CS241: Spring 2015 -- Lecture 17, quick sort and merge sort

1. Quick sort pseudocode

   	quickSort(List aList) {
   		if (size>1) {
   			select a pivot and split aList into head and tail (where everything in head is < pivot...) with the pivot in between
                           quickSort(head)
                           quickSort(tail)
   		}
   	}
           

2. quicksorting 3,0,1,8,7,2,5,4,9,6 (in place)
3. Merge sort pseudocode

   	mergeSort(List aList) {
   		if (size>1) {
   			split aList roughly in half creating head and tail
                           mergeSort(head)
                           mergeSort(tail)
   			merge head and tail
   		}
   	}
           

4. mergesorting 3,0,1,8,7,2,5,4,9,6 (in place)
5. From pseudocode to code; details! It's all about the details! No. First it's about the representation; after you choose that, it's about the details.
   1. starting -- also known as initializing
   2. stopping
   3. in between
6. Proactive debugging
   1. souts on enter and exit to recursive routines; like this:

          void foo(int startI, int endI) {
              System.out.print("Top of foo: ");
              System.out.print(" startI = " + startI);
              System.out.print(" endI = " + endI);
              System.out.println(toString(startI, endI));
              
              ...foo code...
              
              System.out.print("bottom of foo: ");
              System.out.print(" startI = " + startI);
              System.out.print(" endI = " + endI);
              System.out.println(toString(startI, endI));       
          }
                      

   2. toString(firstI, lastI)
   3. shift(firstI, lastI)