Lab 3: Structs

The goal of this lab is to learn basic C structs.

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).

I have 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 /homes/tvandrun/Public/cs245/lab3/* .

3. Given 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

4. 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.

5. 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)));

-----------------------
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