Saving index values with non-zero elements in Python

Question

I have a one-dimensional array containing values and I'm trying to use a for loop to identify the index values associated with non-zero elements.

For the code below, non_zero_elements should contain the values 0, 1, and 4, but what I am getting is [1, 0, 0, 4, 0].

I tried referencing a similar thread (Finding Non-Zero Values/Indexes in Numpy) but couldn't identify the bug in my code.

a = [1,2,0,0,4,0]
non_zero_elements = []
i = 0
for i in a:
    if a[i] != 0:
        non_zero_elements.append(i)
        print('The value',a[i],'in index',i,'is a non-zero element.')
        i = i + 1
print('Non-zero elements: ',non_zero_elements)

Nick · Accepted Answer · 2020-09-08 04:05:33Z

5

An easy way to solve this is with a list comprehension over an enumerate, which gives you the index and the value, allowing you to filter on non-zero values:

a = [1,2,0,0,4,0]
non_zero_elements = [i for i, v in enumerate(a) if v != 0]
print(non_zero_elements)

Output:

[0, 1, 4]

answered Sep 8, 2020 at 4:05

Nick

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Spencer Tibbitts · Accepted Answer · 2020-09-08 04:05:43Z

1

I think the way you were trying to do it, you meant to iterate over the indices and not the elements of list a.

a = [1,2,0,0,4,0]
non_zero_elements = []
i = 0
for i in range(0,len(a)):
    if a[i] != 0:
        non_zero_elements.append(i)
        print('The value',a[i],'in index',i,'is a non-zero element.')
        i = i + 1
print('Non-zero elements: ',non_zero_elements)

answered Sep 8, 2020 at 4:05

Spencer Tibbitts

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1 Comment

cielo_azzuro Over a year ago

Thank you very much for the feedback, it was spot on.

Surender · Accepted Answer · 2020-09-08 04:05:45Z

1

for i in a: in this i is the value of items in a and not their index.

You can use for i, v in enumerate(a): to iterate over the list where i will be index and v will be the value at that index.

Alternately, you can use for i in range(len(a)): to iterate using the index and access a[i].

answered Sep 8, 2020 at 4:05

Surender

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1 Comment

cielo_azzuro Over a year ago

Thank you, Surender!

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