How to get all indices of the highest number in an array?

Question

For example, I have an array

arr[5] = {1, 5, 2, 3, 5}

and the highest number is obviously 5.

My question is how do I get both the indices of the highest number (which is 5).

The expected result is 1 and 4.

This is more of algorithmics. First, assume the first two are the largest two, then read the others and adjust. — Paul Stelian, Apr 25 '17 at 06:51
Can you clarify what exactly do you expect as a result? Do you need to find all indices of 5 in the array? — default locale, Apr 25 '17 at 06:55
I guess you already know how to find the maximal element. This question will help you to find all occurrences: https://stackoverflow.com/questions/10443461/c-sharp-array-findallindexof-which-findall-indexof — default locale, Apr 25 '17 at 07:00

score 2 · Answer 1 · answered Apr 25 '17 at 06:55

2

var arr = new int[] { 1, 5, 2, 3, 5 };
        int max = arr.Max();
        List<int> indexes = new List<int>();
        for (int i = 0; i < arr.Length; i++)
        {
            if (arr[i] == max)
                indexes.Add(i);
        }
int highindex = indexes.LastOrDefault();

answered Apr 25 '17 at 06:55

Narek Arzumanyan

616
3
11

You don't need a list, a simple LINQ query will return the same results – Panagiotis Kanavos Apr 25 '17 at 07:04

score 1 · Answer 2 · answered Apr 25 '17 at 07:04

Thats how you get all indexes of your highest number

var numbers = new int[] { 1, 5, 2, 3, 5 };

int max = numbers.Max();

var indexes = numbers.Select((c, i) => new 
                                       { 
                                          character = c, index = i 
                                       })
                     .Where(list => list.character == max)
                     .ToList();

Panagiotis Kanavos · Answer 3 · 2017-04-25T07:22:23.160

You can use a LINQ query to find the numbers and index that equal the maximum number :

var arr=new[] {1, 5, 2, 3, 5};
var max    = arr.Max();
var indexes= arr.Select( (n,idx)=>n==max?idx:-1)
                .Where(idx=>idx!=-1)
                .ToArray();

This will return {1,4}.

This query uses the Enumerable.Select overload that provides the index of the current element and returns that index if the number is equal to the maximum.

The original title was a bit confusing - how to find the two largest values. You can use a similar query to get the N largest values by selecting the value and index, ordering the results and taking first N items:

var indexes = arr.Select( (val,idx)=> (val:val,idx:idx) )
                 .OrderByDescending(p=>p.val)
                 .Take(2)
                 .Select(p=>p.idx);

This query uses C# 7 tuples to hold the intermediate results and give them a name

Simple and efficient solution..... – csharpbd Apr 25 '17 at 07:28 — csharpbd, Apr 25 '17 at 07:28

How to get all indices of the highest number in an array?

3 Answers3