CODEWITHSUNDEEP
pythonlist-comprehension
Mohamed Abdelrazek
Mohamed Abdelrazek

Reputation: 108

list comprehension Error " IndexError: list index out of range "

I am trying to make list comprehension for the below code, the code works fine if comprehension not used.

This code is used to return the missing number in a sequence of numbers that have a constant difference between the consecutive terms of a given series of numbers.

def find_missing(sequence):
    difference = min(sequence[1] - sequence[0], sequence[-1] - sequence[-2])
    # for index in range(len(sequence)):
    #     if sequence[index] + difference != sequence[index+1]:
    #         return sequence[index] + difference
    # return [index for index in range(len(sequence))]
    return [sequence[index]+difference for index in range(len(sequence)) if sequence[index]+difference != sequence[index+1]]


print(find_missing([1, 2, 3, 4, 6, 7, 8, 9]))  # ==5
print(find_missing([1, 3, 5, 9, 11]))  # == 7
print(find_missing([1, 3, 4]))  # == 2

I have an error message when using comprehension. Error:

IndexError: list index out of range

Upvotes: 1

Views: 847

Answers (1)

Mark
Mark

Reputation: 92440

You are getting an error because index+1 in sequence[index+1] is one bigger than the array on the last iteration. Python generally discourages using indexes for loops — partially because it's so easy to create these kind of bugs.

If you want to compare adjacent elements, you can instead loop over zip(sequence, sequence[1:]). This will give you pairs that you can then compare — it's easier to read and you don't need to worry about index errors:

def find_missing(sequence):
    difference = min(sequence[1] - sequence[0], sequence[-1] - sequence[-2])

    return [ f + difference 
             for f, n in zip(sequence, sequence[1:]) 
             if  f + difference != n ]


print(find_missing([1, 2, 3, 4, 6, 7, 8, 9]))  # == [5]
print(find_missing([1, 3, 5, 9, 11]))  # == [7]
print(find_missing([1, 3, 4]))  # == [2]

Upvotes: 3

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