Python: Most efficient loop until condition

Question

I have got to a point in my code where I need to do this:

while True:
  if first_number == second_number:
    do_something()
    break

The first number will keep incrementing beyond my control and I need to wait until this number reaches the value of the second number before I can continue.

I am looking for the best way to do this that will use up the least amount of resources (CPU).

Skip the loop and execute `first_number = second_number`? – chepner Mar 02 '14 at 16:36 — chepner, Mar 02 '14 at 16:36

score 4 · Accepted Answer · answered Mar 02 '14 at 16:55

4

What you are doing is called "busy waiting". In order to do this without eating up CPU time, you should sleep for a short interval in each iteration of the loop:

while True:
  if first_number == second_number:
    do_something()
    break
  time.sleep(0.1)

Adjust the sleep interval as appropriate.

But there's likely a better way to accomplish what you want. How is first_number changing value if it's "out of your control"? Is your program receiving input? Reading from a socket? Are you sending a network request for data? Is this data shared among processes using multiprocessing's facilities?

answered Mar 02 '14 at 16:55

Alp

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This is one of the solutions that I've looked at but wanted to see if there were any others. – A_Porcupine Mar 02 '14 at 16:58
The actual problem I'm trying to solve currently is: first_number is current time position through a song and second_number is the total duration of the song. I need to run some code when the song finishes but the built-in function to deal with this doesn't seem to be working (another question) and I was wondering about this anyway. – A_Porcupine Mar 02 '14 at 17:01
1

Still hard to recommend alternatives without knowing more about the use case (is the song playing in a subprocess? in a separate thread via some library function?). In any case, use of `sleep` for this purpose is entirely standard. – Alp Mar 02 '14 at 17:08
Is there any way you can make the thing asynchronous? Might take the load of needing the loop in your user code. – Will Mar 02 '14 at 17:12

score 0 · Answer 2 · answered Mar 02 '14 at 16:36

0

Simply check if first number is not equal to the second number, in the while loop's condition itself.

while first_number != second_number:

answered Mar 02 '14 at 16:36

thefourtheye

233,700
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But won't this run the CPU at 100%? – A_Porcupine Mar 02 '14 at 16:38
The first number is beyond my control and will keep incrementing, I need to do something in my code once this reaches the value of the second number. – A_Porcupine Mar 02 '14 at 16:42
@A_Porcupine If the difference between `first_number` and `second_number` is too big, this is the best you could do :) – thefourtheye Mar 02 '14 at 16:51

score 0 · Answer 3 · answered Mar 02 '14 at 16:36

0

You can try:

while first_number != second_number:
    #Do what you want

answered Mar 02 '14 at 16:36

Ruben Bermudez

2,293
14
22

Python: Most efficient loop until condition

3 Answers3