Sort a character array in ascending order using Counting sort

Question

Input char a[10] = {'c','b','c','d','E','C','a','A','b','C'};

Output : A a b b C C c c d e

I have been given a character array and I have to sort it in ascending order and I must use Counting sort to do that

I have tried so far:

#include <stdio.h>
#include <stdlib.h>
#define RANGE 255


void countingSort(char a[], char b[], int n) // a = array, b = empty array, n = size
{
    int i;

    int c[RANGE +1];
    memset(c, 0, sizeof(c));


    for( i = 0; i< n; i++)
    {
        c[a[i]] = c[a[i]] + 1;
    }
    for(i = 1; i<RANGE; i++)
    {
        c[i] = c[i] + c[i-1];
    }
    for(i = n-1; i>=0; i--)
    {
        b[c[a[i]] - 1] = a[i];
        c[a[i]] = c[a[i]] - 1;
    }

}

int main()
{
    char a[10] = {'c','b','c','d','E','C','a','A','b','C'};
    int n = 10;
    char b[10];
    int i;
    for( i = 0; i<n;i++)
    {
        printf("%c",a[i]);
    }
    printf("\n");
    countingSort(a,b,n);
    for( i = 0; i<n;i++)
    {
        printf("%c",b[i]);
    }
    printf("\n");

    return 0;
}

I have used ASCII table to sort the array and my output is

ACCEabbccd

I managed to sort the array in ascending order but I DO NOT know how to put a right after A and so on.

Must countingSort() be used unedited and than apply code after calling countingSort() OR can countingSort() change? — chux
– chux, Commented Apr 28, 2020 at 14:13
countingSort() can be edited but it has to be in the program. Actually we are learning counting sort algorithm in class, so we need to implement it in the code. — Shehab
– Shehab, Commented Apr 28, 2020 at 14:17

chux · Accepted Answer · 2020-04-28 15:20:37Z

One approach simply doubles the c[] size and forms an index where all even indexes are uppercase and odd ones are lowercase.

#if 1
#define RANGE (255*2 + 1)
#include <ctype.h>
#define CH_TO_INDEX(ch) \
    (2*toupper((unsigned char)ch) + !!islower((unsigned char) ch))

#else
// Original
#define RANGE 255
#define CH_TO_INDEX(ch)    (ch)

#endif

void countingSort(char a[], char b[], int n) {
  int i;
  int c[RANGE + 1];
  memset(c, 0, sizeof(c));

  for (i = 0; i < n; i++) {
    //c[a[i]] = c[a[i]] + 1;
    c[CH_TO_INDEX(a[i])]++;
  }
  for (i = 1; i < RANGE; i++) {
    c[i] = c[i] + c[i - 1];
  }
  for (i = n - 1; i >= 0; i--) {
    // b[c[a[i]] - 1] = a[i];
    b[c[CH_TO_INDEX(a[i])] - 1] = a[i];
    // c[a[i]] = c[a[i]] - 1;
    c[CH_TO_INDEX(a[i])]--;
  }
}

Output

cbcdECaAbC
AabbCCccdE

A more complex char --> index could be had that does not double the size of c[]. Such mappings tend to make assumptions that there are only letters A-Z. Such a mapping may use an auxiliary mapping array:

unsigned char map[256] - {
    0, 1, 2, ...., 31, ' ', ... 'A', 'a', 'B', 'b', ... 'Z', 'z', 
    ASCII characters after 'Z' and before 'a'
    ASCII characters after 'z', .... 255 };

OP requested

Output : A a b b C C c c d e

But based on {'c', 'b', 'c', 'd', 'E', 'C', 'a', 'A', 'b', 'C'}, I think OP wants

Output : A a b b C C c c d E

Note that original code fails when a[i] < 0. Code needs re-work for negative char. Re-code using unsigned char.

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