upvotes: 9

Martijn Pieters
Martijn Pieters

Reputation: 1122122

There is no do...while loop because there is no nice way to define one that fits in the statement: indented block pattern used by every other Python compound statement. As such proposals to add such syntax have never reached agreement.

Nor is there really any need to have such a construct, not when you can just do:

while True:
    # statement(s)
    if not condition:
        break

and have the exact same effect as a C do { .. } while condition loop.

See PEP 315 -- Enhanced While Loop:

Rejected [...] because no syntax emerged that could compete with the following form:

    while True:
        <setup code>
        if not <condition>:
            break
        <loop body>

A syntax alternative to the one proposed in the PEP was found for a basic do-while loop but it gained little support because the condition was at the top:

    do ... while <cond>:
        <loop body>

or, as Guido van Rossum put it:

Please reject the PEP. More variations along these lines won't make the language more elegant or easier to learn. They'd just save a few hasty folks some typing while making others who have to read/maintain their code wonder what it means.

Upvotes: 84

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