Programming Basics for Logistics Algorithms:

Review Session

Tatiana Polishchuk

Associate Professor, 

Linköping University, KTS�

itn.liu.se/~tatpo46/

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Course Overview

• Introduction to programming
• Variables and data types
• Algorithms and scripts
• Input/output statements
• Conditional statements
• Loops, nested loops
• Vectors
• Matrices and graphs
• Plotting: graphical functions
• Good programming practices
• Data Management

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And finally….

What is Matlab?

Matlab is both a program and a programming language ...

… it has lots of tools you can just use to analyse data and images. But you can also write code to extend it to do any analysis you like (although unlike a ‘pure’ language, it will only work if the Matlab program is installed on the computer).

+

+

+

=

A really powerful tool for work and research!

Command window

• A fancy scientific calculator

>> 2+3

>> 2*3

>> 2/3

>> 2^3

>> 2^0.5

>> 2+3*6/2

>>(2+3)*6/2

• + check for more complicated math functions: sqrt, power, mod, abs

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Matlab help commands

• help

>> help sin - displays documentation for the function sin

• doc

>> doc sin - opens Matlab documentation

• lookfor

>> lookfor convert - displays functions with convert in the first help line

• start Matlab help documentation

>> helpdesk

>> doc

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Variables and Data Types

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Data type = class

Workspace

• Display contents of workspace

>> whos

Name Size Bytes Class

a 1x1 8 double array

b 1x1 8 double array

c 1x1 8 double array

Grand total is 3 elements using 24 bytes

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• Delete variable(s) from workspace

>> clear a b; % delete a and b from workspace

>> whos

>> clear all; % delete all variables from workspace

>> whos

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Variables and Data Types

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>> default: double

Variable, script and function names

• Must start with a letter
• Followed by any number of letters, digits, or underscores.
• Matlab is Case Sensitive
• A and a, my_fun and My_fun, etc are not the same name
• Be expressive: try and use names that
• Describe what functions do
• Describe what variables are

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Scripts

• Matlab editor
• Use scripts to execute a series of Matlab commands

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Press + to create new m-file in the matlab editor

Press Run button to run the script

Comments

• Anything following the % sign is interpreted as a comment

>> % This is my first script: LAB1, day 1

>> a = 10; % the number of passengers in the bus

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●

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Hello world! This is my first Matlab script!

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Input/Output

>> disp (‘Hello world!’)

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Displays the string in parentheses

Remember to put text into ‘ ’

Exercise:

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Input/Output

>> a = input (‘What is your name? >’)

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Expects user’s response and assigns the expression as a value to the variable a

Exercise:

Add to your helloWorld.m script the following lines:

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>> myname=input('What is your name?>', 's’);

>> disp ('Written by ')

>> disp (myname)

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Input/Output

>> a = input (‘What is your name? >’)

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Expects user’s response and assigns the expression as a value to the variable a

Exercise:

Add to your helloWorld.m script the following lines:

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>> myname=input('What is your name?>', 's’);

>> disp ('Written by ')

>> disp (myname)

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Use when you expect a string

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Input/Output

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Formatted output can be printed to the screen using the fprintf function (all in one line! unlike disp)

>> fprintf('The value is %d, for sure!', 4^3)

The value is 64, for sure!

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Conversion character:

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%d integer (it stands for decimal integer)

%f float (real number)

%c single character

%s string

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integer number

place holder

for integer number

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Input/Output

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\n - provides printing from new line

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>> fprintf('The value is %d, \n for sure!', 4^3)

The value is 64,

for sure! >>

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More escape characters:

\b Backspace '' Single quotation mark

\f Form feed %% Percent character

\t Horizontal tab \\ Backslash

\r Carriage return \N Octal number N

new line

Questions?

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Next: Flow control

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if/else/elseif

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• Basic flow-control, common to all languages
• MATLAB syntax is somewhat unique

IF

if cond

commands end

ELSE

if cond

commands1 else

commands2 end

ELSEIF

if cond1

commands1 elseif cond2

commands2 else

commands3 end

Conditional statement: evaluates to true or false

• No need for parentheses: command blocks are between reserved words

Relational Operators

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• MATLAB uses mostly standard relational operators

short-circuit (scalars)

&

|

• greater or equal >=
• less or equal <=
• Logical operators elementwise

&&

||

• And
• Or
• Not
• Xor
• All true
• Any true

~

xor all any

• Boolean values: zero is false, nonzero is true
• See help for a detailed list of operators

for

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for

n=1:100

commands

end

• for loops: use for a known number of iterations
• MATLAB syntax:

Loop variable

for i = range commands

end

• The loop variable
• Is defined as a vector
• Is a scalar within the command block
• Does not have to have consecutive values (but it's usually cleaner if they're consecutive)
• The command block
• Anything between the for line and the end

for

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Print ‘Hello world’ 10 times with numbering the lines, each line in new row

>>

Hello world 1

Hello world 2

Hello world 3

Hello world 4

Hello world 5

Hello world 6

Hello world 7

Hello world 8

Hello world 9

Hello world 10

for

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Print ‘Hello world’ 5 times with numbering the lines, for odd numbers only, starting from 1, each line in new row

>>

Hello world 1

Hello world 3

Hello world 5

Hello world 7

Hello world 9

while

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• The while is like a more general for loop:
• No need to know number of iterations

WHILE

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while cond commands

end

• The command block will execute while the conditional expression is true
• Beware of infinite loops! CTRL+C?!

