Sort 2 arrays in C by a joint logic - c

Question

Given two arrays
a = [1,3,2,8,5,6]
b = [4,3,2,5,1,2]
I would like to sort them in "one pass" where the "logic" involves the two arrays, meaning:
if
int sortBySum(index i, index j){
if (a[i]+b[i] > a[j]+b[j]){
return 1;
}
return -1;
}
The output of the two arrays will be
a = [2,1,3,5,6,8]
b = [2,4,3,1,2,5]
Because the original arrays sum is a+b=[5,6,4,13,6,8]

Answer 1

When a task is about sorting, qsort is - in most cases - your friend.
But qsort can only sort one array. So you need to copy the elements from array a and b into an array of structs containing the data from a and b. Then use qsort and then copy the data back.
Like:
#include <stdio.h>
#include <stdlib.h>
typedef struct
{
int a;
int b;
} common_data;
// Compare function for qsort
int cmp(const void *p1, const void *p2)
{
common_data* pa = (common_data*)p1;
common_data* pb = (common_data*)p2;
return ((pa->a + pa->b) - (pb->a + pb->b));
}
int main(void)
{
int a[] = {1,3,2,8,5,6};
int b[] = {4,3,2,5,1,2};
size_t sz = sizeof(a)/sizeof(a[0]);
common_data carr[sz];
// Copy to common array
for (size_t i = 0; i < sz; ++i)
{
carr[i].a = a[i];
carr[i].b = b[i];
}
// Sort common array
qsort(carr, sz, sizeof(carr[0]), cmp);
// Copy to back to original array
for (size_t i = 0; i < sz; ++i)
{
a[i] = carr[i].a;
b[i] = carr[i].b;
}
// Print array a
for (size_t i = 0; i < sz; ++i)
{
printf("%d ", a[i]);
}
printf("\n");
// Print array b
for (size_t i = 0; i < sz; ++i)
{
printf("%d ", b[i]);
}
printf("\n");
return 0;
}
Output:
2 1 3 5 6 8
2 4 3 1 2 5

Answer 2

A primitive approach can look the following way.
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
int a[] = { 1, 3, 2, 8, 5, 6 };
int b[] = { 4, 3, 2, 5, 1, 2 };
const size_t N = sizeof( a ) / sizeof( *a );
printf( "a: " );
for ( size_t i = 0; i < N; i++ )
{
printf( "%d ", a[i] );
}
putchar( '\n' );
printf( "b: " );
for ( size_t i = 0; i < N; i++ )
{
printf( "%d ", b[i] );
}
putchar( '\n' );
putchar( '\n' );
for ( size_t n = N, last; !( n < 2 ); n = last )
{
for ( size_t i = last = 1; i < n; i++ )
{
if ( a[i] + b[i] < a[i-1] + b[i-1] )
{
int tmp = a[i];
a[i] = a[i-1];
a[i-1] = tmp;
tmp = b[i];
b[i] = b[i-1];
b[i-1] = tmp;
last = i;
}
}
}
printf( "a: " );
for ( size_t i = 0; i < N; i++ )
{
printf( "%d ", a[i] );
}
putchar( '\n' );
printf( "b: " );
for ( size_t i = 0; i < N; i++ )
{
printf( "%d ", b[i] );
}
putchar( '\n' );
putchar( '\n' );
return 0;
}
The program output is
a: 1 3 2 8 5 6
b: 4 3 2 5 1 2
a: 2 1 3 5 6 8
b: 2 4 3 1 2 5
The code that sorts the arrays using the method bubble sort can be moved in a separate function.