Lecture note#9

Report
AMS597 Spring 2011
Hao Han
April 05, 2011
1
Introduction to MATLAB
 The name MATLAB stands for MATrix LABoratory.
 Typical uses include:
 Math and computation
–
–
–
Algorithm development
Data acquisition
Modeling, simulation, and prototyping
 Data analysis, exploration, and visualization
– Scientific and engineering graphics
– Application development, including graphical user interface
(GUI) building
 We will focus on the statistical computing in MATLAB.
2
Desktop Tools &
Development Environment
 Workspace Browser – View and make changes
to the contents of the workspace.
 Command Windows – Run MATLAB
statements (commands).
Hotkey: Ctrl+c -> break while the status is busy
 M-file Editor – Creating, Editing, Debugging
and Running Files.
3
MATLAB Variables
 Variable names are case sensitive. Variable names must start with a
letter and can be followed by digits and underscores.
 MATLAB does not require any type of declarations or dimension
statements. When it encounters a new variable name, it automatically
creates the variable and allocates the appropriate amount of storage.
For example: New_student = 25;
To view the matrix assigned to any variable, simply enter the variable
name.
 Special Variables:
 pi
value of π
 eps
smallest incremental number
 inf
infinity
 NaN
not a number
 realmin
the smallest usable positive real number
 realmax
the largest usable positive real number
4
MATLAB Matrices
 MATLAB treats all variables as rectangular matrices.
 Separate the elements of a row with blanks or commas.
 Use a semicolon ‘;’ to indicate the end of each row.
 Surround the entire list of elements with square brackets ‘[ ]’.
Claim a scalar:
x = 2;
Claim a row vector:
r = [1 2 3]
r = [1,2,3]
Claim a column vector:
c = [1;2;3]
c = [1 2 3]’
Claim a matrix:
a = [1 2 3; 4 5 6; 7 8 9]
a =
1 2 3
4 5 6
7 8 9
Subscripts: the element in row i and
column j of A is denoted by A(i,j).
a(3,2)=8 or a(6)=8
5
Matrix Manipulations
 The Colon Operator:
1:5 is a row vector containing
integers from 1 to 5.
To obtain non-unit spacing,
specify an increment.
For example, 100:-7:50
 Extracting a sub-matrix:
Sub_matrix = matrix(r1:r2,c1:c2);
sub_a = a(2:3,1:2)
sub_a =
4 5
7 8
 Replication:
b = [1 2; 3 4];
b_rep = repmat(b,1,2)
b_rep =
1
2
1
3
4
3
2
4
 Concatenation:
c = ones(2,2);
c_cat = [c 2*c; 3*c 4*c]
c_cat =
1
1
2
1
1
2
3
3
4
3
3
4
c_cat = cat(DIM,A,B);
2
2
4
4
 Deleting rows or columns:
c_cat(:,2)=[];
6
Structures and Cell Arrays
Structure
Cell Array
 Way of organizing related data
 Cell arrays can have entries of
 Create a structure, s, with fields,
x, y, and name
s.y = 1;
s.x = [1 1];
s.name = 'foo';
% or equivalenty
s2 = struct('y',1,'x',[1
1],'name','foo');
 Test for equality:
% works for any s1, s2
isequal(s1,s2);
arbitrary datatype
% create 3 by 2 cell array
a = cell(3,2);
a{1,1} = 1;
a{3,1} = 'hello';
a{2,2} = randn(100,100);
 Using cell arrays with other
datatypes can be tricky
% create 2 by 1 cell array
a = {[1 2], 3};
y = a{1}; % y is 1 by 2 numeric array
ycell =a(1); % is 1 by 1 cell array
x = y+1; % allowed
xcell = ycell+1; % not allowed
onetwothree = [a{1:2}]; % = [1 2 3]
7
MATLAB Operators
 Relational operators:





Less than
Less than or Equal
Great than or Equal
Equal to
Not equal to
<
<=
>=
==
~=
 Element wise operators:
+
Addition
-
Subtraction
 not ~
.*
Element-by-element multiplication
% highest precedence
 and &
% equal precedence with or
 or |
% equal precedence with and
./
Element-by-element division
.\
Element-by-element left division
.^
Element-by-element power
.'
