I have a task "You are given an array of integers. You should find the sum of the integers with even indexes (0th, 2nd, 4th...). Then multiply this summed number and the final element of the array together. Don't forget that the first element has an index of 0.

For an empty array, the result will always be 0 (zero).

Input: A list of integers.

Output: The number as an integer."

My solution is:

def checkio(array: list) -> int:
    """
        sums even-indexes elements and multiply at the last
    """
    summary = 0
    if len(array) > 0:
        for i in array:
            if array.index(i) % 2 == 0:
                summary  = i
                print('sum = '   str(summary)   ' index = '   str(array.index(i)))
        res = summary * array[-1]
        return res
    else:
        return 0

And when it doesnt work correct, when i print indexes to see whats wrong, i got that some index is missing (in this example 16th)

print(checkio([-37,-36,-19,-99,29,20,3,-7,-64,84,36,62,26,-76,55,-24,84,49,-65,41]))

Output:

sum = -37 index = 0
sum = -56 index = 2
sum = -27 index = 4
sum = -24 index = 6
sum = -88 index = 8
sum = -52 index = 10
sum = -26 index = 12
sum = 29 index = 14
sum = -36 index = 18
-1476

What is the reason?

CodePudding user response：

The issue is that array.index() will return the first index of the value

You have a 84 at position 9, then when you call it on the second 84, the vall is the same array.index(84) so result is same : 9 and it won't count it

Use enumarate that make an iterator with both position and value

def checkio(array: list) -> int:
    """
        sums even-indexes elements and multiply at the last
    """
    summary = 0
    if array: # true is non-empty
        for i, value in enumerate(array):
            if i % 2 == 0:
                summary  = value
                print('sum =', summary,'index =', i)
        res = summary * array[-1]
        return res

    return 0

CodePudding user response：

As mentioned in the answer by @azro, the problem with your code has to do with array.index() always returning the first index of a given element in your list, so that it will return undesired results for duplicates.

enumerate() is one way to go, but there's still room for some optimization here. The following bit is verbose, since it will loop over all the elements, while we already know we only need elem 0, 2, etc.

        for i, value in enumerate(array):
            if i % 2 == 0:

We could just write:

array = [-37,-36,-19,-99,29,20,3,-7,-64,84,36,62,26,-76,55,-24,84,49,-65,41]

def checkio1(array: list) -> int:
    """
        sums even-indexes elements and multiply at the last
    """
    summary = 0
    if array: # true is non-empty
        for i in range(0,len(array),2): # 2 is the step, so we enter elem 0, 2, etc.
            summary  = array[i]
        res = summary * array[-1]
        return res

    return 0

print(checkio1(array))
1968

Let me share two alternative methods as well.

1. using numpy:

import numpy as np

def checkio2(array: list) -> int:
    """
        sums even-indexes elements and multiply at the last
    """
    if not array:
        return 0
    arr = np.array(array)
    return arr[np.arange(0,len(arr),2)].sum()*arr[-1]

print(checkio2(array))
1968
print(checkio2([]))
0

2. using itemgetter from the operator module:

from operator import itemgetter

def checkio3(array: list) -> int:
    """
        sums even-indexes elements and multiply at the last
    """
    if not array:
        return 0
    return sum(itemgetter(*range(0,len(array),2))(array))*array[-1]

print(checkio3(array))
1968
print(checkio3([]))
0

CodePudding user response：

Here is my attempt, just for fun reference. All previous posts work great.

I don't like to use array here, since it has different meaning in Python.

def checkio(nums: list) -> int:
    """
        sums even-indexes elements and multiply at the last
    """
    if not nums:     #  not empty
       return sum(nums[0::2]) * nums[-1]   # . just rely on range slicing here & save some modulo computations later...
    else:
       return 0