Chapter 3 - Programming in Matlab. aaaapptx

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Chapter 3
Programming in MATLAB

• A computer program is a sequence of computer commands.
• In a simple program the commands are executed one after the
other in the order they are typed.
• Script Files:
Input Command
Output Commands
• Relational and Logical operators
• Conditional statements
• The Switch-Case statement
• Loops (for and while)
Introduction
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In MATLAB script files are created and edited in the
Editor/Debugger Window. This window is opened from the
Command Window. In the File menu, select New, and then
select Script or press the shortcut key ( Ctrl + N ).
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3.1 Script Files
Creating and Saving a Script File & Running a Script File
Figure 1-6: Creatie

3.1.1 Input Command
The form of the input command is:
4
The string is actually a vector that contains the numeric codes for the characters.
The length of S is the number of characters. A quotation within the string is
indicated by two quotation marks.

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3.1.2 Output Commands
CHATER
1- The disp command is used to display the elements of a variable without
displaying the name of the variable, and to display text. The format of the disp
command is:
2- The fprintf command is used to display text and data of variables on the screen
with the same line.
The format of the fprintf command is:

CHAPTER 5
3.2 Relational and Logical Operators
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• A relational operator compares two numbers by determining whether a
comparison statement is true or false.
• Relational operators are used as arithmetic operators within a mathematical
expression. The result can be used in other mathematical operations.
• If two arrays are compared (only arrays of the same size can be compared),
the comparison is done element-by-element, and the result is a logical array
of the same size with 1s and 0s.

CHAPTER 5
8

CHAPTER 3
9
Logical operators
• Logical operators have numbers as operands. A nonzero number
is true, and a zero number is false.

CHAPTER 3
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Logical operators

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Expression Evaluates As Because
A < B false A(1,1) is not less than B(1,1).
A < (B + 1) true Every element of A is less
than that same element
of B with 1 added.
A & B false A(1,2) & B(1,2) is false.
B < 5 true Every element of B is less
than 5.
Example
Given matrices A and B
A = [1 0 ; 2 3]
B = [1 1 ; 3 4]

Logical Operators
(Scalar logical)
Operator Name Description
&& AND A&&B
Returns True if both A and B evaluate to true
And False if they do not
conditional
|| OR A||B
Returns True if either A or B or both evaluate to
True And False if they do not
conditional

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Built-in logical functions

CHAPTER 5
14
Built-in logical functions

3.3 Conditional Statements
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• A conditional statement is a command that allows MATLAB to make a decision
of whether to execute a group of commands that follow the conditional
statement, or to skip these commands.

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If statements
•Syntax:
•If logical expression
•Statements
•end

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Example
Calculate the square root y of the variable x only when the
value of x is non-negative.
The command y = sqrt(x) is only executed if x >=0 is true.
The variable y is not defined when x >=0 is false.
>> if x >= 0
y = sqrt(x)
end

Example
A worker is paid according to his hourly wage up to 40 hours, and 50%
more for overtime. Write a program in a script file that calculates the pay
to a worker. The program asks the user to enter the number of hours and
the hourly wage. The program then displays the pay.

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3.3.2 The if-else-end Structure

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Example
Give MATLAB code to calculate y where y = -1 when x < 0 and
y = 2 when x > 2.
>> if x < 0
y = -1
elseif x > 2
y = 2
end
The statement y = -1 is executed only if the condition x<0 is
true.
The statement y = 2 is executed only if the condition x<0 is
false and the condition x>2 is true.
The variable y is not defined when both conditions are false,
that is, when 0 <= x <= 2.

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3.3.3 The if-elseif-else-end Structure

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Example
The value of f(x) is -2x when x < 0; x(x-2) when x is in [0, 2] and
log(x-1) otherwise. Calculate f(x).
>> if x < 0
f = -2*x
elseif x <= 2
f = x*(x-2)
else
f = log(x-1)
end
The first block is executed when x < 0 is true.
The elseif is only reached if x < 0 is false, that is x >= 0, so you
only need to check x <=2.
The else is reached when all previous conditions are false, that is x
> 2.

