Transcript Introduction to Matlab & Data analysis

Introduction to Matlab
& Data Analysis
Tutorial 6:
Boolean Logic, Flow Control
and the debugger
Please change directory to directory E:\Matlab (cd E:\Matlab;)
From the course website
(http://www.weizmann.ac.il/midrasha/courses/MatlabIntro//course_outline.htm )
Download:
debugExample.m, drawGasPrices.m, tryCatch.m, gas.mat,
Shown on board:
t6.m , trueOrFalse.m , rockPaperScissors.m, , estimatingEpsilon.m
Weizmann 2010 ©
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Topics
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Boolean logic
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Relational and logical operators
What is true and what is false
Flow control:
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if-else-end constructions
Switch-case-end constructions
for loops
while loops
First
Loops flow control:
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break
continue
try-catch
retrun
use of
Matlab debugger
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Lecture Reminder
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What is true and what is false?

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Type “logical”
 True represented by 1
 False represented by 0
Keywords: true, false.
What is False?
 false
 0
What is true? anything else …
Special cases:
 Empty string ‘’
 Array with false values [1 0 1]
 Empty matrix []
What About NaN?

Not true and not false – NaN
 if (NaN)
??? NaN's cannot be converted to logicals.
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Reminder - Relational operators
<
Less than
>
Greater than
<= Less than or equal to
>= Greater than or equal to

Element by element:


Similar dimensions
Scalar expansion
== Equal to
~= Not equal to
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Reminder - Logical Operators
Logical operator
Description
Logical operator
Description
&&
And (short-circuiting)
&
And
||
OR (short-circuiting)
|
OR
~
Not
~
Not
x
y
X && Y
x || y
xor(x,y)
0
0
0
0
0
0
1
0
1
1
1
0
0
1
1
1
1
1
1
0
x
~x
0
1
1
0

Logical functions
(column-wise)
 any(x)
 all(x)
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Reminder - Flow Control
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Flow control:
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if-else-end constructions
switch-Case
constructions
For loops
While loops
Loops flow control:
If road == the_one_less_traveled_by
 Break
 continue
disp(‘Made all the difference’);
Return
Else
Try-Catch
disp(‘Kept for another day’);
end
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Reminder - If-else-end constructions
if (expression I)
statement1a;
statement1b;
else
statement2a;
statement2b;
end
Use indentation!
if (expression I)
statement1a;
statement1b;
elseif (expression II)
statement2a;
statement2b;
elseif (expression III)
statement3a;
statement3b;
else
statement4a;
statement4b;
end
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Switch-Case constructions
if (expression I)
statement1a;
statement1b;
elseif (expression II)
statement2a;
statement2b;
elseif (expression III)
statement3a;
statement4b;
switch expression
case test_exp1
statement1a;
statement1b;
case {test_exp2, test_exp3}
statement1a;
statement1b;
end


Scalar:
expression == test_exp1
String:
strcmp(expression, test_exp1)
otherwise
statement1a;
statement1b;
end
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Example - Switch-Case
day='Tue';
switch day
case 'Mon'
disp('Matlab class');
case {'Tue','Wed','Thu'}
disp('Matlab tutorial');
otherwise
disp('Free');
end
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Reminder - For and While Loops
for index = array
while expression
statement1;
statement2;
statement1;
statement2;
end

In every iteration of
the loop the index
equals to one column
of the array.
end


If the expression is
true -> run another
iteration
Pitfall – endless loop
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Reminder - Nested loops
for index1 = array1
statement1;
statement2;
for index2 = array2
statement3;
statement4;
end
end
Indentation!
for index1 = array1
statement1;
statement2;
while expression
statement3;
statement4;
if expression1
statement5;
end
end
end
No limitations of nesting
order and # levels
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
break – immediately breaks the
loop
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continue – jump to the end
statement.
Does not break the loop
return – returns control to the
command line (or to the calling
function).
Anywhere


Breaks only one loop
Only inside loops!
Reminder - Break, Continue
and return affect the flow
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End Of lecture Reminder
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Boolean Logic
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Boolean Logic - Pitfalls
a = []; b = []; c = 1;
a == b Empty array
a == c Empty array


