Introduction to matlab

Introduction to Matlab

By: TARUN GEHLOT

Outline:

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
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What is Matlab?
Matlab Screen
Variables, array, matrix, indexing
Operators (Arithmetic, relational, logical )
Display Facilities
Flow Control
Using of M-File
Writing User Defined Functions
Conclusion

What is Matlab?




Matlab is basically a high level language
which has many specialized toolboxes for
making things easier for us
How high?
Matlab

High Level
Languages such as
C, Pascal etc.

Assembly

What are we interested in?




Matlab is too broad for our purposes in this
course.
The features we are going to require is

Series of
Matlab
commands

Matlab
m-files

functions
Input
Output
capability

Command
Line
Command execution
like DOS command
window

mat-files

Data
storage/
loading

Matlab Screen


Command Window

type commands



Current Directory

View folders and m-files



Workspace

View program variables

Double click on a variable
to see it in the Array Editor



Command History

view past commands

save a whole session
using diary

Variables


No need for types. i.e.,
int a;
double b;
float c;



All variables are created with double precision unless
specified and they are matrices.
Example:
>>x=5;
>>x1=2;



After these statements, the variables are 1x1 matrices
with double precision

Array, Matrix


a vector

x = [1 2 5 1]

x =
1


2

5

a matrix

1

x = [1 2 3; 5 1 4; 3 2 -1]

x =
1
5
3


transpose

2
1
2

3
4
-1
y = x’

y =
1
2
5
1

Long Array, Matrix


t =1:10
t =

1

2
3
4
k =2:-0.5:-1



5

6

7

8

k =

2

B



1.5

1

0.5

0

-0.5

= [1:4; 5:8]

x =
1
5

2
6

3
7

4
8

-1

9

10

Generating Vectors from functions


zeros(M,N) MxN matrix of zeros



ones(M,N)

MxN matrix of ones

rand(M,N)

MxN matrix of uniformly
distributed random
numbers on (0,1)



x = zeros(1,3)
x =
0
0
0
x = ones(1,3)
x =
1
1
1
x = rand(1,3)
x =
0.9501 0.2311 0.6068

Matrix Index



The matrix indices begin from 1 (not 0 (as in C))
The matrix indices must be positive integer

Given:

A(-2), A(0)
Error: ??? Subscript indices must either be real positive integers or logicals.
A(4,2)
Error: ??? Index exceeds matrix dimensions.

Concatenation of Matrices


x = [1 2], y = [4 5], z=[ 0 0]
A = [ x y]
1

2

4

5

B = [x ; y]
1 2
4 5
C = [x y ;z]
Error:
??? Error using ==> vertcat CAT arguments dimensions are not consistent.

Operators (arithmetic)
+
*
/
^
‘

addition
subtraction
multiplication
division
power
complex conjugate transpose

Matrices Operations
Given A and B:

Addition

Subtraction

Product

Transpose

Operators (Element by Element)

.* element-by-element multiplication
./ element-by-element division
.^ element-by-element power

The use of “.” – “Element” Operation
A = [1 2 3; 5 1 4; 3 2 1]
A=
1 2 3
5 1 4
3 2 -1

x = A(1,:)
x=

y=
1 2 3

3 4 -1

c=x./y

d = x .^2

b=

y = A(3 ,:)

b = x .* y

c=
0.33 0.5 -3

d=

3 8 -3

K= x^2
Erorr:
??? Error using ==> mpower Matrix must be square.
B=x*y
Erorr:
??? Error using ==> mtimes Inner matrix dimensions must agree.

1

4

9

Basic Task: Plot the function sin(x)
between 0≤x≤4π


Create an x-array of 100 samples between 0
and 4π.
>>x=linspace(0,4*pi,100);



Calculate sin(.) of the x-array
1
0.8
0.6

>>y=sin(x);



Plot the y-array
>>plot(y)

0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1

0

10

20

30

40

50

60

70

80

90

100

Plot the function e-x/3sin(x) between
0≤x≤4π


Create an x-array of 100 samples between 0
and 4π.



