If statement with no else branch while making array with for loop

Question

I want to create an array inline like this: [x if arr[arr.index(x)+1] < x for x in arr] but when i do it python tells me i cant do that without having an else branch to my if statement.

The only "solution" I found was this: list(filter(lambda x: x != None, [x if arr[arr.index(x)+1] < x else None for x in arr]) but that is just ugly, and unnessecary. The reason i want to initialize the array this way is to keep the code short and clean, but now it would just be better to do a normal for loop.

Hopefully there are better ways to do this ;)

`[x for x in arr if arr[arr.index(x)+1] < x]`. No idea if that actually makes sense to me, but the `if` condition comes at the end if it's not accompanied by an `else`. — roganjosh, Jan 02 '19 at 23:16
What if there are duplicate items in `arr`? `index` will only find the first. — Peter Wood, Jan 02 '19 at 23:18
Possible duplicate of [How does this "\[.. for .. in ..\]" work in Python?](https://stackoverflow.com/questions/11875364/how-does-this-for-in-work-in-python) — Eb946207, Jan 02 '19 at 23:18
Possible duplicate of [list comprehension vs. lambda + filter](https://stackoverflow.com/questions/3013449/list-comprehension-vs-lambda-filter) — Code-Apprentice, Jan 02 '19 at 23:19
Disagree with the dupes, I think this is better served by numpy syntax but we don't have enough context — roganjosh, Jan 02 '19 at 23:20
Are you actually using numpy arrays or just python lists here? "Arrays" 99% of the time refers to numpy — roganjosh, Jan 02 '19 at 23:22

score 2 · Accepted Answer · answered Jan 02 '19 at 23:17

2

Your if is in the wrong position to correctly implement filter() with a list comprehension. Try this instead:

[x for x in arr if arr[arr.index(x)+1] < x]

See list comprehension vs. lambda + filter for details.

answered Jan 02 '19 at 23:17

Code-Apprentice

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This crashes with `arr = [1, 3, 2, 4]` with `IndexError: list index out of range` – Peter Wood Jan 03 '19 at 17:05
@PeterWood Good point. I was only showing how to fix the syntax error from the OP's code. I didn't worry about testing if this fix reveals a logic error. I leave the fix as an exercise for the OP and any other reader. – Code-Apprentice Jan 03 '19 at 20:35

Peter Wood · Answer 2 · 2019-01-03T17:51:43.980

2

You can loop through pairs from the list:

[a for a, b in zip(arr, arr[1:])
 if a < b]

For example:

>>> arr = [1, 3, 2, 4]
>>> [a for a, b in zip(arr, arr[1:])
...  if a < b]
[1, 2]

The accepted answer crashes with these values:

>>> [x for x in arr if arr[arr.index(x)+1] < x]
Traceback (most recent call last):
IndexError: list index out of range

edited Jan 03 '19 at 17:51

answered Jan 02 '19 at 23:24

Peter Wood

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This is a very elegant solution that seems to almost match what the OP is doing. Note that this will eliminate the last element of the list because `zip()` terminates on the shortest of the input lists. This may not be the desired behavior. – Code-Apprentice Jan 03 '19 at 16:37
@Code-Apprentice The only way OP's code (and your fixed solution) wouldn't crash is if the last element was duplicated earlier in `arr`. But then the solution would be wrong as `index` would find the earlier value. – Peter Wood Jan 03 '19 at 17:04

If statement with no else branch while making array with for loop

2 Answers2