In Python list comprehension is it possible to access the item index?
PythonListList ComprehensionPython Problem Overview
Consider the following Python code with which I add in a new list2 all the items with indices from 1 to 3 of list1:
for ind, obj in enumerate(list1):
if 4 > ind > 0:
list2.append(obj)
How would you write this using list comprehension, if I have no access to the indices through enumerate?
something like:
list2 = [x for x in list1 if 4 > ind > 0]
but since I have no ind number, would this work?
list2 = [x for x in enumerate(list1) if 4 > ind > 0]
Python Solutions
Solution 1 - Python
list2 = [x for ind, x in enumerate(list1) if 4 > ind > 0]
Solution 2 - Python
If you use enumerate, you do have access to the index:
list2 = [x for ind, x in enumerate(list1) if 4>ind>0]
Solution 3 - Python
Unless your real use case is more complicated, you should just use a list slice as suggested by @wim
>>> list1 = ['zero', 'one', 'two', 'three', 'four', 'five', 'six']
>>> [x for ind, x in enumerate(list1) if 4 > ind > 0]
['one', 'two', 'three']
>>> list1[1:4]
['one', 'two', 'three']
For more complicated cases - if you don't actually need the index - it's clearer to iterate over a slice or an islice
list2 = [x*2 for x in list1[1:4]]
or
from itertools import islice
list2 = [x*2 for x in islice(list1, 1, 4)]
For small slices, the simple list1[1:4]. If the slices can get quite large it may be better to use an islice to avoid copying the memory
Solution 4 - Python
For those wondering, you can just as easily store the matching indices as the result of the list comprehension if your use-case requires it. It may be that you need to evaluate the value of each item in the list to determine if it matches some value or pattern, and that you just want to obtain a list of the matching indices for later processing rather than the matching values.
This can be done as shown below:
>>> values = ['zero', 'one', 'two', 'three', 'four', 'five', 'six']
>>> search = ['one', 'three', 'five']
>>> matches = [i for i, x in enumerate(values) if x in search]
>>> print(matches)
[1, 3, 5]
The important part above is the use of the [i for i, x in enumerate(l) if ...] pattern in the list comprehension, where the index i is being captured rather than the value x which would be achieved using the more typical [x for x in l if ...] or the alternate [x for i, x in enumerate(l) if ...] enumeration list comprehension patterns.