Graphs, Continued

ECE 297

Review: Map as a Graph

• Intersections Nodes– unique id and unique name

• Street Segments Edges– unique id only

0

1

2

3

4

Bloor & Yonge

Bay & College

Bloor & Bay

College & Spadina

DVP & unknown(20)

01

2 3

4

5

ids for intersections & street segments

overlap remember which is which in your

code

Graph Type?

• Undirected or directed?

• One way streets directed• A two way street makes 2 edges

Graph Representation?• Adjacency List or Adjacency Matrix?

0

1

2

3

4

Adjacency ListNode 0: out_edge {3}Node 1: out_edge {0, 3, 2}Node 2: out_edge {3, 4}…

Might also have “in_edge” list if you want/need it

Adjacency Matrix

0 1 2 3 4

To NodeF

rom N

ode

0 0 0 0 1 0

1 1 0 1 1 0

2 0 0 0 1 1

3 0 1 0 0 0

4 0 0 1 0 0

Memory Complexity• N intersections (N ~ 166,000 for Toronto)

0

1

2

3

4

Adjacency ListNode 0: out_edge {3}Node 1: out_edge {0, 3, 2}Node 2: out_edge {3, 4}…

Adjacency Matrix

0 1 2 3 4

To NodeF

rom N

ode

0 0 0 0 1 0

1 1 0 1 1 0

2 0 0 0 1 1

3 0 1 0 0 0

4 0 0 1 0 0

• Adjacency matrix:– N rows x N columns– Memory required: O(N2)– 26 GB (for 1 byte / entry)!

• Adjacency list:– N lists– Average: 4 ints / list– O(N)– 2.6 MB!

libstreetsdatabase: More Than Connectivity

• Intersections (Nodes)– id– name– lattitude, longitude

• StreetSegments (Edges)– id– Street id What street are you part of?

• This has a name

– Speed limit– One way (from node to node)– Curvepoints shape of street segment

StreetSegment: Curve Points

• Not all StreetSegments (city blocks) are straight

• If not straight, have CurvePoints list of (lat,lon) points followed between intersections

libstreetdatabase: Streets & POI

• Streets– list of StreetSegments– unique id– unique name (e.g. “Yonge”)

• Points of Interest– id– location– non-unique name (e.g. “Tim Horton’s)

• Natural Features– ponds, parks, etc.– Not used in milestone1

Graph Reference

• Chapter 22 of Introduction to Algorithms by Cormen et al

Milestone 1

• List of functions to implement in m1.h– Comments say what each should do– Good practice: documentation is in your code!– Header file: comments say how to use

• Not how it is built

• Detailed unit tests in ece297exercise– These give the detailed specification– “Test-driven development”– Most test functionality– Some also test speed too slow fail– One runs valgrind to check memory OK

Milestone 1 Unit Tests

// Load anything you want hereload_map (“toronto.bin”);

int id = find_intersection_id_from_name (“Yonge & Bloor”);if (id != 22333) { <your code failed the test, record it>}// lots more tests

// clean up here; all memory should be de-allocatedclose_map ();

main.cpp(made by unit tests)

load_map

// Load anything you want hereload_map (string map_name);

What should I do in load_map ()?3 basic approaches

1. load_map does nothing • Implement all your higher-level api calls by calling lower level api

(libstreetdatabaseapi) directly• Stateless• Often easier to code & debug• But may be slow for some calls

2. load_map builds everything you could ever want• Never have to call lower-level api again• You have full control make beautiful, fast data structures

load_map

// Load anything you want hereload_map (string map_name);

3. Blended approach• load_map creates some new data structures• Use them for api calls that need them for speed• Call lower level api when it is fast enough

• All 3 approaches are reasonable• Your choice which to use• If you fail speed tests, change your approach

• load_map not automatically speed tested• Spec: keep speed interactive• Your TA will notice in the demo if it is very slow (> 1 s or so)• Otherwise OK

Clean Up

// Load anything you want hereload_map (“toronto.bin”);

int id = find_intersection_id_from_name (“Yonge & Bloor”);if (id != 22333) { <your code failed the test, record it>}// lots more tests

// clean up here; all memory should be de-allocatedclose_map ();

main.cpp(made by unit tests)

ece297exercise runs valgrind on a load_map(), use, close_map() testMust free all memory or will lose marks

Automatic Memory Checking• Memory checking:

valgrind slowdown ~10x

• Testing: want fast & thorough

• Small test case: almost as thorough as big

• St. Helena

load_map How Do I Get Its Data?

// Load anything you want hereload_map (“toronto.bin”);

int id = find_intersection_id_from_name (“Yonge & Bloor”);if (id != 22333) { <your code failed the test, record it>}// lots more tests

// clean up here; all memory should be de-allocatedclose_map ();

main.cpp(made by unit tests)

?

Easy way global variable

But Aren’t Global Variables Bad?

• Yes, if you overuse them– E.g. could pass argument but use global

variable to avoid typing!

• But almost all programs have some– Or something very similar– Limit their number

• Group related data into classes (e.g. StreetGraph?) organizes data and avoids lots of namespace pollution

StreetGraph

load_map()close_map()

find_intersection_id_from_name (“B & Y“);

Global Variable Pitfalls

• Anyone can use / call functions of / delete our global object– Would be nice if we could check it was set up first

StreetGraph *streetGraph; // Pointer to your data!

int find_intersection_id_from_name (string intersection_name){ streetGraph->answerMe (intersection_name); // What if streetGraph isn’t loaded yet? // Crash!

delete streetGraph; // Thought I should clean up, but my teammates will be // surprised when they try to use streetGraph next!}

Object-Oriented Alternative

• Design pattern: singleton– Have one loaded copy of a certain object– Many parts of the program need it

StreetGraph

load_map()close_map()

find_intersection_id_from_name (“B & Y“);

• When function asks for access– Return a pointer to the singleton object

• But can check that it is loaded first

StreetGraph as a Singletonclass StreetGraph {private: static StreetGraph *theObjectPtr; // One copy per class

public: StreetGraph (string mapName) { if (theObjectPtr != NULL) { cerr << “StreetGraph already loaded!” << endl; . . . // take some action; else { ... // Load up stuff theObjectPtr = this; } }

static StreetGraph *getObjectPtr ( ) { if (theObjectPtr == NULL) { cerr << “StreetGraph not loaded yet!” << endl; . . . // could end the program, or could load stuff up } return (theObjectPtr); }}

Can call without an object

Replaces our global variable: like a controlled

access global variable

Singletonclass StreetGraph {...public: ~StreetGraph () { ... // Clean everything up theObjectPtr = NULL; // Says we’re not loaded }}

StreetGraph.h

#include “StreetGraph.h”

void myFunc ( ) { StreetGraph *streetGraph = StreetGraph::getObjectPtr (); streetGraph->someQuery ( ... ); ...}

somefile.cpp

Calling without an object use full name

Further Reading

• Common problems, and how to map them to software designs

• E.g. singleton pattern

• Inspired by patterns in architecture

Team Formation

Dr. Tallman

ECE297 Tutorials, Jan 21 & Jan 23

• Your team will meet your Communication Instructor (CI) and schedule a weekly 30-minute meeting beginning the following week.

• Wed, Jan 21, 1-3pm, students go to GB404• Fri, Jan 23, 9-11am, students go to GB412• Fri, Jan 23, 3-5pm, students go to GB412 • If you do not have a full team formed by these

dates, come to your scheduled tutorial, and Dr. Tallman will assist you.