How to show random numbers in descending order in array

Question

I want to generate numbers from 1 to 10 in an array, then print them in descending order. The method I’ve used so far doesn’t work properly. Sometimes the number 10 is at the bottom, and all other numbers are correctly ordered.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void)
{
    int a[7], j, i;
    srand(time(NULL));
    a[0] = rand() % 10;

    int length = 7;
    for(int i = 0; i < 7; i++) {
        int duplikat = 0;
        a[i] = rand() % 10+1;
        
        for (int j=0; j<i; j++) {
            // descending order: Just change (a[j] > a[j+1])
            if(a[i] == a[j]) duplikat = 1;
          
            if (a[j] < a[j+1]) {
               int temp = a[j];
               a[j] = a[j + 1];
               a[j + 1] = temp;
            }
            if (duplikat) i--;
        }
    }

    for(int i = 0; i< length; i++) {
        printf("a[%d] = %d\n", i, a[i]);
    }

    printf("\n");
    for(j=0;j<7;j++) printf("\n%d ", a[j]);
    return 0;
}

Answer 1

Some observations:

• The line if (duplikat) i--; should be outside the inner loop over j, so that you decrease i at most once when a dupicate is found.

  Fiddling with the index of a for loop to implement some skipping logic is also very brittle. Instead, consider a while loop until your array a is full where you add items only if they are no duplicates.

• If you want to "bubble down" the new number so that the part of the array is sorted, you must start from the right-hand side of the array. Otherwise, the only swap that may take place is of the new item with the last item in the array.

You could do something similar to insertion sort, where you keep the generated array sorted at all times:

• Determine a new random number to add, the "pick".

• Find the place i where the pick goes; if the number was already generated, pick again.

• Otherwise, create an empty slot at the end of the array by incrementing the actual length, then move all elements right of the i one place to the right. There is now a gap at i, where you place your pick.

• Repeat until your array has enough items.

In code:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void)
{
    int a[7];
    int length = 7;     // desired length of array
    int n = 0;          // current length of array

    srand(time(NULL));
    
    while (n < length) {
        int pick = rand() % 10 + 1;
        int i = 0;
        
        // skip greater elements
        
        while (i < n && pick < a[i]) i++;
        
        // shift smaller elements and add pick if it is unique
        
        if (i == n || pick != a[i]) {
            int j = n++;
            
            while (i < j) {
                a[j] = a[j - 1];
                j--;
            }
            
            a[i] = pick;
        }
    }
    
    // print array

    for (int i = 0; i< length; i++) {
        if (i) printf(", ");
        printf("%d", a[i]);
    }

    printf("\n");

    return 0;
}