Sorting of an array

I am programming in C. I have an array of structures. I need to print the array in sorted order based on an element of the structure. The main problem where I am stuck is I do not want to modify the original array.

For example: My array is proctab[10]. This is the array of structure named pentry.

struct pentry
{
    int a;
    int b;
    char c;
}

I need to print as follows:

a = 1, b = 2, c = a
a = 2, b = 1, c = d
a = 3, b = 0, c = e
a = 4, b = 1, c = a
a = 4, b = 2, c = a

and so on.. i.e. the result is sorted on a. but if a has same value for two structures in the array, the array should be sorted on b as well.

I want the original array proctab to remain intact.

Is there any way to do this?

Solution

To sort the items without messing with the original array, create a second array of references:

struct pentry plist[]= { { 1, 2, 'a' }, { 4, 8, 'z' }, { 2, 7, 'c' }, { 2, 1, 'e' }, { 5, 6, 'b' } ;
struct pentry (* pref)[5] ;

int compare( const struct pentry ** px, const struct pentry ** py )
{
  return ( (** px).a == (** py).a ) ? ( (** py).b - (** px).a ) : ( (** py).a - (** px).a ) ;
}

void dosort( struct pentry ** zdest, struct pentry * asrc, int n )
{
  int i ;
  struct pentry ** fill ;

  for ( i= n, fill= zdest ; ( i -- ) ; ) { *( fill ++)= asrc ++ ; }
  qsort( zdest, n, sizeof( * zdest ), compare ) ;
}

void show_sorted( struct pentry ** aref, int n )
{
  while ( n -- )
  {
    printf("%d %d %c\n", (** aref).a, (** aref).b, (** aref).c ) ;
    ++ aref ;
  }
}

int main()
{
  dosort( pref, plist, sizeof( plist) / sizeof( * plist)) ;
  show_sorted( pref, sizeof( plist) / sizeof( * plist)) ;
  return 0 ;
}