Finding number of repeated elements in an array using C

One of the things that you can do is, keep an extra array for the numbers you have checked and each time when you compare numbers you would check if you have already seen this number or not.

I also want to include a few another approach to this problem. If we know that numbers in the list won't be so big we can use an array to keep track of counts.

//numbers = {4, 3, 4, 4, 3, 4, 5}
int max = findMaxValue(numbers);
int counts[max]; //should be done with dynamic memory allocation
for(i=0;i<max;i++){
    counts[max] = 0;
}
for(i=0;i<numbers.size;i++){
    counts[numbers[i]]++;
}
int sum = 0;
for(i=0;i<max;i++){
    if(counts[i] > 1){
        sum += counts[i];
    }
}

Another thing that you can do is, sort the numbers first and then compare adjacent elements.

I think it is a idea to put the same elements together first by sorting them.

#include<stdio.h>

int main() {
    int n = 0, count = 0;

    printf("Enter the length of array: ");
    scanf("%d", &n);

    int arr[n];

    for (int i = 0; i < n; i++) {
        printf("Enter a number: ");
        scanf("%d", &arr[i]);
    }

    //sort
    for (int j = 0; j < n - 1; j++) {
        for (int k = j + 1; k < n; k++) {
            if (arr[j] >= arr[k]) {
                arr[j] = arr[j] ^ arr[k];
                arr[k] = arr[j] ^ arr[k];
                arr[j] = arr[j] ^ arr[k];
            }
        }
    }

    // count
    int num = 1; // If you don’t want to include the repeated number itself, replace all "num = 1" with "num = 0"
    for (int j = 0; j < n - 1; j++) {
        if (arr[j] == arr[j + 1]) num++;
        else {
            printf("The num %d repeats %d times\n", arr[j], num); // You can delete this line
            count += num;
            num = 1; //Initialize num to avoid repeated accumulation of num and prepare to enter the next loop
        }
    }
    printf("Total repeats: %d\n", count);
    
    return 0;
}

Let me learn you how to deal with such questions, by adding some lines to your source code:

#include <stdio.h>
int main()
{
    int n,i,j,count=1,p=0;
    printf("Enter the length of array");
    scanf("%d",&n);
    int arr[n];
    for(i=0;i<n;i++)
    {
        printf("Enter a number\n");
        scanf("%d",&arr[i]);
    }
    for(i=0;i<n;i++)
    {
        printf("Start for-loop for i, i=[%d]",i);
        for(j=i+1;j<n;j++)
        {
            printf("  Start for-loop for j, j=[%d]", j);
            if(arr[i]==arr[j])
            {
                printf("    Both arr[%d] and arr[%d] equal [%d]", i, j, arr[i]);
                count++;
                printf("      As a result, count=[%d], p=[%d]", count, p);
                break;
            }
            else printf("    Arr[%d]<>arr[%d]: arr[i]=%d, arr[j]=%d, count=[%d], p=[%d]", i, j, arr[i], arr[j], count, p);
        }
        if(count!=1)
        {
            printf("  count=[%d] which is different than 1 while p=[%d]", count, p);
            p=p+count;
            printf("  p has been increased by count and now, p=[%d]", p);
            count=1;
            printf("  count is brought back to 1");
        }     
    }
    printf("Number of repitions are %d",p);

}

JEZUS? All those printf() commands?
Yes: as a beginner you better put too much such commands than too less. One important thing: don't just read the output and start from there but first, try to imagine what you expect the output to be like. Like this, you'll immediately see where your expectations don't meet the real outcome and you'll know what you did wrong.

... and another important remark: once you're finished, don't remove those printf() lines, but put them into comment. If ever you might need to adapt your code to a changed requirement, you might need those printf() lines again.

You are on the right track but your logic is incorrect: you do not count the last element of each repeated value.

• you should initialize count to 0 instead of 1
• you should iterate the inner loop from 0 instead of from i + 1 to count the number of occurrences of arr[i] and continue to the end of the array, removing the break statement.
• you should just increment p if the count is not exactly 1, ie: if the element is repeated.

The time complexity of this approach is the same as yours, O(N²). You could lower the complexity to O(N.log(N)) by sorting the array and counting the duplicates in a single scan.

An even more efficient approach may reach linear time complexity: using a hash table, you would count the number of occurrences of each element read from the user. You would then enumerate the hash table, counting the number of non zero counts.

Here is a modified version:

#include <stdio.h>

int main() {
    int n, i, j, p = 0;

    printf("Enter the length of array: ");
    if (scanf("%d", &n) != 1)
        return 1;

    int arr[n];

    for (i = 0; i < n; i++) {
        printf("Enter a number: ");
        if (scanf("%d", &arr[i]) != 1)
            return 1;
    }
    for (i = 0; i < n; i++) {
        int count = 0;
        for (j = 0; j < n; j++) {
            if (arr[i] == arr[j]) {
                count++;
            }
        }
        if (count != 1) {
            p = p + 1;
        }     
    }
    printf("Number of repetitions: %d\n", p);
    return 0;
}