Problem Statement

An equilibrium index of a sequence is an index into the sequence such that the sum of elements at lower indices is equal to the sum of elements at higher indices.

For example, in a sequence

A

{\displaystyle A}

: 3   is an equilibrium index, because: 6   is also an equilibrium index, because: (sum of zero elements is zero) 7   is not an equilibrium index, because it is not a valid index of sequence

A

{\displaystyle A}

.

Write a function that, given a sequence, returns its equilibrium indices (if any). Assume that the sequence may be very long.

Let's start with the solution:

eqIndex: function [row][
    suml: 0
    delayed: 0
    sumr: sum row
    result: new []
    loop.with:'i row 'r [
        suml: suml + delayed
        sumr: sumr - r
        delayed: r
        if suml = sumr -> 'result ++ i
    ]
    return result
]

data: @[
    @[neg 7, 1, 5, 2, neg 4, 3, 0]
    @[2 4 6]
    @[2 9 2]
    @[1 neg 1 1 neg 1 1 neg 1 1]
]

loop data 'd ->
    print [pad.right join.with:", " to [:string] d 25 "=> equilibrium index:" eqIndex d]