Start Python 'while' loop before condition is defined

Question

I've got a piece of code that executes a function, and then based on that output decides if it should reiterate:

while (function_output) > tolerance:
    function_output = function(x)

The problem is that the while loop won't start until "function_output" is defined - but it's defined in the loop. Currenly I've got:

function_output = function(x)
while (function_output) > tolerance:
    function_output = function(x)

but is there a way to get this loop to start without having to iterate the function once already?

Emulate a `do while`? `while True: stuff() if fail_condition: break` — Robert Harvey, Nov 23 '18 at 16:36
@RobertHarvey of course, you can also just do what to OP is doing already — juanpa.arrivillaga, Nov 23 '18 at 16:37
@TIF: For what it's worth, your second block of code is the canonical way to do it. — Robert Harvey, Nov 23 '18 at 16:38
The OP is correct, in thinking that the 2nd way is bad: It has repeated code. — ctrl-alt-delor, Nov 23 '18 at 16:41
@ctrl-alt-delor meh. I find it better than doing an extra comparison each iteration. In any event, it's sort of a common idiom in python to emulate a do-while — juanpa.arrivillaga, Nov 23 '18 at 16:43
Thanks, I didn't know a "do-while" loop was would I should have been looking for! — TIF, Nov 23 '18 at 17:05
@juanpa.arrivillaga I don't see any extra comparisons, anywhere. — ctrl-alt-delor, Nov 23 '18 at 17:09
@ctrl-alt-delor the while-statement comparison and the `if` statement inside the while-loop body. Now, I believe the CPython may optimize `while True:` specifically. Not sure. — juanpa.arrivillaga, Nov 23 '18 at 17:10

blue_note · Accepted Answer · 2018-11-23T16:41:24.593

2

There is no such thing in python. Neither a do-while, nor a

while (x = f() > 5):
  dostuff

like in C and similar languages.

Similar constructs have been proposed, but rejected. What you are already doing is the best way to do it.

On the other hand, if you want to do it in a do-while style, the proposed way is

while True:
    if f() > tolerance:
       break

edited Nov 23 '18 at 16:41

answered Nov 23 '18 at 16:40

blue_note

1

[soon](https://stackoverflow.com/a/52722801/3620003) – timgeb Nov 23 '18 at 16:49
@timgeb: useful (though a bit ugly looking). Let's hope for a `do-while` too... – blue_note Nov 23 '18 at 16:52

Tobey · Answer 2 · 2018-11-23T16:50:52.333

1

Use a break statement to escape from a loop

while True:
    if function_output(x) > tolerance:
        break

edited Nov 23 '18 at 16:50

answered Nov 23 '18 at 16:38

Tobey

2

I think your condition is inverted - that was the condition to continue the loop in the OPs code – SpoonMeiser Nov 23 '18 at 16:40

score 0 · Answer 3 · answered Nov 23 '18 at 16:39

0

How about this pattern. You may want to choose a better variable name.

should_continue = True
while should_continue:
    should_continue = ( function(x) > tolerance )

answered Nov 23 '18 at 16:39

ctrl-alt-delor

While this is certainly an alternative, it isn't much better than OP's implementation. – r.ook Nov 23 '18 at 16:41
1

@Idlehands is has less repeated code. – ctrl-alt-delor Nov 23 '18 at 16:42
1

@ctrl-alt-delor, What's repeated, a function call? If I had to weight up a repeated function call versus creating a new variable, I know which I'd prefer. – jpp Nov 23 '18 at 16:43
@Jpp I agree if just a function call, but if there are lots or arguments, the I would do it this way. Or encapsulate. – ctrl-alt-delor Nov 23 '18 at 17:07

3 Answers3