There are many circumstances where you are working on a program but have access to a working (or nearly working) executable.

• In the real world, very few programming assignments are to write something new from scratch. Much more often, you will be adding new features to an existing program, or fixing bugs in an existing program, or trying to improve the performance of an existing program.

• Even if you are developing a whole new program, there may be an older one already in use that is being replaced because it is in a older programming language that no one in the company cares to maintain, or runs only on older hardware or operating systems that are due for replacement, etc.

• And in the academic world, you will often find that instructors[Like me!](#ret-Like me!): provide executable solutions to assignments so that you can see what the finished product of an assignment is supposed to look like.

Whenever you find yourself working on a system for which you already have a (nearly) working executable, you can take advantage of this by doing head to head testing, in which you

1. Read the problem description below.

• You will find files for this lab here.

  Unpack the archive (unzip head2headFiles.zip).

  Compile the program on Linux (use the command “make”). The executable should be named triangle. Rename it to triangleA.

• Now introduce a simple bug of some kind into triangle.cpp. For example, swap the “else if” and output statements for the isosceles and equilateral cases in report().

  Recompile the program and rename the executable to triangleB.

• Run the input files provided through each version of the program. To start with, try running

  ```sh ./triangleA < test0.in

  ```

  just to be sure things are working.

  We saw the use of < to redirect input so we would not have to type everything manually in a much earlier lab.

  We can also use > to redirect output into a file that otherwise would have gone to our screen. So run

  ./triangleA < test0.in > test0A.out
  ./triangleA < test1.in > test1A.out
  ./triangleB < test0.in > test0B.out
  ./triangleB < test1.in > test1B.out
  
  

  You now have four output files. Inspect these with more or an editor.

• Next we are going to explore some of the common tools for quickly and automatically comparing output files head-to-head to see if any differences exist and, if so, what those differences are.

  Unfortunately, although “diff” tools like these are widely available, they are not a standard part of Windows distributions. They are, however, available in just about any Unix or Linux system.

  • The most basic of the comparison tools is diff. Try running

    diff test0A.out test0A.out
    
    

    Now, since we are comparing a file against itself, obviously there are no differences to be reported. Then try

    diff test0A.out test0B.out
    diff test1A.out test1B.out
    

    diff gives you a very quick report on the differences, if any, between two files.

    The behavior of diff can be tweaked by various command options. For example,

    diff -i test0A.out test0B.out
    
    

    will ignore differences in upper and lower-case characters when comparing the two files. The option -w causes diff to ignore differences in white space (blanks and tabs).

• diff is pretty straightforward, but the output isn’t particularly pretty. Try this next:

  sdiff test0A.out test0B.out | more
  sdiff test1A.out test1B.out | more
  

  (Hit ‘q’ to exit the “more” program.)

  sdiff shows the two files “side by side”. You may find it useful to make the window for your ssh session a bit wider when using this program. Lines that match exactly are shown side by side with no marker in between them. Lines that appear to have been changed have a ‘|’. Lines that appear to have been added or removed from one file but not the other are marked with a ‘<’ or ‘>’ pointing at the “extra” line.

• If you have progressed in CS252 as far as the exercises on X, you might want to try the emacs in X assignment next. As part of that, you will be using a more interactive difference viewer in emacs that steps from one change to another, highlighting the differences in color.

• Although this lab has focused on programs that you would use in a Unix or Linux environment, you can perform head-to-head testing in Windows environments as well. Even if your program writes to standard output, you can use output redirection in a Windows cmd window to capture the output into a file, just as you uses input redirection to a program in the earlier “Supplying Inputs to Programs” lab.

  If you are seated as a CS Dept lab Windows PC, run the program winmerge. (If you are not in a CS lab, use remote desktop to visit the CS Virtual PC lab and run winmerge there.

  From the File menu, select “Open.”. Use the “Left” and “Right” controls to select the two files that you want to compare, the test0A.out and test0B.out files that you created in your Linux ssh session. ( These should be available in your Z: drive.)

  If all you get is a simple report saying that the files are “different”, try selecting “Expand” from the View menu.

• Repeat the winmerge process with the test1A.out and test1B.out files

Winmerge is quite a useful program. You can get your own copy at WinMerge. It is also available in a portable version that can be installed on a USB flashdrive and executed on almost any Windows machine).]