1. ENEE150 Discussion 09
Section 0101
Adam Wang

2. Overview
Hash tables and Trees
Project 3
Quiz

3. Hash Tables An array of linked lists (called buckets)
Each piece of data has a hash code
This code gets mapped to an index in the array
The entry gets added to the linked list
Ideally, each linked list has very few nodes
Very fast for looking up data pieces quickly
Other variations: hashmap, treemap, linkedhashmap

4. Trees Like linked lists, except each node now has two pointers
Binary Search tree: Every child to the left is less than the parent, and every child to the right is greater than the parent
When searching for an element, we only have to search on average log(N) nodes
We’ll see later that it’s natural to use recursion for trees

5. Project 3: Bus Simulator
Simulates a bus moving through bus stops and adding/removing passengers
The program will take in a passengers file and a log file
The passenger file contains all the data about when each passenger will come and which bus stop they will get on/get off
You will write to the log file who gets on/gets off at each time step
You will use a counter to simulate time moving; think of it as printing out what has happened during each hour of the simulation

6. Sample passenger file First number is number of bus stops
Each line after that represents a passenger
1st number: arrival time (“arrival hour”)
Ascending order since it’s a simulation
2nd number: Starting bus stop
3rd number: Destination bus stop
Bus only goes forwards; some passengers have to go all the way around

7. Recommended data structures
First thing you’ll do is malloc an array of bus stops
You know the size from the first number in the passengers file
Then you’ll initialize the bus
This doesn’t need to be malloc’d
At each time step you’ll add/remove passengers to/from the bus/bus stop

8. Global variables you might want to use

9. Recommended methods

10. In main Open the passenger and log files to be read and written
argv[1] is the passenger file and argv[2] is the log file; “r” for read and “w” for write
Read in number of bus stops and initialize array of bus stops, as well as the bus
Each bus stop you have to initialize the ID as well
While Loop: runs until done reading passenger file and no more passengers in the ENTIRE system
Each iteration of the loop simulates one timestep/one “hour”
When done, remember to print stats of the simulation

11. Inside while loop (continued)
Print to log file the status of each bus stop
Use method fprintf
1st time step won’t have any passengers
Read in new passengers from file that are arriving at this time
Use fscanf
The FILE* will recall where you last left off reading it at
Remove any passengers getting off at this stop (disembark_bus)
Add any passengers at this stop getting on (board_bus)
Increase wait times of passengers at all the bus stops (age_passengers)
Move bus to the next station
Increase time counter

12. void initialize_system()
Malloc the array of bus stops
Initialize the bus
Remember to initialize all the fields to 0/NULL
Each bus stop also has a unique ID representing its stop #

13. int get_passengers(FILE *inFile)
Reads in any passengers arriving at the current time step
May be more than one, or none at all!
For each passenger, malloc and initialize each of its fields
Insert that passenger into the proper bus stop (insert_passenger)
Increment total passengers, # waiting at that station, etc.
Return value: whether any new passengers were read in
Obviously this method can return whatever you want it to

14. void disembark_bus()
Remove any passengers on the bus getting off at the current bus stop
Iterate through passenger linked list
If passenger’s destination is this stop,
Remove from the list (delete_passenger)
Decrement the bus's number of passengers
Decrement total number of passengers
Record wait time of passenger

15. void board_bus()
Adds all passengers waiting at the current bus stop to the bus
Look up list concatenation
Remember: we can just copy over pointers, because memory has already been allocated
We also need to increment # of passengers on the bus, and set number of passengers at bus stop to 0

16. void age_passengers()
Increment wait time of all the passengers waiting at each bus stop
Simple loop for each bus stop, and loop for each passenger

17. void write_stops (FILE *logFile)
Writes the status of each bus stop to the log file
Remember, the format has to match EXACTLY what’s in the expected output

18. Void insert_passenger (passenger **head, passenger *to_add)
Inserts “to_add” into the linked list “head”
Make this easy for you: just insert at the head

19. Void delete_passenger (passenger **head, passenger *to_del)
Deletes “to_del” from the linked list
You don’t have to follow this format; use the delete method from class if you want
You don’t even need this separate method at all

20. void write_stats(FILE *logFile)
Writes final stats of the simulation
Make use of global variables sim_time, total_passengers, etc. for this

21. Extra
Only the passenger linked lists and the array of bus stops needs to be dynamically allocated
Up to you what to do with the bus and each bus stop
Lucky for you, you are not required to free any memory
Add to the head of the linked list whenever you can (easy)
Hardest part is just keeping track of all the variables

22. Quiz wget ece.umd.edu/~aw97/quizzes/quiz3.c
Dynamically allocate a struct and insert it into the tail of a linked list
submit 2017 fall enee quiz3.c