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I have the following test code in Python:

lst1 = ["1", "2", "3"]
lst2 = ["4", "5", "6"]
lst3 = ["7", "8", "9"]
numbers = [lst1, lst2, lst3]
for lst in numbers:
    lst = list(map(int, lst))
print (numbers)

What I'm trying to do is convert all the string variables in lst1, lst2 and lst3, which are all sublists inside the list numbers, into integers. I'm using the map() function to achieve this by iterating through each list and converting the list, but somehow, it's not working.

Now, I tried a different method using the following code:

numbers = [list(map(int, lst)) for lst in numbers]

For some reason, this works. Does anyone know why the former code doesn't work and the latter does? I'd just like to wrap my head around it so that I fully understand why it doesn't work.

Thanks!

JohnyNich
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  • hi, i found you are not doing anything in the first code similar to second code . In second code it is similar to iterating the whole list converting each element to int and adding it to list. But in first code you are totally ruining the concept where the number list is iterated and then user creating a new list assign it to a new variable then printing the original list . where lst will be override on each iteration i hope now you got where you lag .. – Midhun Mohan Oct 03 '17 at 06:01
  • you can added the data generated into a newlist using append method and now both looks simillar in code but second code will be much faster compared to first one if you have a large array – Midhun Mohan Oct 03 '17 at 06:02

5 Answers5

4
for lst in numbers:
    lst = list(map(int, lst))

creates a reference lst that iterates through numbers, then at each iteration you reassign that reference to a new list, this doesn't change the content of the original list.

This is the same behavior as

x = 4
y = x
y = 2

which doesn't change the value of x

Julien
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1

In the code you wrote the value is being updated in the lst variable an not in the numbers list. But the other code works because you have create a new array with the desired values and assigned it to numbers array itself. You could also do this - 

numbers = map(lambda x: map(int, x), numbers)
Sachin Biradar
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You are printing numbers which is original list of list instead of lstwhich is modified new list.
Try:

for lst in numbers:
    lst = list(map(int, lst))
    print (lst)



 >>> 
[1, 2, 3]
[4, 5, 6]
[7, 8, 9]
>>>
Anil_M
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This is about updating your numbers variable inside the loop (which simply does not work). I normally work around that doing this:

lst1 = ["1", "2", "3"]
lst2 = ["4", "5", "6"]
lst3 = ["7", "8", "9"]
numbers = [lst1, lst2, lst3]
for ind,lst in enumerate(numbers):
    numbers[ind] = list(map(int, lst))
print (numbers)

Also found this questions which appears to respond a similar problem. How to modify list entries during for loop?

Anton vBR
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List Comprehension would be faster in this case

integer_list = [int(i) for j in numbers for i in j]

print(integer_list)  # [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

timeit output:

100000 loops, best of 3: 3.5 µs per loop

You can also use generators

list_generator = (int(i) for j in numbers for i in j)
print(list(list_generator)  # [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

Timeit result:

The slowest run took 5.31 times longer than the fastest.

This could mean that an intermediate result is being cached. 1000000 loops, best of 3: 359 ns per loop

theBuzzyCoder
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