Unconjugated array transpose
 Logical operators:
 Matrix computations:
+ - * / ^
A’; % transpose
A \ b; % returns x s.t. A*x=b
A / b; % returns x s.t. x*A=b
8
MATLAB Functions
 MATLAB provides a large number of standard elementary
mathematical functions, including abs, sqrt, exp, and sin.
 For a list of the elementary mathematical functions, type:
help elfun
 For a list of more advanced mathematical and matrix functions:
help specfun
help elmat
 Seek help for MATLAB function references, type:
help somefun
or more detailed
doc somefun
9
Flow Control (‘if’ statement)
 The general form of the
‘if’ statement is
if expression
…
elseif expression
…
else
…
end
 Example 1:
if i == j
a(i,j) = 2;
elseif i >= j
a(i,j) = 1;
else
a(i,j) = 0;
end
 Example 2:
if (common>60)&&(area>60)
pass = 1;
end
10
Flow Control (‘switch’ statement)
 switch Switch among
several cases based on
expression
 The general form of the
switch statement is:
switch switch_expr
case case_expr1
…
case case_expr2
…
otherwise
…
end
 Example :
x = 2, y = 3;
switch x
case x==y
disp('x and y are equal');
case x>y
disp('x is greater than y');
otherwise
disp('x is less than y');
end
% x is less than y
11
Flow Control (‘for’ loop)
 for Repeat statements a
specific number of times
 The general form of a
for statement is
for variable=expression
…
…
end
 Example 1:
for x = 0:0.05:1
fprintf('%3.2f\n',x);
end
 Example 2:
a = zeros(3,4);
for i = 1:3
for j = 1:4
a(i,j) = 1/(i+j);
end
end
12
Flow Control (‘while’ loop)
 while Repeat statements
an indefinite number of
times
 The general form of a
while statement is
while expression
…
…
end
 Example 1:
n = 1;
y = zeros(1,10);
while n <= 10
y(n) = 2*n/(n+1);
n = n+1;
end
 Example 2:
x = 1;
while x
%execute statements
end
13
Flow Control (‘break’ statement)
 break terminates the execution of for and while loops
 In nested loops, break terminates from the innermost
loop only
 Example:
y = 3;
for x = 1:10
fprintf('%d\n',x);
if (x>y)
break;
end
end
% Question: what is the output?
14
Graphics: 2-D plot
 Basic commands:
plot(x, 's')
plot(x,y, 's')
plot(x1, y1, 's1', x2,y2, 's2', …)
title('…')
xlabel('…') ylabel('…')
legend('…', '…')
 Example 1 [plot(vector)]:
x=0:pi/10:2*pi;
x=[sin(x)' cos(x)'];
figure;
plot(x)
15
Graphics: 2-D plot (cont’d)
 Example 2:
x = 0:0.01:2*pi;
y = sin(x);
z = cos(x);
hold on;
plot(x,y, 'b');
plot(x,z, 'g');
hold off;
 Example 3 [plot(vector,matrix)]:
t=(0:pi/50:2*pi)';
k=0.4:0.1:1;
Y=cos(t)*k;
plot(t,Y)
16
Graphics: 2-D plot (cont’d)
• plot(x1, y1,’s1’, x2,y2,’s2’, …)
t=(0:pi/100:pi)';
y1=sin(t)*[1,-1];
y2=sin(t).*sin(9*t);
t3=pi*(0:9)/9;
y3=sin(t3).*sin(9*t3);
plot(t,y1,'r:',t,y2,'b',t3,y3,'bo')
axis([0,pi,-1,1])
• Linetype • Color
: -- -.
b g r c m y k w
• Markertype
. + * ^ < > v d h o p s x
plot(t,y1,'.r',t,y2, 'b+',t3,y3,'ob:')
17
Subplots
>>
>>
>>
>>
>>
>>
>>
>>
subplot(2,2,1)
…
subplot(2,2,2)
…
subplot(2,2,3)
…
subplot(2,2,4)
…
18
Graphics: 3-D plot
• plot3(x,y,z)
t=(0:0.02:2)*pi;x=sin(t);y=cos(t);z=cos(2*t);
plot3(x,y,z,'b-',x,y,z,'bd');
view([-82,58]);
box on;
legend('Chain','Gemstone')
19
20
Basic Data Analysis
 Import/Export data:
Basic Data Analysis Function (help datafun)
 Use the system import wizard
Function
Description
File -> import data -> find and open files -> finish
cumprod
Cumulative product of elements.
