OK 12,10...9,11

#include <stdio.h>
#define SIZE 12

//to sort array, so it goes as follows: 12,10,8,6,4,2,1,3,5,7,9,11
//works fine - with even and neven SIZE, with if/else and with trenary instead,
//with max_index=s/2 as well
//to work on brushing programm, plus making good comments
//code BEFORE it worked fine here: https://pastebin.com/23gEiBWA
//mistake was assigining s-1 to up, instead of s!


void pr_arr(int s,int *a);
void sort_arr(int s,int *a);
void swap_el(int s,int *a,int ind1,int ind2);

int main()
{
    int arr[SIZE]= {1,2,10,3,9,4,8,5,12,6,7,11};
    pr_arr(SIZE,arr);
    sort_arr(SIZE,arr);
    pr_arr(SIZE,arr);

    return 0;

}

void pr_arr(int s,int *a)
{
    int i;
    for(i=0;i<s;i++)
    {
        printf("%d\t",a[i]);

    }
    printf("\n\n");
}
void sort_arr(int s,int *a)
{
    int temp,i,j,index_max,up,down =-1;
    up = s;
    index_max = s/2;


   for(i=0;i<s;i++)//i -just a cycle counter
   {
      //index_max = down;//better to use middle-array index all the time!
       /*if(!(i%2))//i is even, SO we use down as target index
        index_max=++down;
       else//      i is not even,SO we use up as target index
        index_max=--up;*/

       (!(i%2))? ++down,index_max=down:--up,index_max=up;//almost good, only order
       //11,9,..1,2..12 (instead of 12,10..2,1..11),or starts with a[11] not a[0]

       printf("\ni %d\t,down %d\t,up %d\t,index_max %d\n",i,down,up,index_max);
       for(j=down;j<=up;j++)//looking for the max element between down & up
       {
           //printf("j is %d\t",j);
           if(a[j]>a[index_max])
            swap_el(SIZE,a,j,index_max);
       }
       //swaps a[down] or a[up](depending on i)
       //with a[index_max], OR swaps target element with max element
       (i%2)? swap_el(SIZE,a,down,index_max):swap_el(SIZE,a,up,index_max);
       pr_arr(s,a);
   }
}
void swap_el(int s,int *a,int ind1,int ind2)
{
    int temp = a[ind1];
    a[ind1]= a[ind2];
    a[ind2]= temp;

}