Which is more efficient? A nested if in a for loop or a separate if with a separate for loop?

Question

So I'm trying to decide which of these is more efficient, or if there is a difference between them. The program I'm writing iterates through a for loop (python) and does some stuff, and then, depending on a flag, will write to a file.

Example A:

for element in list:
    Do stuff
    if(write_to_file):
       write to file

Vs example B:

for element in list:
    Do stuff
if(write_to_file):
    for element in list:
        write to file

In the case of A it has to check every time if it's true, but in the case of B if it is true it has to then re-do the for loop. My thought is that they're equal, but I'd like opinions of more experienced programmers

You are writing to a file, ergo, you are worrying over something that'll fall into nothingness compared to the time the I/O takes. :-) — Martijn Pieters
– Martijn Pieters, Commented May 22, 2014 at 13:36
Use timeit and find out: docs.python.org/2/library/timeit.html — kylieCatt
– kylieCatt, Commented May 22, 2014 at 13:37
@IanAuld, Henry Keiter: Thanks, I didn't know that function existed. Will do — Mitch
– Mitch, Commented May 22, 2014 at 13:42

James Mills · Accepted Answer · 2014-05-22 13:50:09Z

2

Based on the Big O complexity analysis of your example(s):

for element in list:
    Do stuff
if(write_to_file):
    for element in list:
        write to file

The above is less efficient because you are iterating over list twice.

i.e: O(n) x 2

But please test yourself, use the timeit module and most importantly of all you are worrying prematurely about optimization. get it working first!

6 Comments

chepner Over a year ago

If write_to_file is false, this is more efficient, because there's only a single loop iteration and the condition is checked only once, rather than each time through the loop.

James Mills Over a year ago

Except for when it's True. Yes it all depends. The OP has not provided any specifics so this is the best answer I can provide based on what I see :)

Mitch Over a year ago

@JamesMills, it's based on user input so completely random. Based on your input and the input in the comments on the OP I tried using timeit on it: Example A: Write_to_file = false: 18.05 ms write_to_file = true: 18.60 ms Ex B: Write to file = true:19.4 ms Writeto file = false: 18.27 ms Not much difference. And don't worry, it was working before I asked!

Benjamin Hodgson Over a year ago

@musher: user input is anything but completely random!

Mitch Over a year ago

@BenjaminHodgson I'm actually curious about this - what do you mean?

|

rbutcher · Accepted Answer · 2014-05-22 18:28:09Z

1

Example C:

if(write_to_file):
    for element in list:
        Do stuff
        write to file
else:
    for element in list:
        Do stuff

As others have said, you will probably not be able notice the difference between any of these approaches.

answered May 22, 2014 at 18:28

rbutcher

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