difference between list comprehension and for loop

Question

There is a question with exactly the same title in stackoverflow but the problem is not what I want to ask. I was solving a leetcode problem and found there is a quite interesting difference between the list comprehension and for loop. Please compare the following two approaches.

Approach 1

set1 = [[]]
num = [1,2,3]
for n in num:
    for s in set1:
        set1 += [ s + [n] ]
print(set1)

Approach 2

set1 = [[]]
num = [1,2,3]
for n in num:
    set1 += [ s + [n] for s in set1]
print(set1)

Approach 1 hangs while Approach 2 does not and produces a correct result. The reason I think is that:
1) Approach 1 adds element to set1 for each member of set1. Thus, the for loop never ends because set1 list is ever growing.
2) Approach 2 updates set1 after all the elements in set1 are processed. Am I on the right track in understanding the difference between two approaches? Also, can I consider [ s + [n] for s in set1] as a list resulted from the following pseudo code?

tmp = []
for s in set1:
   tmp += [s + [n]]

Yes. That's pretty much it.

juanpa.arrivillaga
– juanpa.arrivillaga

2019-03-06 01:40:04 +00:00
Commented Mar 6, 2019 at 1:40 — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Mar 6, 2019 at 1:40

Alain T. · Accepted Answer · 2019-03-06 01:45:34Z

1

The problem is that you're modifying the list you're iterating on in the for s in set1 loop. This could be avoided by using copy() to ensure that you iterating on a separate instance of the list : for s in set1.copy(). you could also constrain the length of the loop: for s in set1[:len(set1)]

answered Mar 6, 2019 at 1:45

Alain T.

42.2k4 gold badges36 silver badges57 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

scriptboy · Accepted Answer · 2019-03-06 01:46:45Z

1

I think your two approaches are not equal.

The approach two is "extend set1 after compute the list". So the equivalent to approach 2 should like below:

set1 = [[]]
num = [1,2,3]
for n in num:
    tmp = []
    for s in set1:
        tmp += [ s + [n] ]
    set1 += tmp
print(set1)

Why you approach 1 hang? It's not hang, it's looping forever, because you extend your list while traverse it.

    for s in set1:
        set1 += [ s + [n] ]

Every time you get next element of set1, your set1 get longer.

It is really a bad idea to change the element while traverse it. Don't do that. Creating a middle variable like your pseudo code is more safe and clear.

answered Mar 6, 2019 at 1:46

scriptboy

8661 gold badge9 silver badges28 bronze badges

Collectives™ on Stack Overflow

difference between list comprehension and for loop

Approach 1

Approach 2

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

Approach 1

Approach 2

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related