希尔排序

#include "stdio.h"
void shellsort(int k[],int n)
{
    int i, j, flag ,gap = n;
    int tmp;
    while(gap > 1){
    gap = gap/2;               
        for(i=1;i<=n-gap;i++){
            j = i + gap;
            if(k[i]<k[j]){
                tmp = k[i];
                k[i] = k[j];
                k[j] = tmp;
                }
            }
        
    }
}

main()
{
    int i,a[11] = {-111,2,5,6,3,7,8,0,9,12,1};         
    printf("The orginal data array is\n") ;
    for(i=1;i<=10;i++)                           
        printf("%d ",a[i]);
    shellsort(a,10);                              
    printf("\nThe result of Shell's sorting for the array is\n");
    for(i=1;i<=10;i++)
        printf("%d ",a[i]);                            /*输出排序后的结果*/
  
}
请问        for(i=1;i<=n-gap;i++){
            j = i + gap;
            if(k[i]<k[j]){
                tmp = k[i];
                k[i] = k[j];
                k[j] = tmp;
                }
            }
        
这个代码我不太明白啊！尤其是i<=n-grap我实在是看不懂