c sorting with recursion - c

Question

I'm trying to write a sorting function with just recursion. I keep getting the error:
lvalue required as unary '&' operand
This are the functions I'm using:
void sorter_rec (int a[], int n) {
if (n ==1 ) return;
else {
swap( &(maximumrec(a,n)), &a[n-1]);
sorter_rec(a,n-1);
return;
};
}
The error is in sorter_rec.
void swap(int *px, int *py)
{ int z = *px;
*px = *py;
*py = z;
return;
}
int maximumrec(int ar[], int n)
{
if (n == 1) {
return ar[0];
} else {
int max = maximumrec(ar, n-1);
return ar[n-1] > max ? ar[n-1] : max;
}
}
How can I solve this?

Answer 1

You may not apply the operator & to the temporary object returned by the function maximumrec.
Also if you are using the selection sort starting from the end of array then the maximum element should be also searched starting from the end of array. In this case the sorting algorithm will be more stable.
Here is a demonstrative program that uses your approach but instead of the searching maximum element it searches minimum element. You can rewrite it such a way that it would search the maximum element if you want.
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#define N 20
int * min_element( int a[], size_t n )
{
int *min = a;
if ( !( n < 2 ) )
{
min = min_element( a + 1, n - 1 );
min = *min < *a ? min : a;
}
return min;
}
void swap( int *a, int *b )
{
int tmp = *a;
*a = *b;
*b = tmp;
}
void sort( int a[], size_t n )
{
if ( !( n < 2 ) )
{
int *min = min_element( a, n );
if ( min != a ) swap( a, min );
sort( a + 1, n - 1 );
}
}
int main(void)
{
int a[N];
srand( ( unsigned int )time( NULL ) );
for ( size_t i = 0; i < N; i++ ) a[i] = rand() % N;
for ( size_t i = 0; i < N; i++ ) printf( "%d ", a[i] );
printf( "\n" );
sort( a, N );
for ( size_t i = 0; i < N; i++ ) printf( "%d ", a[i] );
printf( "\n" );
return 0;
}
The program output might look like
17 9 12 15 1 17 19 17 6 2 14 19 2 8 5 19 0 12 16 8
0 1 2 2 5 6 8 8 9 12 12 14 15 16 17 17 17 19 19 19

Answer 2

Your basic algorithm works. All I had to do was fix up the way you passed pointers around, and it worked the first time! Hope this helps. I changed some names to fit my own style a bit, and added some convenience things (macro and logging method), and main() so I could test it.
Note that the name of an array is the same as &array[0]
#include <stdio.h>
#define INT_COUNT(n) (sizeof(n) / sizeof(int))
void dumpIntArray(int *array, int n) {
printf("{ ");
for (int i = 0; i < n; i++) {
printf("%d ", array[i]);
}
printf(" }\n");
}
void swap(int *px, int *py) {
int z = *px;
*px = *py;
*py = z;
return;
}
int *ptrToMax(int *sortable, int n) {
if (n == 1) {
return sortable;
} else {
int *maximum = ptrToMax(sortable, n - 1);
return sortable[n - 1] > *maximum ? &sortable[n - 1] : maximum;
}
}
void quicksort(int *sortable, int n) {
if (n == 1 ) {
return;
} else {
swap(ptrToMax(sortable, n), &sortable[n - 1]);
quicksort(sortable, n - 1);
return;
}
}
int main(int argc, char **argv) {
int foo[] = { 1, 5, 3, 2, 4, 9, 10, 8, 7 };
printf("Before:\n");
dumpIntArray(foo, INT_COUNT(foo));
quicksort(foo, INT_COUNT(foo));
printf("After:\n");
dumpIntArray(foo, INT_COUNT(foo));
}