CS 106X – Programming Abstractions in C++Cynthia Bailey Lee

              CS2 in C++ Peer Instruction

Materials by Cynthia Bailey Lee is licensed under a 

Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

Permissions beyond the scope of this license may be available at 

http://peerinstruction4cs.org.

2

Today’s Topics1. Queues contrasted with Stacks2. Queues example: Mouse events3. Queue example: Josephus Survivor

Puzzle4. Iterating using range-based for loop

Queues

Stacks vs. Queues Stack is referred to as a LIFO data structure

“Last in, first out” Queue is referred to as a FIFO data structure

“First in, first out”

How many of these operate as FIFO/Queue? Patients waiting in a hospital Emergency Room Passengers boarding and exiting an airplane Passengers boarding and exiting an elevator (assume all

board on ground floor and exit at the same destination) People waiting in line for an amusement park ride

(A) None (B) 1 (C) 2 (D) All 3

LIFO:

FIFO:

Image is modified by Cynthia Lee based on original from Wikimedia Commons “A seatmap over the Airbus A380” Source=self-made |Date=24 September 2007 |Author= S. Solberg J. Used under Creative Commons License

Queue ApplicationsMouse events

Event queues While your code executes, a separate program is

constantly listening for events and recording them Mouse moved, mouse clicked, mouse dragged,

keyboard key pressed, etc Every once in a while, your code can call

getNextEvent() to see what has happened getNextEvent returns the events one at a time in the

same order they happened In other words, returns them in FIFO order! When it is “recording” events, it is enqueuing

events in an event QUEUE Very common use of the Queue ADT

Queue ApplicationsJosephus Survivor Puzzle

Josephus Survivor Puzzle N people seated in a circle Skip M living people and kill

the (M+1)th, repeatedly around the circle, until two remain

Question:Where do you sit?

12

3

4

56

789

10

11

12

Josephus Survivor Puzzle Question: (you will need paper & pencil)

N = 12, M = 3Where do you sit?A. 1 or 2B. 1 or 6C. 5 or 10D. 5 or 6E. Other/none/more

12

3

4

56

789

10

11

12

Queues and Josephus Puzzle1. Enqueue everybody2. Dequeue and immediately re-enqueue M people. 3. Dequeue somebody forever. If more than two

alive, repeat steps 2&3

N = 5, M = 2

1. 2. 3. 2. 3. 2. 3.

?

Queues and Josephus Puzzle1. Enqueue everybody2. Dequeue and immediately re-enqueue M people. 3. Dequeue somebody forever. 4. If more than two alive, repeat steps 2&3

N = 5, M = 2

12345

Contents of the queue, reading from top down:A. 1, 2, 4B. 2, 4, 5C. 2, 4, 5, 1D. 4, 5, 1, 2E. Other

4512

1245

?

1. 2. 3. 2. 3. 2.34512

3.

Queues and Josephus Puzzle:What does that look like in code?

1. Enqueue everybody2. Dequeue and immediately re-enqueue M people. 3. Dequeue somebody forever 4. If more than two alive, repeat steps 2&3

Iterating over a collection

C++11 has a nice for loop just for collections ADTs Range-based for loop gives you each item in a

collection, one at a time For ADTs with a clear linear order (Vector,

Stack, Queue), they are given to you in that order

[Spoiler for Wednesday!] Some ADTs are not linear. In theory, we often consider the ordering of iteration on these to be “undefined,” but in practice some implementations allow you to rely on a certain order

C++11 range-based for loop One way to iterate over a Vector:for (int i=0; i<classlist.size(); i++){

cout << classlist[i] << endl;}

Range-based for loop makes the code cleaner:for (string name : classlist){

cout << name << endl;}

Other iteration techniques in C++ (NOT recommended for this class) STL vector and iteratorsfor (vector<int>::iterator it = classlist.begin(); it != classlist.end(); ++it){cout << *it << endl;}

Stanford library “foreach” loop (obsoleted by range-based for loop in C++11):foreach (string name in classlist){cout << name << endl;}