using .index(" ") for nested list in python

Question

I want to get the index of the "4" in the nested list, it is working cause I suppress the error. any idea for a alternative?

a = [[1,2,3],[4,5,6],[7,8,9]]

for i in range(len(a)):
    try:
        index = a[i].index(4)
        print(i, index)  
    except:
        pass

Asked By: guinab

||

Source

Answer 1

The index method will raise in a ValueError if the element is not found in the list. This is why the first iteration throws an exception.
You can either catch the error or check if the element is in the list before using the index method.

Here’s an example:

a = [[1,2,3],[4,5,6],[7,8,9]]

def find_index(list_of_lists, item):
    for sublist in list_of_lists:
        try:
            return sublist.index(item)
        except ValueError:
            continue

print(find_index(a, 4))

Which yields 0 as expected.

Alternatively, you could implement a look-before-you-leap approach and check if the element exists in the list before attempting to use the index method. This is less efficient however, because checking if an element exists in a list is a linear time operation.

def find_index(list_of_lists, item):
    for sublist in list_of_lists:
        if item in sublist:
            return sublist.index(item)

This is less efficient and less "pythonic", but still viable.

Answered By: Tomer Ariel

Answer 2

This is code that won’t throw an error and doesn’t use index() function.

for i in range(len(a)):
    for j in range(len(a[i])):
        if a[i][j] == 4:
            print("Found at", i, j)
            break

Answered By: 0xRyN

using .index(" ") for nested list in python

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