c# sum of the sums in the array

Question

Got stuck on quite simple problem in my code. I need to count what I would call a nested sums of an array. Let's take as an example the array: [1,2,2,3,6] I want to sum them as:

0 + 1 = 1
1 + 2 = 3
1 + 2 + 2 = 5
1 + 2 + 2 + 3 = 8
1 + 2 + 2 + 3 + 6 = 14

sum = 1 + 3 + 5 + 8 + 14 = 31

Edit: I tried to do it with stack, but it failed

            int sums = 0;
            Stack<int> sum = new Stack<int>();

            for (int i = 0; i < queries.Length; i++)
            {
                sum.Push(queries[i]);
            }

            for (int i = 0; sum.Count != 0; i++)
            {
                if(i != 0)
                {
                    sums += sum.Pop();
                }
            }

Please add an attempt you have made and we will be glad to help. HINT You could use a recersive method or nested for loops, etc. — Ryan Wilson
– Ryan Wilson, Commented Jul 6, 2022 at 18:15
@gunr2171 yea, I know how to do single array sum. But not this one :( — raicha
– raicha, Commented Jul 6, 2022 at 18:17
Kind of yea, tried to put it in some loop to do 2 times, but it failed — raicha
– raicha, Commented Jul 6, 2022 at 18:18

aminrd · Accepted Answer · 2022-07-06 18:26:38Z

2

You can run this task in a single loop considering when you have an array of size 5, the first element is repeating 5 times, second element repeating 4 times and etc.

int[] arr = { 1, 2, 2, 3, 6 };

int mysum = 0;
for (int i = 0; i < arr.Length; i++)
    mysum += (arr.Length - i) * arr[i];

Console.WriteLine(mysum);

Output:

Explanation:

1                 = 1
1 + 2             = 3
1 + 2 + 2         = 5
1 + 2 + 2 + 3     = 8
1 + 2 + 2 + 3 + 6 = 14
========================
5*1 + 4*2 + 3*2 + 2*3 + 1*6 = 31

edited Jul 6, 2022 at 18:26

answered Jul 6, 2022 at 18:20

aminrd

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3 Comments

raicha Over a year ago

Ohh thanks. Could you explain to me, how it works? It looks a bit odd ... You are just multiplying index by a element?

aminrd Over a year ago

@raicha I rewrite your expected sum and as you can see, first element is repeating 5 times (which is length of array minus index) and etc.

Charlieface Over a year ago

Looping backwards would make more sense probably

Charlieface · Accepted Answer · 2022-07-06 18:56:17Z

1

You can do this much more efficiently and more easily, by multiplying each value by its reversed index + 1

For example, using LINQ

int[] arr = { 1, 2, 2, 3, 6 };
var result = arr.Reverse().Select((val, i) => val * (i + 1)).Sum();

Note that .Reverse on an array (or other Collection<T>) does not actually move any items, it just reads them backwards. So this is therefore an O(n) operation, as opposed to your original solution which is O(n² / 2)

dotnetfiddle

You can also do this procedurally, this is almost the same as @aminrd's answer.

int[] arr = { 1, 2, 2, 3, 6 };
var result = 0;
for (var i = arr.Length - 1; i >= 0; i--)
    result += arr[i] * (i + 1);

edited Jul 6, 2022 at 18:56

answered Jul 6, 2022 at 18:48

Charlieface

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1 Comment

raicha Over a year ago

Thanks :) More efficient way

gunr2171 · Accepted Answer · 2022-07-06 18:25:50Z

1

Take advantage of Linq! .Sum() will add up everything in your collection. You run that twice, once per each slice, and once per each subtotal.

var input = new [] { 1, 2, 2, 3, 6 };
var totalSum = Enumerable.Range(1, input.Length).Sum(len => input[0..len].Sum());
// totalSum is 31

Enumerable.Range gets you a collection of numbers between (and including) 1 and 5 - the possible lengths of each slice of your sub arrays. You then use the range operator [0..#] to get increasingly larger slices.

Yes, this is not as clever as aminrd's solution - it's doing all the computations manually and you're performing many slices.

answered Jul 6, 2022 at 18:25

gunr2171

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1 Comment

Alexei Levenkov Over a year ago

input.Take(len).Sum() would avoid slices (and possibly easier to read for old non-python folks).

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