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Exercise
Write a MATLAB if statement to calculate y where y = 1 if
x > pi/2, y = sin(x) if x is in [0, pi/2] and y = 0
otherwise.
Answer:
if x > pi/2
y = 1
elseif x >= 0
y = sin(x)
else
y = 0
end

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3.4 The switch-case Statement
• The switch-case statement is another method that can be used to direct the
flow of a program.
• It provides a means for choosing one group of commands for execution out of
several possible groups.
• If there is more than one match, only the first
matching case is executed.
• If no match is found and the otherwise
statement is present, the group of
commands between otherwise and end is
executed; if otherwise not present, none of
the command groups is executed.
• A case statement can have more than one
value: {value1, value2, value3, ...}.
• Break statement not required as C++.

3.5 Loops
3.5.1 for-end Loops
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P
A
S
S
k = 1:2:9
PASS 1: k=1
PASS 2: k=1+2=3
PASS 3: k=3+2=5
PASS 4: k=5+2=7
PASS 5: k=7+2=9
can be +ve or –ve
Default value is 1
for k=1:3:10
x = k^2
end
x=
1 16 49 100
EXAMPLE
LAST PASS:
If s is +ve (<=)
If s is –ve (>=)
Number of passes is known ahead of time

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Example
Given an integer n, calculate the sum of the integers k2
for k =
1,...,n.
The variable s is initialised to 0.
The index variable k starts at 1, then increases in steps of 1 until it
reaches n.
Each time through the loop the value of k2
is added to s.
Sol:
s = 0;
n=input('Enter the number of terms');
for k = 1:n
s = s + k^2;
end
fprintf('The sum of the series is: %f','S')

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CHAPTER 5
False
A variable should be assigned
the variable again must be assigned
a new value
x=1
while x<=15
x=2*x
end
EXAMPLE
x = 1
x = 2
x = 4
x = 8
x = 16
stop the execution of an indefinite
loop by pressing the Ctrl + C or Ctrl
+ Break keys.
Indefinite loop
Number of passes is NOT known in advance
3.5 Loops
3.5.2 while-end Loops

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3.6 The Break and Continue Commands
Write a program in a script file
that finds the smallest odd
integer that is dividable by 11
and whose square root is
greater than 132. Use a loop in
the program. The loop should
start from 1 and stop when the
number is found. The program
prints the message “The
required number is:” and then
prints the number.
for x=1:inf
if rem(x,11)==0 & sqrt(x)>132
break
else continue
end
end
fprintf('n The required number is: %2.3f
n',x)
x=1;
while true
if rem(x,11)==0 & sqrt(x)>132
break
end
x=x+1;
end
fprintf('The required number is: %0.2f
n',x)

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CHAPTER
FOUR
EXAMPLE 1
Use a for-end loop in a script file to calculate the
sum of the first n terms of the series:
Execute the script file for n = 4 and n = 20.
TIPS:
1. k=1 and n=4 & n=20
2. Use input command to
enter n separately.
3. Define S=0;
4. Write for-end loop and
enter the math expression
in between.
5. (-1)^k*k/2^k
6. Use an output command
to display the result
S=0;
n=input(‘Enter the number of terms’);
for k=1:n;
S=S+(-1)^k*k/2^k;
end
fprintf(‘The sum of the series is: %f',S)

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EXAMPLE 2
A vector is given by V = [5, 17, –3, 8, 0, –
7, 12, 15, 20, –6, 6, 4, –7, 16].
Write a program as a script file that
doubles the elements that are positive
and are dividable by 3 or 5, and, raises
to the power of 3 the elements that are
negative but greater than –5.
TIPS:
1. Define vector V
2. Use for-end loop command, k=1,
n=length(V)
3. If V elements are +ve & divisible by 3 or
5: make them double
4. If V elements are –ve & greater than -5:
^3
5. disp(V)

Chapter 3 - Programming in Matlab. aaaapptx

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