Counting the number of times a value appears in an array

Question

So what's a good, simple algorithm to create a loop in C# where every time a certain value appears in an array it adds 1 to a counter in another array?

For example I have this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication22
{
    class Program
    {
        const int SIZE = 12;

        static void Main(string[] args)
        {
            int[] numbers = new int[SIZE] {5, 5, 5, 7, 7, 7, 9, 7, 9, 9, 9, 1};
           string[] letters = new string[SIZE] { "m", "m", "s", "m", "s", "s", "s", "m", "s", "s", "s", "s" };
            int[] values = new int[SIZE] {15, 22, 67, 45, 12, 21, 24, 51, 90, 60, 50, 44};
            string[] status = new string[SIZE] { "f", "m", "f", "a", "m", "f", "f", "f", "m", "f", "m", "f" };

            int[] Count = new int[4];
            int x = 0;
            int i = 0;

            for (i = 0; i < SIZE - 1; i++)
            {
                if (numbers[i] > 0 && numbers[i] < SIZE)
                {
                    x = Count[i];
                    Count[x]++;
                }
            }

            for (i = 0; i < 4; i++)
            {
                Console.WriteLine("{0}", Count[4]);
            }
        }
    }
}

I am only counting the number of times 4 numbers appear in the numbers array. Someone suggested I use the method in the first loop but it doesn't seem to be working and creates an error that the index is out of bounds in the array. I want to display the number of times each of those numbers(5, 7,9 and 1) appear in 4 rows.

EDIT: Without using LINQ or any other fancy thing like Dictionary or whatever.

Solution 1

You're getting an index out of bounds error because of this section:

for (i = 0; i < SIZE - 1; i++)
{
    if (numbers[i] > 0 && numbers[i] < SIZE)
    {
        x = Count[i];

Notice that you're iterating through 0 to SIZE - 1 (11) when Count only has a size of 4.

---

You can do this task pretty easily with LINQ though.

int[] numbers = new int[SIZE] { 5, 5, 5, 7, 7, 7, 9, 7, 9, 9, 9, 1 };

var count = numbers
    .GroupBy(e => e)
    .Where(e => e.Count() == 4)
    .Select(e => e.First());

So it groups the numbers by their value, we then refine the list to only include groups of 4, then select the first of each to be left with a collection of ints.

---

Here is a non-LINQ based solution using a Dictionary to store the count of numbers.

int[] numbers = new int[SIZE] { 5, 5, 5, 7, 7, 7, 9, 7, 9, 9, 9, 1 };
var dictionary = new Dictionary<int, int>();
var numbersWithFour = new List<int>();

foreach (var number in numbers)
{
    if (dictionary.ContainsKey(number))
        dictionary[number]++;
    else
        dictionary.Add(number, 1);
}

foreach (var val in dictionary)
{
    if (val.Value == 4)
    {
        numbersWithFour.Add(val.Key);
    }
}

---

With a little modification to your program you can get some results.

int[] numbers = new int[SIZE] { 5, 5, 5, 7, 7, 7, 9, 7, 9, 9, 9, 1 };
string[] letters = new string[SIZE] { "m", "m", "s", "m", "s", "s", "s", "m", "s", "s", "s", "s" };
int[] values = new int[SIZE] { 15, 22, 67, 45, 12, 21, 24, 51, 90, 60, 50, 44 };
string[] status = new string[SIZE] { "f", "m", "f", "a", "m", "f", "f", "f", "m", "f", "m", "f" };

// Set the size of Count to maximum value in numbers + 1
int[] Count = new int[9 + 1];
int x = 0;
int i = 0;

for (i = 0; i < SIZE - 1; i++)
{
    if (numbers[i] > 0 && numbers[i] < SIZE)
    {
        // Use value from numbers as the index for Count and increment the count
        Count[numbers[i]]++;
    }
}

for (i = 0; i < Count.Length; i++)
{
    // Check all values in Count, printing the ones where the count is 4
    if (Count[i] == 4)
        Console.WriteLine("{0}", i);
}

Output:

7
9

Solution 2

Use LINQ to do the work

using System.Linq;

var numQuery =
        from num in numbers
        where num == 5
        select num;

Console.WriteLine("Count of 5: " + numQuery.Count);

Or use the method syntax

var numQuery = numbers.Where(num => num == 5);
Console.WriteLine("Count of 5: " + numQuery.Count);

See here for the overview and here for query vs method-syntax.
Found a sample for GroupBy, look here.

Solution 3

I used Regex for my solution since I only had three values.

String results = "" + one.ToString() + " " + two.ToString() + " " + three.ToString();
int count1 = Regex.Matches(results, @one.ToString()).Count;
int count2 = Regex.Matches(results, @two.ToString()).Count;
int count3 = Regex.Matches(results, @three.ToString()).Count;

Seems 'hacky', but worked for me. It'll work with strings or numbers but only if you're working with a few values. Pretty efficient in that case. If not, I think the other answer would be a better option.

Comments

[Matthew Watson]

This is for research/homework, so you must use a loop? If so, better say so else you'll get a million answers all about using Linq.

[Destrictor]

This would only show the number of times one number would appear. He wants to see them all at once, so use GroupBy for that.

[Sabotenderizer]

I'm not allowed to use those yet. )=

[Sabotenderizer]

I put 4 because there are only 4 different numbers. I would like to display how many times each number appears.

[Sabotenderizer]

Printing the ones where the count is 4? Hmm how about printing how many times each number appears?

[Daniel Imms]

Then change it to Console.WriteLine("{0}", Count[i]); you would need to remove the if (Count[i] == 4) for them all to show up though.

[Daniel Imms]

The best would be combining both: Console.WriteLine("{0}: {1}", i, Count[i]);

[Sabotenderizer]

Thanks, but one more thing =). Exactly how does Count[numbers[i]]++; work? How are the arrays entangled together in that way to count the number of times a value appears?

[Daniel Imms]

We use numbers[i] as an index for Count[] and then add one to the value in Count[] using the post-increment operator ++. It could also be done like this Count[numbers[i]] += 1