Python Lists Wrong Output

Question

Below I have the code. At ex2 and ex3, lst[2] and lst[3] show the different output. For example:

At ex3, lst[2] shows the output of 5 which is correct but
at ex2, lst[2] shows the output of 4 which is not correct because lst[2] should be added by 2, not by one

Why is that? What did I type wrong? I am new to python so any help would be nice. Thanks

def add_indexes(lst):
    for x in range(len(lst)):
        if lst[x] == lst[0]:
            lst[x] += 0

        elif lst[x] == lst[1]:
            lst[x] += 1

        elif lst[x] == lst[2]:
            lst[x] += 2

        elif lst[x] == lst[3]:
            lst[x] += 3

        elif lst[x] == lst[4]:
            lst[x] += 4

    return lst

ex1 = [0, 0, 0, 0, 0]
ex2 = [1, 2, 3, 4, 5]
ex3 = [5, 4, 3, 2, 1]
print(add_indexes(ex1))
print(add_indexes(ex2))
print(add_indexes(ex3))

I don't understand what the code is supposed to do or why the result is supposed to be meaningful. — Karl Knechtel
– Karl Knechtel, Commented Jul 17, 2020 at 9:59
I have a task which requires: "Given a list of numbers, create a function which returns the list but with each element's index in the list added to itself. This means you add 0 to the number at index 0, add 1 to the number at index 1, etc..." — Arimmjow
– Arimmjow, Commented Jul 17, 2020 at 10:01
@Arimmjow: Your code does not do what you described. You don't need to do any tests (if statements). Just a loop over the range of the list, and add lst[x] += x. — Aziz
– Aziz, Commented Jul 17, 2020 at 10:04

CanciuCostin · Accepted Answer · 2020-07-17 10:12:56Z

1

The reason is that you are first updating the values for lst[1] from 2 to 3, meaning that at the next iteration lst[1] == lst[2] will apply instead.

Starting

lst = [1,2,3,4,5]

Iteration 0:

x=0 => nothing changes

Iteration 1:

x=1 => lst[1] == lst[1] => lst[1]+=1 => lst[1] =3

Iteration 2:

x=2 => lst[2] == lst[1] => lst[2]+=1 => lst[2] =3

For your requirement you can simply use list comprehension:

ex2 = [1, 2, 3, 4, 5]
result = [x+ind for ind, x in enumerate(ex2)]

Output:

>>> result
[1, 3, 5, 7, 9]

edited Jul 17, 2020 at 10:12

answered Jul 17, 2020 at 10:06

CanciuCostin

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Comments

rozumir · Accepted Answer · 2020-07-17 10:08:56Z

0

That's because you increase the list on the fly, after 2 loops you have something like that:

lst = [1,3,3,4,5]

so condition:

elif lst[x] == lst[1]:
    lst[x] += 1

gets real and you increase 3 by 1 instead of increasing by 2. Try adding operation on a copy of the list or on an empty list and append items.

answered Jul 17, 2020 at 10:08

rozumir

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Comments

Aziz · Accepted Answer · 2020-07-17 10:09:18Z

0

From the OP's comment:

Given a list of numbers, create a function which returns the list but with each element's index in the list added to itself. This means you add 0 to the number at index 0, add 1 to the number at index 1, etc...

The function can be as simple as this:

def add_indexes(lst):
    for x in range(len(lst)):
        if lst[x] += x:
    return lst

answered Jul 17, 2020 at 10:09

Aziz

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Comments

Sai · Accepted Answer · 2020-07-17 10:13:24Z

0

Mistake:

You are changing the values iteratively, so when x = 2 already lst[1] is 3, so it's adding only 1 to lst[2]

I think you're trying to add numbers based on their position in the list. You can simply do this:

ex1 = [0, 0, 0, 0, 0]
for i in range(len(ex1)):
    ex1[i] += i
print(ex1)

output:

[0, 1, 2, 3, 4]

edited Jul 17, 2020 at 10:13

answered Jul 17, 2020 at 10:07

Sai

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Comments

Akshay Sehgal · Accepted Answer · 2020-07-17 10:15:09Z

0

I would recommend doing this another way -

[np.sum(i) for i in enumerate(ex2)]

[1, 4, 6, 9, 13]

[np.sum(i) for i in enumerate(ex2)]

[5, 6, 7, 8, 9]

edited Jul 17, 2020 at 10:15

answered Jul 17, 2020 at 10:06

Akshay Sehgal

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Comments

AAEM · Accepted Answer · 2020-07-17 10:56:25Z

I see a lot of correct answers here,but I wanted to point out the mistake you have made.

From what I could see ,you're trying the check for each individual case of the index in your list and adding the corresponding index to the value stored at that particular list. Considering from this point of view ,the "if" condition you have choose:

if lst[x] == lst[0]:
elif lst[x] == lst[1]:
elif lst[x] == lst[2]:
elif lst[x] == lst[3]:
elif lst[x] == lst[4]:

This checks if the value at the current index is equal to the value at each indices 0,1,2,3,4. This makes the list dynamic as explained by @CanciuCostin.

The right way to do it will look something like this:

   for x in range(len(lst)):
        if x == 0:
            lst[x] += 0

        elif x == 1:
            lst[x] += 1

        elif x == 2:
            lst[x] += 2

        elif x == 3:
            lst[x] += 3

        elif x == 4:
            lst[x] += 4

But this is an inefficient way.Thanks to @Sai Sreenivas for his improved efficient code to tackle your problem.

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