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>>

i =

4

i =

16

i =

256

Without ; to print output

while

Step by step:

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iteration 0: i = 2

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Iteration 1: check (2<100)

i = 2*2= 4

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Iteration 2: check (4<100)

i = 4*4 = 16

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Iteration 3: check (16<100)

i = 16*16 = 256

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Iteration 4: check (256>100)

exit

Questions?

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Next: Arrays

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1D Arrays: vectors

• Creating
• Explicitly:
• a = [1 2 3 4]; a = [1; 2; 3; 4]; a = [0.2 3 2.7]
• Using the ‘:’ operator:
• a = 1:4; a = 4:1; a = 4:-1:1; a = 1:10:40; a=0:0.2:1;
• Using ‘ones’ or ‘zeros’
• a = ones(5,1); a = ones(1,5); a = zeros(10,1);
• Using random sampling functions ‘rand’, ‘randn’
• a = rand(5,1); a = randn(5,1); a = randi(5,5,1)

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1D Arrays: vectors

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More examples how to create vectors:

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>> a = [1 2 3 4]

>> a = [1, 2, 3, 4]

>> a = 1:4 % no need for []

>> b = 1:2:9

b =

1 3 5 7 9

>> ? how to define the vector c:

9 7 5 3 1

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1D Arrays: Indexing and

accessing elements

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Indexing vectors: starting from “1”

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>> newvector=[1 3 5 7 9 3 6 9 12 15]

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Accessing elements of the vector: vector(index) or vector(index vector)

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>> newvector(3)

ans = 5

>> newvector(4:6)

ans = 7 9 3

>> newvector([1 10 5])

ans = 1 15 9 index vector

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first element

1D Arrays: Concatenation

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Add elements to the empty vector

>> myvector=[]

>> myvector = [myvector 4]

ans=

4� >> myvector = [myvector 2]

ans=

4 2

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Can be continued as many times as needed!

2D Arrays: Matrices

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>> A = zeros (5,5)

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A =

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

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>> B = ones (3,4)

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B =

1 1 1 1

1 1 1 1

1 1 1 1

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>> C = rand (2,3)

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C =

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0.5504 0.0791 0.8982

0.5963 0.5766 0.4633

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range

Matrix Modification

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Modifying matrix elements assigning a new value:

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EXAMPLE:

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A = 3 3 4 7 A(1,2)= 13 3 13 4 7

5 3 2 1 5 3 2 1

2 3 2 1 2 3 2 1

1 3 1 1 1 3 1 1

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A = 3 3 4 7 A(:,4)= [ 5; 5; 5; 5] 3 13 4 5

5 3 2 1 5 3 1 5

2 3 2 1 2 3 2 5

1 3 1 1 1 3 1 5

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Removing Elements from Matrix

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It is not possible to remove just 1 element to the matrix!

But we can remove entire rows or columns:

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EXAMPLE:

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A = 3 3 4 4 A(3, : )= [ ] A = 3 13 4 4

5 3 2 4 5 3 2 4

2 3 2 3 1 3 1 1

1 3 1 1

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B = 1 2 1 B (4) = [ ] B = 1 1 1 1 1 1 1 1

1 1 1 reshaping

1 1 1

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Size, Length and Numel

• Size function returns the vector [#rows, #columns]:

A =

10 2 9

7 4 6

• Length function returns either # rows, or #columns, whichever is largest:

>> length(A) =

ans =

3

• Numel function returns the total number of elements in the array

>> numel(A) =

ans =

6

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>> size(A)

ans =

2 3

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Vector & Matrix Algebra

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IMPORTANT: there are two types of matrix operations

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• linear algebra operations

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• element-wise operations

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Element-wise functions

Element-wise operations

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Summary

• Understanding how arrays work is the key to successful programming
• [] – square brackets create new arrays
• () – access parts of an array
• *,^ etc. perform linear algebra (matrix) operations
• .*,.^ etc. perform element-wise operations

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Quiz from previous lecture: Arrays and Matrices

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MATLAB makes arrays easy!

That’s its power!

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Next: Graphs

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Questions?

Graphs

• A graph is a pair (V, E), where
• V is a set of nodes, called vertices
• E is a collection of pairs of vertices, called edges
• Vertices and edges are positions and store elements
• Example:
• A vertex represents an airport and stores the three-letter airport code
• An edge represents a flight route between two airports and stores the mileage of the route

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ORD

PVD

MIA

DFW

SFO

LAX

LGA

HNL

849

802

1387

1743

1843

1099

1120

1233

337

2555

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Operations on Graphs

• What operations do you think we can do to a graph
• add an edge
• add a node
• Others……

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Representation

• There are different ways to represent a graph
• List of edges
• Adjacency matrix

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Visualizing Graphs

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digraph objects represent directed graphs, which have directional edges connecting the nodes

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>>clear ;

>>A = [

0 1 0 1;

1 0 1 0;

0 1 0 1;

1 0 1 0

];

>>figure;

>>G = digraph (A);

>>plot (G);

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Visualizing Graphs

Defining edge list - another way of creating graphs

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>> E_list = [

1 2;

1, 3;

2, 3

]; % edge list

>> E_weights = [1 1 2];

>> G = graph(E_list(:,1), E_list(:,2), E_weights);

>> plot (G)

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Visualizing Graphs

graph objects represent undirected graphs, which have direction-less edges connecting the nodes

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>>clear ;

>>A = [

0 1 0 1;

1 0 1 0;

0 1 0 1;

1 0 1 0

];

>>figure;

>>G = graph (A);

>>plot (G);

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Graph created using adjacency matrix

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Questions?

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Thank you!

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