CS210 Lab: Libraries and Running Time Measurement


Prelab Questions:

For a review of relevant topics click here.

Highlights of This Lab:

Lab Exercise:


Click the little computer above for a detailed description.
In the exercise, the students can add the library, which has been created in this lab, to existing code to test the runtime of three different search algorithms and three different sorting algorithms.

1. Running Time Measurements

A routine's performance can be judged in many ways. In CS210, you have been/will be learning Big Oh. The idea is that you can estimate the routine's projected execution times as a function of the number of data items (N) that it manipulates as it performs its task. The results are estimates of the form O(N), O(LogN) and so on.

Big Oh allows us to group routines based on their projected performance under different conditions (best case, worst case, etc). Remember that Big Oh is only an estimate. It does not take into account factors specific to a particular environment, such the specific implementation, the type of computer system we are using, and the data that we are processing.

To determine how well or poorly a routine will perform in a particular environment, we need to evaluate the routine in that environment.

In "Part 1" of the laboratory exercise, you will measure the performance of three search routines:

The first two search routines were discussed in CS170. For a review, click
here

In summary, for the binary search, you are given a sorted array. The idea is to divide the array into two equal parts and narrow down the range of the array to be searched either to the low part of the array or high part of the array. We will continue narrowing down the range until the value is located, or until we realize that the value searched for is not in the array.

The linear search is also known as the sequential search. The idea is start with the first element, and sequentially search through the array until we find a match.

Before we can measure the performance of these routines, we must first develop a set of tools that allow us to measure the execution time.

The general approach will be the following:

If the routine executes very rapidly, then the difference between startTime and stopTime may be too small for your computer system to measure. If that is the case, we can execute the routine several times (say n) and then divide the resulting time by the number of repetions (n) as follows: To capture the startTime and stopTime, we will use a class called Timer which has the following member functions: To add some complexity to this lab, we will turn this Timer class into a library and use this library for the lab exercise at the end.

2. Creating a Library and Using it

This section is broken into four subsections:

2.1 Testing that a potential library works

We are going to build a library out of the Timer class, but first we want to make sure that it will work. We will download the Timer class and a sample program just to try it out
  1. Download the C++ programs Ftp the files from /pub/class/210/ftp/Library

    The three files are :

  2. complexity.cpp:
  3. This is the main program.
  4. timer.cpp:
  5. This is the implementation of the Timer module.  It can be compiled into a library once it is bug-free.
  6. timer.h:
  7. This is the header file of the Timer module

    Store the three files on your local PC:

       D:\Workarea

  8. Compile the C++ programs

    Using the procedures discussed in Lab 1 do the following:

  9. Let's have a look at this code:

2.2 Definition of a library

  1. What is a library?

    A library is a collection of functions or classes. Unlike an object file, a library file stores each function or class, individually. When your program references a function or class contained in a library, it links that function or class and adds its code to your program. This way only functions or classes that you actually use in your program are added to the executable file.

  2. What are header files?

    Each function or class defined in the C or C++ standard library has a header file associated with it. The header files that relate to the functions that you use in your programs should be included (using #include) in your program.
    There are two reasons for this:

    1. to get the data types that work with many functions or classes in the standard library. Your program must have access to these data types that are defined in the header file related to each function or class
    2. to obtain the prototypes for the standard library functions

  3. Why create your own Library?

    Libraries can be regarded as pre-compiled functions or classes that do not need to be re-compiled if other parts of your project change. If you have a "bug-free" module (a piece of self-contained code) that could be reused with other projects, you can compile this module to a library. After you link this library to your new project, you will then be able to use any of the functions or classes in this library as if they are defined locally in your project.

    One major advantage is that the functions or classes in the library will not be recompiled, which saves time compiling your entire project.

    In our example, "timer" is a perfect candidate: it does not contain any bugs and we can use it again in other projects.

2.3 Creating a library

In this program, timer.h and timer.cpp define and implement a time measuring class Timer. This class is independent of the kind of program it is measuring. In other words, we can reuse this Timer class to measure the running time of other programs. It naturally follows that once we make this class bug-free, we can compile it into a library.  To use this time-measuring class, we simply include the header file (i.e.timer.h) in the program and link the library to this program.

The following shows you how we do this:  

Step 1. Create the library

Step 2. Add the necessary files to mylibrary and build the library

Step 3. Build mylibrary 

The following message should be displayed in the output area:

------ Build started: Project: mylibrary, Configuration: Debug Win32 ------

Compiling...
timer.cpp
Creating library...

Build log was saved at "file://d:\Workarea\mylibrary\Debug\BuildLog.htm"
mylibrary - 0 error(s), 0 warning(s)

---------------------- Done ----------------------

    Build: 1 succeeded, 0 failed, 0 skipped

Let's just stop for a second. We've created a libray with timer.cpp and timer.h. The library is stored in a file called mylibrary.lib, under the D:\Workarea\mylibrary\Debug folder.

We've got this library, but what do we do with it? It has member functions and data members, but we aren't making use of them--we are just defining them.

2.4 Using the library

Step 4. Link the library to your project

You now have to do two things to add the library that you created to the "link line".
  1. Add mylibrary.lib to the Project Properties

  2. Add the corresponding path (or location of this library) to the project. The path is used to locate the library files when the VC++ project is being built.


You should see the following output if you have followed the above steps correctly.
Select Build->Build lab4

------ Build started: Project: lab4, Configuration: Debug Win32 ------

Compiling...
complexity.cpp
Linking...

Build log was saved at "file://d:\Workarea\lab4\Debug\BuildLog.htm"
lab4 - 0 error(s), 0 warning(s)

---------------------- Done ----------------------

    Build: 1 succeeded, 0 failed, 0 skipped

Finally, click on Debug->Start Without Debugging to run your program.
Your output should look like the following:

 

 


3. Lab Exercise

There are two parts to this exercise. In the first part, you will plot the execution times of three searching routines. In the second part, you will plot the execution times of three sorting routines.

Part 1

Part 2


4. Postlab Exercises

For postlab exercices, click
here.

This page last modified by Nova Scheidt:
Thursday, 27-Oct-2016 16:01:45 CST
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