Comparing to empty array
gives empty array
Use isempty(A)
NaN == NaN
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NaN is not equal to itself. Use isnan.
2 = 2 Error
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Major Pitfall! = ~= ==
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Use Indentation & Parentheses
x = 0; y = 1; z = 2;
% The statement below is unclear
x^2 - y == -1 && y*2 - z == 0 || z*2 - x == 5
% Use parentheses!
% Use Indentation –
% Do not split a logical expression into two lines
( (x^2 - y == -1) && …
(y*2 - z == 0 )
) || …
(z*2 - x == 5)
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Flow control
Examples:
 if-else-end
 for loops
 while loops (break, continue)
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Example - Rock-Paper- Scissors
%%Paper-Rock-Scissors
% we use
How do we compute
% P for Paper
% R for Rock
who won? Ideas?
% S for Scissors
Hint: use strcmp
% get players hands
clc;
player1_hand = input('Player 1 hand:');
clc;
player2_hand = input('Player 2 hand:');
clc;
disp(['Player 1 hand is: ' player1_hand]);
disp(['Player 2 hand is: ' player2_hand]);
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If Example –
Rock-Paper- Scissors
%Calculate who won
if ( ( strcmp(player1_hand,'P') && strcmp(player2_hand,'R') ) || ...
( strcmp(player1_hand,'R') && strcmp(player2_hand,'S') ) || ...
( strcmp(player1_hand,'S') && strcmp(player2_hand,'P') ) )
disp('Player 1 won!');
elseif ( ( strcmp(player2_hand,'P') && strcmp(player1_hand,'R') ) || ...
( strcmp(player2_hand,'R') && strcmp(player1_hand,'S') ) || ...
( strcmp(player2_hand,'S') && strcmp(player1_hand,'P') ) )
disp('Player 2 won!');
else
disp('Draw');
end
Notice the indentation
rockPaperScissors.m
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Simple For Loop Example –
Using For Loops to Initiate Matrices
Using a for loop create a 1x10 vector where each
entry is the sum of its index and the square root of
the previous element (the “zero” element value is 0)
x = zeros(1,10); Allocating memory
prev_num = 0;
“end case”: i=1
variables naming: prev_, cur_
for i=1:10
x(i) = i + sqrt(prev_num);
prev_num = x(i);
end
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Simple For Loop Example –
Using For Loops to Initiate Matrices
Alternative code:
x = zeros(1,10);
Using if-else-end to
take care of end cases
for i=1:10
if (i == 1) % end case first element
x(i) = i;
else
x(i) = i + sqrt(x(i-1));
end
end
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Nested For Loop Example –
Using For Loops to Initiate Matrices
Using for loops initiate a 3x3 matrix such that:
Each entry is the sum of its subscripts square
A = zeros(3,3);
% Don’t forget to allocate memory!
A=
2
5
10
for n = 1:3
for m = 1:3
A(n,m) = n^2 + m^2;
end
end
5 10
8 13
13 18
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While Loop Example –
Estimating Epsilon

Epsilon – the smallest number that can be
added to 1 such that the result is greater
than 1.
Can we compute the last line inside the loop?
my_eps= 1;
my_eps = 1;
while (my_eps+1) > 1
my_eps = my_eps/2;
end
while (my_eps+1) > 1
my_eps = my_eps/2;
if (my_eps+1) <= 1
my_eps = my_eps*2;
break;
end
end
Is that ok?
my_eps = my_eps*2;
estimatingEpsilon.m
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Efficiency And Readability


Usually when operations can be performed by either
loops or vector operation – the vector operations are
more efficient and readable.
Always allocate memory in advance.
x = linspace(0,2*pi,1000);
y = sin(x);
==
z = zeros(size(x));
for i=1:length(x)
z(i) = sin(x(i));
end
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The Debugger
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Debugger

Debugging – The process of discovering
and correcting bugs in a program


Matlab has a graphical debugger
We use the debugger to follow a specific
run of the control flow
debugExample.m
/2
A
*2
B
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Debugging Run Time Errors
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Open the example: edit debugExample.m;
Debug buttons
Debug menu
Stop if errors / warn
Break points –
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There are bugs
Lets find them . .
Red
Gray
Modifying a file
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Coding Tips
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An “end case” – a relatively rare input that
might cause bugs (NaN, Inf).
Use indentation:
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Write the “end” before writing the content
Cntl-I
Use variables instead of numbers (avoid
“hard coding”)
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Throwing and Catching errors
error(‘msg’);
try – catch - end
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Try-Catch
try
statement1;
error
statement2;
…
catch
statement3;
statement4;
…
 Try to run the code in
the “try” block
 If no error and reached
the catch, go to the end
statement.
 If an error occur, go to
the catch statement.
end
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Try-Catch - Errors
try
statement1;
error(‘error message’);
statement2;
…
catch


Many Matlab functions
throw errors when
something goes wrong
You can throw error:
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error(‘error message’);
You can also catch errors:
statement3;
error_string = lasterr;
errorStr = lasterr;
error = lasterror;
disp(errorStr);
Using
Structure,
the error error.message next tutorials…
statement4;
end
rethrow(lasterror);
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TryCatch.m
Try and Catch Example
try
A = ones(3,3);
x = input('Input a number by which A will be divided:');
if isnan(x)
error(['NaN was entered: ' num2str(x)]);
Input: 2
end
A = A /x;
A
catch
errorStr = lasterr;
disp(errorStr);
%rethrow(lasterror);
end
Try to uncomment
A=
0.5000
0.5000
0.5000
0.5000
0.5000
0.5000
0.5000
0.5000
0.5000
Input: nan
Error using ==> TryCatch at 7
NaN was entered: NaN
Input:[0, 1]
Error using ==> mrdivide
Matrix dimensions must agree.
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Final Example –
Drawing multiple plots
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File
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edit drawGasPrices.m
you should have gas.mat in the same directory
Reminder:
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Gasoline_prices – US gas prices at each month of
the years 1976-2004
Years – 1976:2004
Months – char matrix. Each row contains a month
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Final Example –
Drawing multiple plots

