About Time and Space Complexity

You did not have much of a chance to analyze the algorithm you used in the first programming assignment this semester. And the problem you had to solve was a small one, you had to find the minimum spanning tree of a connected, weighted graph, with only ten vertices.

It's now time to ask yourself, `What if?...' What if I had to solve the same problem for a graph with ten thousand vertices? What if there were 1 million vertices? How is the running time of my program effected by the number of vertices in the graph? What about the memory my program requires?

This assignment gives you a chance to find answers to these questions and to better understand what time and space complexity of an algorithm means.

A description of what you have to do is included in a memo from your instructor. Some hints about how to do the experimental part are also included.

What you'll turn in

  1. A well commented listing of your program which includes the changes you've done to enable measuring running time. Grading, as described below, considers both the functionality and the readability of your code. You may also want to have a look at a grading sheet used in the past for a similar assignment. If you are unsure about what the documentation of a program means, then you may want to read this document.
  2. A floppy disk (3.5") which contains the source file(s) for your program, a DOS executable named acme_eps and a README file indicating what the program does, how to build the executable, the platform it's been tested on, and how to run it, plus any other information you consider useful - like the name of the author, etc. Also include two input files (use the extension .din for the name), one describing a graph with 10,000 vertices and another one with 20,000 nodes, and the spreadsheed (Microsoft Excel)used to record the running times for various input sizes.
  3. A memo describing your work. You will indicate what you have looked for and how you solved the problem. Direct the memo to your class instructor. A hardcopy of the program will be attached to the memo. Also attach tables with the running times for various input sizes, and the graph(s) that shows the running time versus the input size.


  1. A mark between 0 and 10 for functionality. The mark basically indicates in what measure your program works properly and how well you have followed the initial specification.
  2. A mark between 0 and 10 for readability. This mark indicates how well documented your program is; it also considers the general appearance of your final report.
  3. Multiply the above two marks to get the mark for this programming assignment.

For each business day you turn in your project earlier you receive a 5% bonus. However, penalties increase by 5% as well. You can turn in your assignment up to ten business days ahead of the deadline.

You may be asked to do a code review with your instructor.


  1. Use the the clock() function that comes with timer.h
  2. Don't turn in running times of zero! They do not mean we can find the minimum spanning tree of a graph in no time (though we'd like to): the fact is caused by the granularity of the computer's timer. If processing is very fast we may not accumulate at least one CLK_TCK of time. The solution is to run the program with an input that's large enough for the running time to be at least a few CLK_TCK.
  3. Keep in mind that the program you run has three major parts: Don't include the reading of the graph description and the print statements in the timing. The reason is that in reality the graph description comes from a file (through input redirection), and the output goes into a file (through output redirection), and reading/writing from/to a file is a highly system dependent operation. This does not mean that we can ignore the time it takes to do input/output, it just means it is hard to get accurate and meaningful readings of these values. And it may well be the case that the input and output operations actually determine the running time of your program.

    Therefore the time complexity you will determine experimentally, will be the time complexity of the spanning tree algorithm, and not the time complexity for the whole program.

Department of
Computer Science
Applied Mathematics

DATE:March 26, 1997
TO:CS-330 students
FROM:Your instructor
SUBJECT:Space and Time Complexity

Dear Student,

It just occured to me that the first programming assignment did not call for any analysis of the algorithm you used to solve the problem for the ACME - ExpertSnowPlowers. And the problem was small enough for the running time and the storage needed of your program to not be a problem.

Your program proved to be very successful, to the point that people call asking about how they can get a hold of your program and whether your program can handle their problems, time and space-wise.

I ask you to analyze the implementation of the algorithm you used in the first programming assignment and find its time and space complexity. This is the theoretical part of the problem.

I also ask you to find the time complexity of the algorithm using an experimental approach, i.e. by measuring the running time of your program for various input sizes.

All these findings will be included with future releases of your software.

Virgil Bistriceanu

(c) 1997, Virgil Bistriceanu - virgil@csam.iit.edu
Posted: March 26, 1997. Last update: March 26, 1997