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CSC 301.01, Class 19: Radix sort


  • Preliminaries
    • Notes and news
    • Upcoming work
    • Extra credit
    • Questions
    • PSA
  • Another sorting example
  • Radix sort
  • Analyzing radix sort
  • Bucket sort, revisited
  • Analyzing bucket sort

News / Etc.

  • Exam 1 to be returned Monday, for real.

Upcoming work

Extra credit (Academic)

  • CS Table Tuesday, GHC
  • CS Extras, Tuesday, Alums
  • CS Extras Thursday, ???
  • Nice Fish

Extra credit (Peer)

  • Neverland players (negative extra credit).
  • Cool Art thingy Saturday 1-3 pm
  • RTS tonight 9:30 in Loose Lounge

Extra Credit (Misc)

  • Any “successor to 10/10” event.

Other good things

  • Volleyball vs. Beloit, Friday at 7:00 p.m.
  • Women’s Tennis vs. St. Norbert, Saturday at 9:00 a.m.
  • Volleyball vs. Lake Forest, Saturday at 1:00 p.m.
  • Women’s Tennis vs. Ripon, Saturday at 3:00 p.m.
  • Football, Saturday at 1:00 p.m.


Will you return our homework?
Maybe after break.

Another sorting example

  • Cards set up with “slot” for 1 and a whole for zero.
  • A is 00001, B is 00010, C is 00011, D is 00100, etc.
  • I want to separate the first fifteen cards from the last eleven cards (or maybe sixteen/ten). How can I do so easily?
  • Can I use that capability to sort? Let’s see.
  • Put all of the 0’s in position 0 before all of the 1’s in position 0.
  • Put all of the 0’s in position 1 before all of the 1’s in position 1.
  • Put all of the 0’s in position 2 before all of the 1’s in position 2.
  • Put all of the 0’s in position 3 before all of the 1’s in position 3.
  • Put all of the 0’s in position 4 before all of the 1’s in position 4.
  • This appears to take five steps.
  • As long as I can encode all my values in five bits, I can sort any number of elements in five steps.

Radix sort

Given a collection of values represented in fix-width binary. Binary order is the same as th eorder of values.

  • Start from the lower order bit and work toward upper order bit, putting things with a zero before things with a one. (Preserving order within any bit value.)

Running time is O(n * #bits)

Why does it work?

After each round, the elements are sorted according to their last k bits

After the first round, all the things with zero come before all the things with 1 in the rightmost bit. They are sorted by the rightmost bit.

After the second round, we have 00 before 01 before 10 before 11 because we’ve preserved order within the lower bits..

Analyzing radix sort

Is this stable?

  • Yes

What happens if you go left to right instead of right to left?

  • Nothing sensible that I know.

How do you sort in reverse?

  • Put the zeros after the 1’s

How much space does this use? Big-O, exact?

  • We can’t use the Quicksort “swap from the ends” approach, because that breaks stability, and we need stability, for example, to make sure that the 00’s come before the 01’s.
  • As far as I know, there’s no way to do this in place for an array.

If we have a list, rather than an array, we can build two other lists, one of the zeros and one of the ones. We’ll also have a pointer to the end of each of those lists to make it easy to append.

Node *current; Node *zeros; Node *ones; Node *lastzero; Node *lastone;

while (current)
    Node *next = current->next
    if (current->bitofinterest == 0)
        last0->next = current;
        last0 = current;
        last0->next = null;
        last1->next = current;
        last1 = current;
        last1->next = null;
    current = next;
last0->next = ones;
current = zeros;

For arrays, we probably need an array of size n/2. O(n)

Bucket sort, revisited

Analyzing bucket sort