CODEWITHSUNDEEP
pythonpython-3.xlistinteger
ANURAG VERMA
ANURAG VERMA

Reputation: 55

Unable to convert output to list

How to create a function that takes a list of non-negative integers and strings and return a new list without the strings?

Eg. filter_list([1, 2, "a", "b"]) ➞ [1, 2]

Issue: Not able to convert output to list

def filter_list(a):
    mylist=[]
    if type(a)==list:
        for i in a:
            if type(i)==int:
                output=print(i)
        mylist+=output
        return mylist

What am I doing wrong?

Upvotes: 4

Views: 826

Answers (6)

solid.py
solid.py

Reputation: 2812

You could use a list comprehension which only includes list elements if they have the int type.

def filter_list(lis):
    return [i for i in lis if type(i) == int]

print(filter_list([1, 2, "a", "b"]))

Code Output:

[1, 2]

Upvotes: 2

ubaid shaikh
ubaid shaikh

Reputation: 453

You can use the filter method as follows:

def filter_list(a):
    return list(filter(lambda x: type(x) == int, a))

print(filter_list([1, 2, "a", "b"]))

Output:

[1, 2]

Your code fixed:

def filter_list(a):
    mylist=[]
    for i in a:
        if type(i)==int:
            mylist.append(i)
    return mylist

def filter_list1(a):
    return list(filter(lambda x: type(x) == int, a))

print(filter_list([1, 2, "a", "b"]), filter_list1([1, 2, "a", "b"]))

Output:

[1, 2] [1, 2]

Upvotes: 1

ProgrammingEnthusiast
ProgrammingEnthusiast

Reputation: 304

The mistake you made is that you are assigning the value returned by print() to output (which is None) and then you are adding a None type object to a list object which is an invalid operation. You can fix your code by doing something like this :

def filter_list(a):
    mylist=[]
    if type(a)==list:
        for i in a:
            if type(i)==int:
                print(i)
                mylist+=[i]
        return mylist

However this can be done in a much prettier way using List comprehensions!
List comprehension is a way to create a new list using a relatively much shorter syntax. 

[elem for elem in _list if type(elem) is int]

Upvotes: 2

Jarvis
Jarvis

Reputation: 8564

Use the filter function itself:

>>> list(filter(lambda x: isinstance(x, int), [1, 2, "a", "b"]))
[1, 2]

Issue with your code is that instead of printing and using += (which is used to concatenate two lists, not one list and one int) to add to your list, you should append the element like this:

for i in a:
    if type(i) == int:
        mylist.append(i)

Upvotes: 1

Timur Shtatland
Timur Shtatland

Reputation: 12347

Use isinstance and list comprehension:

def filter_list(lst):
    return [i for i in lst if isinstance(i, int)]

Upvotes: 1

Daniel Hao
Daniel Hao

Reputation: 4980

Can this help:

>>> def filter_list(lst):
    return [item for item in lst if type(item) is not str]

>>> filter_list([1, 2, 'a', 'b'])
[1, 2]
>>>

Upvotes: 2

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