How to split a list created through several iterations of a for loop in python

Question

I am creating a list in a for loop in python but I have no idea how to separate results of each itertaion. For example I want to keep the even numbers from 0 to 10 two times and I want to separate the results. I tried the following code:

res=[]
for i in range (2):
    for j in range (10):
        if j%2==0:
            res.append(j)

But res is

[0, 2, 4, 6, 8, 0, 2, 4, 6, 8]

And I want to have it as:

[[0, 2, 4, 6, 8], [0, 2, 4, 6, 8]]

The point is that in reality results of my iteration have not the same length and I can not simply split my res based on the number of iteration. I do appreciate if anyone help me to do it.

append new empty sublist for every i before the inner loop — buran
– buran, Commented Mar 10, 2021 at 15:03

tbjorch · Accepted Answer · 2021-03-10 15:13:37Z

2

You have to create a list in the first loop level which you then append to the res list to achieve that. Below is your code modified to minimum to achieve what you want.

res=[]
for i in range (2):
    temp = []
    for j in range (10):
        if j%2==0:
            temp.append(j)
    res.append(temp)

alt. use a list comprehension. Note that the range function also has a third argument allowing you to set the step size, which if set to 2 removes the need to check j%2 == 0.

[list(range(0,9,2)) for i in range(2)]

Both of these creates the list [[0, 2, 4, 6, 8], [0, 2, 4, 6, 8]]

edited Mar 10, 2021 at 15:13

answered Mar 10, 2021 at 15:03

tbjorch

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Comments

Yehla · Accepted Answer · 2021-03-10 15:04:24Z

1

How about this?

res=[[],[]]
for i in range (2):
    for j in range (10):
        if j%2==0:
            res[i].append(j)

answered Mar 10, 2021 at 15:04

Yehla

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1 Comment

Justin Over a year ago

This solution is only limited to a range = 2. Check @tbjorch's answer, which works for values > 2

Amir Bahrami · Accepted Answer · 2021-03-10 15:06:50Z

1

Use a generator for each up itrate:

def gen(x):
    for i in range(x):
        if i % 2 == 0 :
            yield i

Then:

array = []
for i in range(2):
     array.append(list(gen(10)))

answered Mar 10, 2021 at 15:06

Amir Bahrami

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2 Comments

tbjorch Over a year ago

the gen function it not necessary since range has the step size argument. This would achieve the same: array.append(list(range(0,9,2)))

Amir Bahrami Over a year ago

Yes, but i meant to mention a maybe new approach @tbjorch

Zeinab Mardi · Accepted Answer · 2021-03-10 15:13:17Z

1

a = [[i for i in range(0,10,2)]]*2
print(a)
[[0, 2, 4, 6, 8], [0, 2, 4, 6, 8]]

answered Mar 10, 2021 at 15:13

Zeinab Mardi

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