How can I retrieve the indices used to generate the max sum such that no adjacent elements are used?

Question

Below is my code, which produces the indices used, but in some cases, finds a value, and takes the first index containing that value, while I need the specific index from which that value came.

import random

numsLength = random.randint(1,100)
a = [random.randint(0, 400) for i in range(numsLength)]
DP = [[] for i in a]  # This will hold bestSeq from i, initially empty
seq = []
mineNum = []

def solveDP(i):
    global DP

    if i >= len(a):
        return []

    # If this value is already computed return that value
    if len(DP[i]) > 0:
        return DP[i]

    # Include the current element and go to i+2
    arr1 = [a[i]] + solveDP(i + 2)
    # Skip the current element and go to i+1
    arr2 = solveDP(i + 1)
    if sum(arr1) > sum(arr2):
        DP[i] = arr1[:]    # Copy arr1 into DP[i]
    else:
        DP[i] = arr2[:]    # Copy arr2 into DP[i]
    return DP[i]

mines = solveDP(0)
for x in mines: # This loop simply assigns the integers' ordered numbers, in order to track which were used
    if a.index(x) + 1 not in mineNum:
        mineNum.append(a.index(x) + 1)

Asked By: ForgivenProdigy

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Source

Answer 1

Ok, so I think I’ve got it. My solution is simply to include the index i along with a[i] when picking an element. The hack needed is in using sum() to recreate the value you are calculating.

import random

numsLength = random.randint(1,100)
print(numsLength)
a = [random.randint(0, 400) for i in range(numsLength)]
DP = [[] for i in a]  # This will hold bestSeq from i, initially empty

def solveDP(i):
    if i >= len(a):
        return []

    # If this value is already computed return that value
    if DP[i]:
        return DP[i]

    # Include the current element and go to i+2
    arr1 = [(a[i],i)] + solveDP(i + 2)
    # Skip the current element and go to i+1
    arr2 = solveDP(i + 1)
    if sum(i for i, _ in arr1) > sum(i for i, _ in arr2):
        DP[i] = arr1[:]    # Copy arr1 into DP[i]
    else:
        DP[i] = arr2[:]    # Copy arr2 into DP[i]
    return DP[i]

mines = solveDP(0)

mineNum = []
for x,idx in mines:
    if idx+1 not in mineNum:
        mineNum.append(idx+1)
print(mineNum)

Update:

DPS = [sum(i for i, _ in d) for d in DP]
print(DPS)
print(max(DPS))

However, it seems that your algorithm leaves the max as the first index in DP

Answered By: quamrana

How can I retrieve the indices used to generate the max sum such that no adjacent elements are used?

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