Apply function to each element of a list
PythonListFunctionPython Problem Overview
How do I apply a function to the list of variable inputs?
For e.g. the filter function returns true values but not the actual output of the function.
from string import upper
mylis=['this is test', 'another test']
filter(upper, mylis)
['this is test', 'another test']
The expected output is :
['THIS IS TEST', 'ANOTHER TEST']
I know upper is built-in. This is just an example.
Python Solutions
Solution 1 - Python
I think you mean to use map instead of filter:
>>> from string import upper
>>> mylis=['this is test', 'another test']
>>> map(upper, mylis)
['THIS IS TEST', 'ANOTHER TEST']
Even simpler, you could use str.upper instead of importing from string (thanks to @alecxe):
>>> map(str.upper, mylis)
['THIS IS TEST', 'ANOTHER TEST']
In Python 2.x, map constructs a new list by applying a given function to every element in a list. filter constructs a new list by restricting to elements that evaluate to True with a given function.
In Python 3.x, map and filter construct iterators instead of lists, so if you are using Python 3.x and require a list the list comprehension approach would be better suited.
Solution 2 - Python
Or, alternatively, you can take a list comprehension approach:
>>> mylis = ['this is test', 'another test']
>>> [item.upper() for item in mylis]
['THIS IS TEST', 'ANOTHER TEST']
Solution 3 - Python
Sometimes you need to apply a function to the members of a list in place. The following code worked for me:
>>> def func(a, i):
... a[i] = a[i].lower()
>>> a = ['TEST', 'TEXT']
>>> list(map(lambda i:func(a, i), range(0, len(a))))
[None, None]
>>> print(a)
['test', 'text']
Please note, the output of map() is passed to the list constructor to ensure the list is converted in Python 3. The returned list filled with None values should be ignored, since our purpose was to convert list a in place