Write a program to find an index of Max item in an array

Question

Write a console app in C# to find an index i in an array that is the maximum number in the array.

If the maximum element in the array occurs several times, you need to display the minimum index.

If the array is empty, output -1.

Please let me know what is wrong in my code?

If I input the array a = { 1, 2, 46, 14, 64, 64 };, for instance, it returns 0, while it should be returning 4.

  public static void Main()
  {
     double[] a = { 1, 9, 9, 8, 9, 2, 2 };
     Console.WriteLine(MaxIndex(a));
  }

  public static double MaxIndex(double[] array)
  {
     var max = double.MinValue;
     int maxInd = 0, maxCount = 0;
     int minIndex = 0;
     var min = double.MaxValue;
     for (var i = 0; i < array.Length; i++)
     {
        if (min > array[i])
        {
           min = array[i];
           minIndex = i;

        }
        if (max == array[i])
           maxCount++;
        if (max < array[i])
        {
           maxCount = 1;
           max = array[i];
           maxInd = i;
        }
     }

     if (array.Length == 0)
        return -1;
     if (maxCount > 1)
        return minIndex;
     return maxInd;
  }

Answer 1

Your calculation is correct, but you return the wrong variable minIndex instead of maxIndex. Don't do more than necessary in a method. This calculates also the min-index and the count how often it appears, then it does nothing with the results. Here is a compact version:

public static int MaxIndex(double[] array)
{
    var max = double.MinValue;
    int maxInd = -1;

    for (int i = 0; i < array.Length; i++)
    {
        if (max < array[i])
        {
            max = array[i];
            maxInd = i;
        }
    }

    return maxInd;
}

It also sets maxInd = -1 which was part of your requirement. Since MatthewWatson had an objection regarding repeating double.MinValue in the array, here is an optimized version:

public static int MaxIndex(double[] array)
{
    if(array.Length == 0)
        return -1;
    else if (array.Length == 1)
        return 0;

    double max = array[0];
    int maxInd = 0;

    for (int i = 1; i < array.Length; i++)
    {
        if (max < array[i])
        {
            max = array[i];
            maxInd = i;
        }
    }

    return maxInd;
}

---

If code-readability/maintainability is more important and you don't care of few milliseconds more or less, you could use LINQ (a version that enumerates only once):

int minIndexOfMaxVal = a.Select((num, index) => new {num, index})
    .OrderByDescending(x => x.num)
    .Select(x => x.index)
    .DefaultIfEmpty(-1)
    .First();

This works with every kind of sequence and types not only with arrays and doubles.

Answer 2

Try this simple line of code:

int[] numbers = { 1, 2, 3, 4, 5, 4, 3, 2, 1 };
var index = numbers.ToList().IndexOf(numbers.Max());

I think code is simple enough to be self-explanatory.

However, if you aim for performance, conversion to List could be omitted and you could wirte own code to find index of maximum number.

Or even simplier, proposed by @fubo:

Array.IndexOf(numbers, numbers.Max());

Answer 3

A lot of simplification is possible in this code:

int? maxVal = null; // null because of array of negative value; 
int index = -1;

for (int i = 0; i < array.Length; i++)
{
  int current = array[i];
  if (!maxVal.HasValue || current > maxVal.Value)
  {
    maxVal = current ;
    index = i;
  }
}

Will get you the first index of the max value. If you just need a short code and Don't mind iterating twice a simple linq can do the trick

var index = array.ToList().IndexOf(array.Max());

Answer 4

It returns zero, because minIndex is indeed zero: Change minIndex to maxIndex:

if (maxCount > 1) return maxIndex;

In order to compare doubles use the following code instead of ==:

if (Math.Abs(a-b)<double.Epsilon)

Answer 5

Your code in general is way too complicated with way too many variables for what it has to do.

You want the index of the first occurence of the highest value, so there isn't any real reason to store anything but a MaxValueIndex and a MaxValue.

All other variables should be local so garbage collection can dispose of them.

As for what is actually wrong with your code, I can't help out, since the variable names are confusing to me. But here is some code that achieves the goal you want to achieve.

double[] a = { 1, 9, 9, 8, 9, 2, 2 };
var highestValueInArray = a.Max();
var arrayLength = a.Length;

for (int i = 0; i < arrayLength; i++)
{
    if (a[i] == highestValueInArray)
    {
        Console.WriteLine(i);
        break;
    }
}

instead of manually calculating the max value, you can just use YourArray.Max() to get the highest value.

then just iterate through it like normal, but at the first occurence, you break; out of the loop and return the index it is at.

I'm pretty sure there's also a way to use FirstOrDefault in combination with IndexOf, but couldn't get that to work myself.