Program: DETAB Assignment 1

    • Code Run. Run, Code, RUN!
PLEASE!!!!

— /usr/games/fortune1

Due by 11:59pm, Wednesday, September 28th.

This assignment is to be done individually.
See the Instructions for Homework Submission for information on how to use handin.


Program: DETAB

(This is exercise 1-20 in Kernighan and Ritchie with more explanation.)

Write a program called detab that replaces tabs in its input with the proper number of blanks to space to the next tab stop. For the purpose of this assignment, assume tab stops are every 8 spaces.

Your program should work as a filter, taking its input from the standard input (stdin) and writing its output to the standard output (stdout).

This program should do the same thing as expand(1), so you can test your program by comparing its output to the output of expand on the same file, as in the following example. (If you are not familiar with IO redirection, see the FAQ entry).

diff(1) is a program that reports differences between two files. If it says nothing, the files are the same.

    • gcc -o detab -Wall detab.c

    • ./detab < TestInput > detab.out

    • ~pn-cs357/demos/detab < TestInput > reference.out

    • diff detab.out reference.out

%


    1 Berkeley Unix (BSD) distrubutions have traditionally included a collection of games stored in /usr/games. for-tune(1) prints a randomly selected adage ranging from the humorous to the truly strange. Sadly, these days, many system administrators choose not to install /usr/games in order to save space or to encourage the doing of actual work.





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Since diff(1) reported no differences, the files must be the same. If there had been differences diff would have displayed the lines that differ between the two files.

In the compile-line above, I used two switches for gcc. The first, -o detab, tells the compiler to name the output “detab” rather than “a.out”. The second switch, -Wall, turns on verbose warnings for “shaky” C programming practices. This catches many common programming errors. All the code you write in this class should compile cleanly with -Wall turned on.

Further Discussion

detab is supposed to read text from its input (stdin) and copy it to its output (stdout), expanding tabs as it goes. Some confusion tends to arise when it comes to the meaning of “expanding tabs.” A tab character causes the cursor to move to the next tab-stop. For this program, tab stops are defined to be every eight columns, so a tab will move the cursor from wherever it is to the next column that is a multiple of 8 (if you start counting from 0, or 8n + 1 if you start at 1). The result

is that, depending on where it is typed, a tab can expand to anywhere between 1 and 8 spaces. Because both tabs and spaces are invisible, it’s hard to give an example here, but if we write tab
as “⊲” and blank spaces as “ ”, you can see the effect of detab in Figure 1.

Input    After Expansion
⊲aftertab    aftertab
0⊲aftertab    0aftertab
01⊲aftertab    01aftertab
012⊲aftertab    012aftertab
0123⊲aftertab    0123aftertab
01234⊲aftertab    01234aftertab
012345⊲aftertab    012345aftertab
0123456⊲aftertab    0123456aftertab
01234567⊲aftertab    01234567aftertab


Figure 1: detab in action

If you have any doubt about what your program is doing relative to what it should do, check its output against the output of /usr/bin/expand on the same input file. They should do the same thing.2

Tricks and Tools

Remember, this assignment is fairly straightforward, but it there are a few tricky edge cases to watch out for. (E.g., one should not be able to backspace beyond the left margin.) Pay close attention to these. Also, be careful not to impose artificial limits on the program’s functionality. There is no reason, for example, to limit either line length or file size. There are a few library routines that may help with implementing detab listed in Figure 2.

Note also that any character that changes the column could have an effect on your tab-stop calculations. Other than “ordinary” letters, the ones of note are tab (’\t’), backspace (’\b’), newline (’\n’), and carriage return (’\r’).


2NOTE: the behavior of expand(1) appears to have changed with respect to carriage return (’\r’). Compare your output to that from my demo version, ~pn-cs357/demos/detab.




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getchar(3)

read a character from stdin



putchar(3)

write a character to stdout



getc(3)

read a character from the specified stream



putc(3)

write a character to the specified stream



gets(3)

never use this function. Not ever.



feof(3)

test to see if a given stream has encountered an end of file




Figure 2: Some potentially useful library functions

Coding Standards

See the pages on coding standards on the cpe 357 class web page.


What to turn in

Submit via handin in the CSL to the Asgn1 directory of the pn-cs357 account:

    • Your well-documented source file, called detab.c.

    • A README file that contains:

– Your name, including your login name(s) in parentheses (e.g. “(pnico)”).

– Any special instructions for running your program.

– Any other thing you want me to know while I am grading it.

The README file should be plain text, i.e, not a Word document, and should be named “README”, all capitals with no extension.

Sample runs

I would generally put some sample runs below, but it’d be silly because in printed form the tabs and spaces would look exactly the same. I will place an executable version of detab in ~pn-cs357/demos so you can run it yourself and compare your output with diff(1).

I have also published a partial test harness in ~pn-cs357/demos/tryAsgn1. When you run this, it will try to compile your program and try running it on a few sample inputs.

Also, and this comes with absolutely zero support, tryAsgn1 is just a shell script meaning you can read it. Why would you want to beyond curiosity? It could lead you to the path where the actual test inputs are.












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