Challenge 160 Task #2 - Steps to recovering the Equilibrium in your lists

Task #2 - Equilibrium Index


Original Description

Submitted by: Mohammad S Anwar

You are give an array of integers, @n.

Write a script to find out the Equilibrium Index of the given array, if found.

For an array A consisting n elements, index i is an equilibrium index if the sum of elements of subarray A[0…i-1] is equal to the sum of elements of subarray A[i+1…n-1].

Example 1:

Input: @n = (1, 3, 5, 7, 9)
Output: 3

Example 2:

Input: @n = (1, 2, 3, 4, 5)
Output: -1 as no Equilibrium Index found.

Example 3:

Input: @n = (2, 4, 2)
Output: 1


Full Source

Phrased a little differently, an equilibrium index is an index, where the sum of all elements left to it is equal to the sum of all elements to the right of it.

With negative numbers and zero being allowed in a list, there might be more than one Equilibrium Index. 1 0 0 1 for example with index 1 and 2 being equilibrium indices, or 0 0 0 0 with all indices being possible candidates. I wrote a solution that will find the first equilibrium index, if there is one.

sub equilibrium_index(@xs) {
    for my $i ( 0 .. $#xs  ) {
        my $lower = sum0( @xs[ 0 .. $i - 1 ] );
        my $upper = sum0( @xs[ $i + 1 .. $#xs ] );
        return $i if $lower == $upper;

    return -1;

The solution walks all indices in order and returns the first where the sum of the left and right subarrays is equal. I’m using sum0 from List::Util to prevent warnings about uninitialized values that would pop up at the beginning and at the end of the list if I were to use sum.

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