Question

How to group elements of an array based on their first occurrence in C

Answer 1

#include <stdio.h>
 
void group_elements(int arr[], int len) {
    int frequency[10] = { 0 };
   
    for (int i = 0; i < len; i++) {
        frequency[arr[i]]++;
        printf("frequency[arr[%d]] = %d arr[%d] = %d\n", i, frequency[arr[i]], i, arr[i]);
    }
   
    for (int i = 0; i < len; i++) {
        if (frequency[arr[i]] != 0) { // -1 should not be in array
            int total_frequency = frequency[arr[i]];
            while (total_frequency--) {
                printf("%d ", arr[i]);
            }
            frequency[arr[i]] = 0;
        }
    }
}
 
int main(void) {
    int arr[] = { 8, 3, 7, 8, 2, 5, 8, 5, 1, 9, 8, 1, 7 };
  
    group_elements(arr, sizeof(arr) / sizeof(arr[0]));
     
    return 0;
}

  
  
/*
run:
  
frequency[arr[0]] = 1 arr[0] = 8
frequency[arr[1]] = 1 arr[1] = 3
frequency[arr[2]] = 1 arr[2] = 7
frequency[arr[3]] = 2 arr[3] = 8
frequency[arr[4]] = 1 arr[4] = 2
frequency[arr[5]] = 1 arr[5] = 5
frequency[arr[6]] = 3 arr[6] = 8
frequency[arr[7]] = 2 arr[7] = 5
frequency[arr[8]] = 1 arr[8] = 1
frequency[arr[9]] = 1 arr[9] = 9
frequency[arr[10]] = 4 arr[10] = 8
frequency[arr[11]] = 2 arr[11] = 1
frequency[arr[12]] = 2 arr[12] = 7
8 8 8 8 3 7 7 2 5 5 1 1 9 
  
*/

 

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Answer 2

#include <stdio.h>

#define MAX_VALUE 100  // Assumes values are between 0 and 99

void groupElements(int arr[], int len) {
    int frequency[MAX_VALUE] = { 0 };
    int order[MAX_VALUE] = { 0 };
    int orderCount = 0;

    // Count frequencies and track order of first occurrences
    for (int i = 0; i < len; i++) {
        int num = arr[i];
        if (frequency[num] == 0) {
            order[orderCount++] = num;
        }
        frequency[num]++;
    }

    // Output grouped elements
    printf("Grouped array: ");
    for (int i = 0; i < orderCount; i++) {
        int num = order[i];
        for (int j = 0; j < frequency[num]; j++) {
            printf("%d ", num);
        }
    }
    printf("\n");
}

int main() {
    int arr[] = {88, 33, 77, 88, 22, 55, 88, 55, 11, 99, 88, 11, 77};

    groupElements(arr, sizeof(arr) / sizeof(arr[0]));

    return 0;
}



/*
run:
   
Grouped array: 88 88 88 88 33 77 77 22 55 55 11 11 99 
   
*/

 

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