Goal:
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Get from the user at most five years
Draw a figure with a subplot for every year
The year subplot displays the prices of gas during
this year as a function of the months.
1. Load the data
Make sure the data is OK
2. Getting which
3. Draw the plot
years to draw
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Final Example –
Drawing multiple plots

Notice:
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Errors:
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Using while, return, break, continue to get input
Using For loop:

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Throwing for error
Try-catch to avoid error exit when the input is wrong
Using the loop index to retrieve relevant data
Using index to set current subplot
Variables naming
Use the debugger to go over the code
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1. Load the data
Drawing Multiple Plots –
Throwing Error
Make sure the data is OK
clear; close all;
load gas.mat;
% checking that the data exist
if (~exist('gasoline_prices','var') || ...
~exist('months','var') || ...
~exist('years','var') )
error('gasoline_prices ... not defined');
end
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2. Getting which
years to draw
Drawing Multiple Plots –
Displaying Instructions to the User
% getting which years to draw
years2draw = [];
disp('Enter which year to draw')
disp('Years should be between 1976 and 2004');
disp('Maximum five years are allowed.');
disp('When done enter D');
D while mean break
disp('If you wish to quit without drawing enter Q');
Q while mean return
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2. Getting which
Drawing Multiple Plots –
Quitting and Breaking a Loop
years to
draw
while length(years2draw) < 5
…
cur_year = [input('Year / D / Q :','s')];
if (strcmp(cur_year, 'Q'))
disp('Bye!');
return;
elseif (strcmp(cur_year, 'D'))
break;
end
Use while
…
end
to iterate
until you get a
satisfying input
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2. Getting which
Drawing Multiple Plots –
Using Continue, for mal input
years to
while length(years2draw) < 5
…
% getting the number
cur_year = str2num(cur_year);
cur_year = floor(cur_year);
if ~isscalar(cur_year)
disp('Input is not a number, try again.');
continue;
end
% year not in range
if (cur_year < 1976 || cur_year > 2004)
disp('Years are not … 1976-2004');
continue;
end
years2draw(end+1) = cur_year;
end
draw
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2. Getting which
years to draw
Drawing Multiple Plots –
Try-Catch Handling of unpredicted error
while length(years2draw) < 5
…
try
<all the code of getting the input>
% adding the year to the input years vector
years2draw(end+1) = cur_year;
catch
We avoid exiting by catching the error
error_str = lasterr;
disp(['Bad input, try again:' error_str]);
continue;
end
…
end
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3. Draw the plot
Drawing Multiple Plots –
Return if Nothing Need to be Done
% end case:
% if no year was entered we are done
if (isempty(years2draw))
disp('No year was entered, exiting.');
return;
end
% opening a new figure
figure;
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3. Draw the plot
Drawing Multiple Plots –
For loop – Variables Naming
for i = 1:length(years2draw)
% getting the data
cur_year
= years2draw(i);
cur_year_prices = gasoline_prices(find(years == cur_year),:);
% setting the current subplot
subplot(length(years2draw),1,i);
% drawing
plot(1:12,cur_year_prices, 'r-x','LineWidth',2)
xlabel('Month', 'FontSize', 14);
ylabel('$/Gallon', 'FontSize', 14);
title(['Monthly Gas Prices during:' num2str(cur_year)], 'FontSize', 16)
set(gca,'XTick',1:12, 'XTicklabel', months);
grid;
Variables naming:
axis tight;
end
i for index
cur_ for current (value is specific for this iteration)
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Summary

Boolean logic



Relational and logical operators
What is true and what is false
Flow control:





if-else-end constructions
Switch-case-end constructions
for loops
while loops
Loops flow control:




break
continue
try-catch
return
First use of
Matlab debugger
44
Extra examples Not shown in the tutorial
Weizmann 2010 ©
45
Boolean Logic Tips and
Pitfalls
quote = ['To err is human - and to blame' …
'it on a computer is even more so. '];
Sub_str1 = 'i am';
Sub_str2 = 'i';

Strings are char arrays:
quote == sub_str1

??? Error using ==> eq
Matrix dimensions must agree.

quote == sub_str2
[ 0 0 0 … 1 0 0 1 … 0 0]

Should have equal sizes
Scalar expansion
String comparison –


strcmp
strncmp
strcmpi
strcmp(quote, sub_str1)

0
(“i” - not case sensitive)
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Examples
Which are similar?
1 || 2 && 0
1 || (2 && 0)
(1 || 2) && 0
Notice the Precedence
Arrays - Which are similar?
[]
[0, 1]
[1, 1]
any([0, 1])
all([0, 1])
TrueOrFalse.m
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Use Logical Expressions for Array Indexing
>> isnan([1, NaN])
[0, 1]
>> v = 1:10;
% Find all elements of v
% that are product of 3 or equal 5
% (hint – use mod(v,3)):
>>
>> find(v == 5 | mod(v,3) == 0)
[3, 5, 6, 9]
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