Calculate sin(.) of the x-array
>>y=sin(x);



Calculate e-x/3 of the x-array
>>y1=exp(-x/3);



Multiply the arrays y and y1
>>y2=y*y1;

Plot the function e-x/3sin(x) between
0≤x≤4π


Multiply the arrays y and y1 correctly
>>y2=y.*y1;



Plot the y2-array
0.7

>>plot(y2)

0.6
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3

0

10

20

30

40

50

60

70

80

90

100

Display Facilities

0.7
0.6



plot(.)

0.5
0.4
0.3

Example:
>>y=sin(x);
>>plot(y)
>>plot(x,y)



stem(.)

0.2
0.1
0
-0.1
-0.2
-0.3

0

10

20

30

40

50

60

70

80

90

100

0

10

20

30

40

50

60

70

80

90

100

0.7
0.6
0.5
0.4
0.3

Example:
>>stem(y)
>>stem(x,y)

0.2
0.1
0
-0.1
-0.2
-0.3

Display Facilities


title(.)
>>title(‘This is the sinus function’)

xlabel(.)
>>xlabel(‘x (secs)’)



0.8

ylabel(.)

0.6
0.4
0.2
sin(x)



This is the sinus function
1

0
-0.2
-0.4
-0.6

>>ylabel(‘sin(x)’)

-0.8
-1

0

10

20

30

40

50
60
x (secs)

70

80

90

100

Operators (relational, logical)






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

== Equal to
~= Not equal to
< Strictly smaller
> Strictly greater
<= Smaller than or equal to
>= Greater than equal to
& And operator
| Or operator

Flow Control






if
for
while
break
….

Control Structures


If Statement Syntax

if (Condition_1)
Matlab Commands
elseif (Condition_2)
Matlab Commands
elseif (Condition_3)
Matlab Commands
else
Matlab Commands
end

Some Dummy Examples
if ((a>3) & (b==5))
Some Matlab Commands;
end
if (a<3)
elseif (b~=5)
end
if (a<3)
else
end

Control Structures
Some Dummy Examples


For loop syntax

for i=Index_Array
Matlab Commands
end

for i=1:100
end
for j=1:3:200
end
for m=13:-0.2:-21
end
for k=[0.1 0.3 -13 12 7 -9.3]
end

Control Structures


While Loop Syntax

while (condition)
Matlab Commands
end

Dummy Example
while ((a>3) & (b==5))
end

Use of M-File
Click to create
a new M-File

• Extension “.m”
• A text file containing script or function or program to run

Use of M-File

Save file as Denem430.m

If you include “;” at the
end of each statement,
result will not be shown
immediately





Functions are m-files which can be executed by
specifying some inputs and supply some desired outputs.
The code telling the Matlab that an m-file is actually a
function is
function out1=functionname(in1)
function out1=functionname(in1,in2,in3)
function [out1,out2]=functionname(in1,in2)



You should write this command at the beginning of the
m-file and you should save the m-file with a file name
same as the function name



Examples
 Write a function : out=squarer (A, ind)
 Which takes the square of the input matrix if the input
indicator is equal to 1
 And takes the element by element square of the input
matrix if the input indicator is equal to 2
Same Name



Another function which takes an input array and returns the sum and product
of its elements as outputs



The function sumprod(.) can be called from command window or an m-file as

Notes:




“%” is the neglect sign for Matlab (equaivalent
of “//” in C). Anything after it on the same line
is neglected by Matlab compiler.
Sometimes slowing down the execution is
done deliberately for observation purposes.
You can use the command “pause” for this
purpose
pause %wait until any key
pause(3) %wait 3 seconds

Useful Commands


The two commands used most by Matlab
users are
>>help functionname

>>lookfor keyword

Questions
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Introduction to matlab

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