Data Structures and Algorithms

     qsort - quicker sort
     qsort(base, nel, width, compar)
     char *base;
     int (*compar)();
qsort() is an implementation of the quicker-sort algorithm. It sorts a table of data in place. base points to the element at the base of the table. nel is the number of elements in the table. width is the size, in bytes, of each element in the table. compar is the name of the comparison function, which is called with two arguments that point to the elements being compared. As the function must return an integer less than, equal to, or greater than zero, so must the first argument to be considered be less than, equal to, or greater than the second.

The pointer to the base of the table should be of type pointer-to-element, and cast to type pointer-to-character. The comparison function need not compare every byte, so arbitrary data may be contained in the elements in addition to the values being compared. The order in the output of two items which compare as equal is unpredictable.

sort(1V), bsearch(3), lsearch(3), string(3)

The following program sorts a simple array:

          static    int intcompare(i,j)
          int *i, *j;
               return(*i - *j);

               int a[10];
               int i;
               a[0] = 9;
               a[1] = 8;
               a[2] = 7;
               a[3] = 6;
               a[4] = 5;
               a[5] = 4;
               a[6] = 3;
               a[7] = 2;
               a[8] = 1;
               a[9] = 0;

               for (i=0; i<10; i++) printf(" %d",a[i]);

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© John Morris, 1996