 Use commands as follows:
cumsum
Cumulative sum of elements.
1. help load & help save
2. help xlsread & help xlswrite
cumtrapz
Cumulative trapezoidal numerical integration.
diff
Difference function and approximate derivative.
max
Largest component.
mean
Average or mean value.
median
Median value.
min
Smallest component.
prod
Product of elements.
sort
Sort array elements in ascending or descending order.
sortrows
Sort rows in ascending order.
std
Standard deviation.
sum
Sum of elements.
trapz
Trapezoidal numerical integration.
cov
Covariance matrix
corrcoef
Correlation coefficients
% Reading into a text file
fid = fopen(‘filename.txt’,‘r’);
X = fscanf(fid,‘%5d’); % or fread
fclose(fid);
% Writing onto a text file
fid = fopen(‘filename.txt’,‘w’);
count = fwrite(fid,x); % or fprintf
fclose(fid);
 Scatter plot
 Statistics Toolbox:
help stats
21
Data Preprocessing
 Missing values:
You should remove NaNs from the data before performing statistical
computations.
Code
Description
i = find(~isnan(x));
x = x(i)
Find indices of elements in vector that are not NaNs, then keep only the non-NaN elements.
x = x(find(~isnan(x)))
Remove NaNs from vector.
x = x(~isnan(x));
Remove NaNs from vector (faster).
x(isnan(x)) = [];
Remove NaNs from vector.
X(any(isnan(X)'),:) = [];
Remove any rows of matrix X containing NaNs.
 Removing outliers:
1.
2.
3.
You can remove outliers or misplaced data points from a data set in
much the same manner as NaNs.
Calculate the mean and standard deviation from the data set.
Get the column of points that lies outside the 3*std. (3σ-rule)
Remove these points
22
Regression and Curve Fitting
 The easiest way to find estimated regression coefficients efficiently is
by using the MATLAB backslash operator.
 Note that we should avoid matrix inversion (from slow to fast…):
% Fit X*b=Y
xx = x’*x;
xy=x’*y;
tic; bhat1 = (xx)ˆ(−1)*xy; toc;
tic; bhat2 = inv(xx)*xy; toc;
tic; bhat3 = xx \ xy; toc;
 Other ways use build-in functions: regress() or glmfit()
 Multiple linear regression model: y = b0 + b1x1 + b2x2 + …
 Example: Suppose you measure a quantity y at several values of time t.
1.4
t=[0 .3 .8 1.1 1.6 2.3]';
y=[0.5 0.82 1.14 1.25 1.35 1.40]';
plot(t,y,'o')
grid on
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0
0.5
1
1.5
2
2.5
23
Regression Example (cont’d)
 Polynomial regression:
 There are six equations in three unknowns, represented by the 6-by-3 matrix
X = [ones(size(t))
t
t.^2]
 The solution is found with the backslash operator.
a = X\y
a = 0.5318 0.9191 -0.2387
 Now evaluate the model at regularly spaced points and overlay the original data
in a plot.
1.5
T=(0:0.1:2.5)';
Y=[ones(size(T)) T T.^2]*a;
plot(T,Y,'-',t,y,'o')
grid on
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0
0.5
1
1.5
2
2.5
24
Regression Example (cont’d)
 Linear-in-the-parameters regression, e.g. exponential function:
X = [ones(size(t)) exp(-t) t.*exp(-t)];
a = X\y
a = 0.1018 0.4844 -0.2847
T=(0:0.1:2.5)';
Y=[ones(size(T)) exp(-T) T.*exp(-T)]*a;
plot(T,Y,'-',t,y,'o')
1.6
grid on
1.4
1.2
1
0.8
0.6
0.4
0
0.5
1
1.5
2
2.5
25
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