Lab 3: Structs and strings

The goal of this lab is to learn the basics about strings and structs in C.

1. Introduction

You are writing software for a Twitter knock-off called Chatter. In the Chatter system, users send messages at most 140 characters long. Additionally, each message has the user id of the user who sent the message (at most 16 characters long), a time stamp indicating the time the message was sent (a long int indicating the number of milliseconds since midnight, Jan 1, 1970), and up to 5 hash codes indicating categories or topics the message falls under (each hash code is at most 10 characters long).

You have been provided some code for producing "random" text components and "random" user names. Your tasks will include designing and implementing a struct to represent messages, writing functions for displaying and sorting messages, and completing drivers to test these messages.

2. Set up

Set up a new directory for this lab and move into it.

mkdir lab3
cd lab3

Copy starter code from the course directory.

cp /cslab/class/csci245/lab3/* .

3. Strings in C

In order to do this lab, you need to know a little more about how strings work in C. Recall that there is not really a string type (in the style of Java class String); all that we have in C are:

1. a way to write string literals (with which you are already familiar), and
2. library functions that can often treat an array of char as a string.

To use an array of characters as a string, it has to contain a special character to mark its end; that character has the integer value 0, which as a char would be written '\0'. The historic name for that character is NUL, though it is more commonly spelled null, and so these strings are usually described as null-terminated.

When you write a string literal the array value created for it includes the terminating zero; so, for example, the literal "foo" specifies an array of four characters: 'f', 'o', 'o', '\0'. So when we talk about a string of length 7 (or containing 7 characters), that requires a char array of size 8.

There are library routines that let you do useful things with strings; they are declared in the include file string.h. The ones you might find useful in this lab are

• int strcmp(char a[], char b[])
  which returns a value less than, equal to, or greater than 0 depending on whether a is less than, equal to, or greater than b.
• strcpy(char dest[], char src[])
  which copies the string value from src to dest. (For the time being, you can ignore the fact that this function does return a value.)
• int strlen(char s[])
  returns the number of characters in s, not including the terminating 0.

There are a few things that require you to be very careful:

1. strcpy() does not make any space for the destination. You must have already declared an array that is large enough to hold the one being copied--including the terminating 0. If you try to copy a string to an array that is too small, you will make a mess of your program.
2. When you are declaring character arrays to hold string values, you must include an extra space for the terminating 0.
3. The library functions all assume that you pass them properly terminated strings, but when you declare a character array, you get no promise about what is in it. Be sure that you initialize them properly. Wierd things can happen when you call library functions with arrays that are not properly null-terminated.
4. When you are declaring character arrays to hold string values, you must include an extra space for the terminating 0.
5. When you are declaring character arrays to hold string values, you must include an extra space for the terminating 0.

Note that because strings are character arrays, you can manipulate the individual characters. That means that you can make the string in a character into a zero-length string (an empty string) by assigning a '\0' to element 0.

Keep in mind that you cannot use the comparison operators (such as > or ==) on string values. Also, remember that strcmp() is not a comparison for equality; if you want to test whether two strings are equal, you have to something like

strcmp(a, b) == 0

You will find more information about strings are in chapter 4 of On to C.

4. Provided code

The chatterUtil library provides the following functions:

• randomInt(): produce a random int between 0 (inclusive) and a given int (exclusive). So randomInt(4) will return 0, 1, 2, or 3, with equal probability.
• getTimeMillis(): return the current time
• fillRandomText(): fill the given char array with a string containing a randomly chosen text for a chatter message
• fillRandomUserName(): fill the given char array with a string containing a randomly chosen text for a user name
• fillRandomHashCode() fill the given char array with a string containing a randomly chosen hash code

5. Tasks

You need to complete the following tasks:

• Design and implement the struct to represent messages (int message.h). I have already set up a dummy struct for a struct message type. You need to complete that struct according to the description of a message given above. Since the struct type needs to be seen by the library and the drivers, it is defined in message.h
• Make a new random message (in message.c). The header file message.h gives the prototype for the function newRandomMessage() which is used by the two driver programs. Write a definition for this function in an implementation file for the message library. Use the functions from chatterUtil to produce the text, user name, and hash codes. Use randomInt() to determine the number of hash codes.
• Display a message (in message.c). The headerfile message.h gives the prototype for the function display() which displays a message to the screen, nicely formatted, showing all the information stored about that message. The driver displayDriver tests this (as well as your newRandomMessage). The display of a message should look something like this:

  -----------------------
  Sent by Verna at 1315499047608
  Q: How can you tell if an elephant is in the refrigerator?
  A: The door won't shut.
  #mischief
  #concert
  #admiration
  #concert
  

  If you want the date to appear as something nice looking instead of a number of milliseconds, see below.
• Sort messages by username (in message.c). The headerfile message.h gives the prototype for the function sortByUserName, and the driver sortDriver.c uses it. This function must be completed so that it sorts an array of messages by user name, alphabetically. You may use any sorting algorithm for this. What will be different is that you are sorting strings, not numbers. To do this alphabetically, first look at the first character of each string---remember that characters are represented by numbers, and the numeric code for characters is consistent with their alphabetical ordering. If the first character is a tie, then look to the second character, etc. Note that sortByUserName() also needs the size of the array as a parameter.
• Finish the sortDriver. The sortDriver program is intended to display all the usernames in the array of messages after sorting, to search the array for a message containing a given hashcode, and then display those messages. You may choose the hashcode to search for. Finish the driver so that it exercises these things. (You may add appropriate functions to the message library if you think it will make the task easier.) Edit your makefile to handle sortDriver.o and sortDriver.

6. Pretty displaying of the time (optional)

If you want to make a readable formatting of the time stamp, do the following things to your code:

• Include sys/time.h in message.h
• Include time.h in message.c
• Make your timestamp in the message struct to be of type time_t instead of long int
• In newRandomMessage(), don't use the getTimeMillis() function that I provided but instead use the function time(). Pass it the timestamp portion of the struct preceded by an ampersand. For example, time(& (toReturn.timestamp));
• In display(), use the function ctime() to convert time to a string. For example,

  printf("Sent by %s at %ld\n", msg.uid, ctime(&(msg.timestamp)));
  

Then the display of a message would look something like

-----------------------
Sent by Willis at Thu Sep  8 12:32:37 2011

Q: Why do elephants stomp on people?
A: They like the squishy feeling between their toes.
#surplus
#moisture

7. Handing in

Be sure that your names are in all of the files you modified. Turn in your files with

/cslab/class/csci245/bin/handin lab3 makefile message.h message.c sortDriver.c