CS240 Spring 2011: Lab03

Arrays and String Manipulation

Topics Covered:

Command line arguments, loops, conditional statements, one dimensional arrays, string manipulation (copy, concatenate, compare, reverse), functions, pointers, recursion, passing arrays to functions, redirection.

Task 0: Downloading the Initial Source Files

Download the file lab3-src.tar and untar it by typing the command:

            tar -xvf lab3-src.tar

The directory lab3-src contains the initial source files as well as the script testall that will test all your programs. To build your programs type "make" inside the directory lab3-src ("Makefile" is provided to you). Make sure you compile and test your programs in lore.cs.purdue.edu.

Note: If you get a permission denied error, type the following in your lab3-src folder:

            chmod 777 *.*
            chmod 777 testall

Task 1: Reverse a String using Recursion

Write a program to reverse a string, entered as a command line argument, recursively. The format of executing your program should be:

            reverse <string>

E.g. reverse Purdue University

The output should be:

            Reverse of String = ytisrevinU eudruP

Note that the above example contains three arguments. You can verify the count using the value of "argc". These three arguments are as follows:

            argv[0] = "reverse"
            argv[1] = "Purdue"
            argv[2] = "University"

Since you have to reverse the complete string "Purdue University", you must first concatenate the arguments together (except "reverse"). You can use the inbuilt string function strcat() for concatenating two strings.

Using strcat():

The strcat() function appends the source string to the destination string, overwriting the null byte ('\0') at the end of destination string, and then adds a terminating  null  byte. The destination string must have enough space for the result.

"string.h" header file must be included before using the string functions.

E.g.

            #include <stdio.h>
            #include <string.h>

            int main(int argc, char ** argv){

                char destination[50], source[50];

                strcpy(source, "Lab"); // copies the String "Lab" in "source" character array
                strcpy(destination, "CS240"); // copies the String "CS240" in "destination" character array

                strcat(destination, " "); // appends a space " " after the string stored in "destination" (which is CS240) and stores the result back to "destination"
                strcat(destination, source); // appends the string stored in "source" (which is Lab) after the string stored in "destination" and stores the result back to "destination"

                printf("%s\n", destination);
            }

The output of the above program is : CS240 Lab

Use man pages for finding more information about the usage of different functions. E.g. man strcat

Now you can make a complete string using the arguments entered at the command line. You have to reverse this string using recursion. The method of solving a problem using recursion involves a function that calls itself again and again till the base case is satisfied. E.g. Factorial of a number can be found out using recursion - fact(5) = 5 * fact(4) = 5*4*fact(3)... and so on.

A file named "reverse.c" has been provided to you which contains the skeleton of the program. The reverse () function, specified in the file,  takes a string as an argument. This function must be called recursively to display the reverse of the string. You must also perform an input validation check which should ensure that the correct number of arguments are passed to the program; if not then it must display an appropriate message on the screen and exit the program:

            Usage: reverse <string>

There is a file named "reverse.org" in your lab3-src folder which is an implementation of this program. Please try to execute this reference program to understand the expected behavior. A "testall" script is given to you to let you know if your program's implementation is correct. To run testall type:

            testall

Few examples for using "reverse.org" are:

Ex.1
            Input:
                        reverse.org
            Output:
                        Usage : reverse <string>

Ex 2.
            Input:
                        reverse.org CS240
            Output:
                        Reverse of String = 042SC

Task 2: Substring Search

Write a program to display all lines in a given file which contains a given string. The format of executing your program should be:

            mygrep {String to Search} <{File Name}

E.g. mygrep CS240 <input.txt

Here, "CS240" is the string to be searched in the file named "input.txt". The program must display all the lines which contain the word "CS240". If the contents of the "input.txt" file are:

            This is CS240 Lab.
            CS240 Spring 2011.
            Purdue University, West Lafayette, Indiana
            Indiana

Then the output should be:

            This is CS240 Lab.
            CS240 Spring 2011.

The symbol "<" is used for redirecting the standard input stream to a file. So, instead of reading the input from the command prompt, the program will take input from the file specified after "<". Also, notice that the mygrep command is case sensitive (Purdue and purdue are different). The program is named "mygrep" as it imitates the behavior of the "grep" command line text search utility for UNIX. Use function getchar to read the input file (one character at a time). Recall that each line in a text file is terminated by the newline ('\n') character. A file named "mygrep.c" has been provided to you which contains the skeleton of the program. You have to implement the isPresent () function which takes the "mainString" and "subString" as arguments and returns a 1 if the "mainString" contains the "subString", else it returns 0. You must also perform an input validation check which should ensure that the correct number of arguments are passed to the program; if not then it must display an appropriate message and exit the program:

            Usage : mygrep {String to Search} <{File Name}

There is a file named "mygrep.org" in your lab3-src folder which is an implementation of this program. Please try to execute this reference program to understand the expected behavior. A "testall" script is given to you to let you know if your program's implementation is correct. To run testall type:

            testall

Few examples for using "mygrep.org" are:

Ex.1
            Input:
                        mygrep.org
            Output:
                        Usage : mygrep {String to Search} <{File Name}

Ex 2.
            Input:
                        mygrep.org Purdue <input.txt
            Output:
                        Purdue University, West Lafayette.

The script testall

A script testall has been provided to test your project. Make sure you run this script since it will be used to grade your project.

Note: We will change the test cases to ensure that your programs do work for all possible test cases and not just the test cases that have been mentioned in the testall provided to you.

Submit

Follow these instructions to turnin lab3:
  1. Login to lore.cs.purdue.edu
  2. Run "make clean" in your lab3-src folder.
  3. Change to the parent directory above lab3-src.
  4. Type "turnin -v -c cs240=XXX -p lab03 lab3-src". Replace XXX with your section number:

    9:30 am - 11:20 am FF930
    11:30 am - 1:20 pm FF1130
    1:30 pm - 3:20 pm FF130
    3:30 pm - 5:20 pm FF330
    9:30 am - 11:20 am RR930
    11:30 am - 1:20 pm RR1130
    3:30 pm - 5:20 pm RR330
    11:30 am - 1:20 pm TT1130

  5. Type "turnin -c cs240=XXX -p lab03 -v" to make sure you have submitted the right files.
The deadline for this lab is Wednesday, February 9th, 11:59 PM