Lecture6 Eng.alhassan

7/28/2019 Lecture6 Eng.alhassan

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Basic of MATLAB

Lecture 6

Eng. Al-Hasan Al-Kuhlani 1


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Control Flow and Operators

MATLAB is also a programming language. Like othercomputer programming languages, has some decision

making structures for control of command execution.

These decision making or control flow structures include

for loops, while loops, if, and switch constructions.

Control flow structures are often used in script and

function M-files.

In a simple program as shown in the previous lessons,

the commands are executed one after the other.

Here we introduce the flow control structures that make

possible to skip commands or to execute specific group

of commands.Eng. Al-Hasan Al-Kuhlani2


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Relational Operators

For more information: help ops.

Relational operators:-

Eng. Al-Hasan Al-Kuhlani3

Function Description Relational

operatoreq Equal ==

ne Not equal ~=

lt Less than le Less than or equal =


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A relational operator compares two numbers by

determining whether a comparison is true or false.

Relational operators can be used to compare two

arrays of the same size or to compare an array toa scalar.

In the second case, the scalar is compared with all

elements of the array and the result has the same

size as the array.



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An output variable assigned to a relational or logicalexpression is identified as logical that is

True represented by 1

False represented by 0

What is False? false or 0

What is true? anything else

Special cases:

Empty string

Empty matrix []

What About NaN? Not true and not false NaN

if (NaN)

??? NaN's cannot be converted to logicals.



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Examples:-



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Logical operators.

Logical operators provide a way to combine or negaterelational expressions.

Function Description operator

- Short-circuit logical AND &&

- Short-circuit logical OR ||and - Element-wise logical AND &

or - Element-wise logical OR |

not - Logical NOT ~

xor - Logical EXCLUSIVE OR

any - True if any element of vector is nonzero

all - True if all elements of vector are nonzero



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Logical operators

The precedence from highest to lowest is relational

operators, followed by logical operators, &, and |.

Parentheses can be used to change the precedence and

should be used liberally to clarify the operations.



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Logical operators

Examples:-


R l i l d L i l


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Relational and LogicalFunctions



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Flow Control

Selection statements that test the results of relational or

logical functions or operators are the decision-makingstructures that allow the flow of command execution to becontrolled.

For more information: help lang .

The``if...end' structures:-

MATLAB supports the variants of if" construct.

if ... end

if ... else ... end

if ... elseif ... else ... End

Example:

reply = input('Would you like to see an echo? (y/n): ', 's');

if strcmp(reply,'y')

disp(reply)end Eng. Al-Hasan Al-Kuhlani11


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If-else-end Constructions


if(expression I)statement1a;

statement1b;

else

statement2a;

statement2b;

end

if(expression I)statement1a;

statement1b;

elseif(expression II)

statement2a;statement2b;

elseif(expression III)

statement3a;

statement3b;else

statement4a;

statement4b;

end


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if...end Structures:-

The if statement evaluates a logical expression and

executes a group of statements when the expression istrue .

The optional elseif and else keywords provide for theexecution of alternate groups of statements.

An end keyword, which matches the if, terminates the lastgroup of statements.

The groups of statements are delineated by the fourkeywords no braces or brackets are involved.

It should be noted that:

elseif has no space between else and if(one word)

no semicolon (;) is needed at the end of linescontaining if, else, end

the end statement is required



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Example (if)

Here we construct a conditional statement which

evaluates the function:

Solution:

If x>=0 & x1 & x


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Switch Selection Structure

The switch selection structure provides an alternative tousing the if, elseif ,and else commands.

The switch statement executes groups of statementsbased on the value of a variable or expression.

Only the first matching case is executed.

The syntax is:


switch expressioncase test expression 1

statementscase {test expression 2, test expression 3}

statements

otherwise

statements

end


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Switch Example 1

SwitchExample.m :

switch x

case 1

disp(x is 1)

case {2,3,4}

disp(x is 2, 3 or 4)

case 5

disp(x is 5)

otherwise

disp(x is not 1, 2, 3, 4 or 5)

end


>>x= 5>>SwitchExamplex is 5


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Switch Example 2

month = input(Enterfirst three letters of the month: ,s);

month = month(1:3); % Just use the first three lettersiflower(month)==feb

leap = input(Is it a leap year (y/n): ,s);

end

switch lower(month)

case {sep,apr,jun,nov}days = 30;

case feb

switch lower(leap)

case y

days = 29;otherwise

days = 28;

end

otherwise

days = 31;end Eng. Al-Hasan Al-Kuhlani17

The number of days in a particularmonth.30 days to September April, Juneand NovemberAll the rest have 31Except February alone

which has 28 days clearAnd 29 on leap year


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Loops

A loop is a structure that allows a group of statements to

be repeated. for Loop:

A for loop repeats a group of commands a fixed,predetermined number of times.

A for loop has the following structure:for variable=expression

statements

end

The commands between the for and end statements areexecuted once for every column in the expression,beginning with the first column and stepping through tothe last column.

At each step, known as an iteration, the appropriate

column of the expression is assigned to the variable.Eng. Al-Hasan Al-Kuhlani18


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For Loop

Rules for writing and using a for loop include:

1. If the expression results in an empty matrix, the loop willnot be executed.

2. If the result of the expression is a scalar, the loop will beexecuted once.

3. If the result of the expression is a vector, then each timethrough the loop, the variable will contain the next valuein the vector.

4. A for loop cannot be terminated by reassigning the loopvariable within the loop.

5. Upon completion of a for loop, the variable contains thelast value used.

6. The colon operator can be used to define the expressionusing the following format

for index = initial:increment:limit Eng. Al-Hasan Al-Kuhlani19


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For Loop

Usually, expression is a vector of the form i:s:j.

A simple example of for loop isfor ii=1:5

x=ii*ii

end

The following statements form the 5-by-5 symmetricmatrix A with (i; j) element i/j forji:

n = 5; A = eye(n);

for j=2:n

for i=1:j-1A(i,j)=i/j;

A(j,i)=i/j;

end

end Eng. Al-Hasan Al-Kuhlani20


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For Example 1

Using a for loop create a 1x10 vector where each entry isthe sum of its index and the square root of the previouselement (the zero element value is 0)


x = zeros(1,10);%Allocating memory

for i=1:10if (i == 1)

x(i) = i;else

x(i) = i + sqrt(x(i-1));end

end

x = zeros(1,10);

prev_num = 0;for i=1:10

x(i) = i + sqrt(prev_num);

prev_num = x(i);end


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For Example 2

Using for loops initiate a 3x3 matrix such that: Each entry

is the sum of its subscripts square

A = zeros(3,3);

% Dont forget to allocate memory!

for n = 1:3

for m = 1:3

A(n,m) = n^2 + m^2;end

end



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Conditional loops

Suppose we now want to repeat a loop until a certaincondition is satisfied.

This is achieved by making use of the MATLAB commandwhile , which has the syntax:

while expression

statements...

end

It is important to note that if the condition inside the

looping is not well defined, the looping will continueindefinitely.

If this happens, we can stop the execution by pressingCtrl-C.


x = 1

while x


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While loop

If all elements in expression are true, the commandsbetween the while and end statements are executed.

The expression is reevaluated and if all elements are stilltrue, the statements are executed again.

If the expression is false, control skips to the statementfollowing the end statement.

The variables modified within the loop should include thevariables in the expression, or the value of the expression

will never change. If the expression is always true (or is a value that is

nonzero), the loop becomes an infinite loop.



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Break, continue, and return

break

immediately breaks the loop.

Breaks only one loop

continue

jump to the next iteration.

return

returns control to the command line

(or to the calling function).


z=1;while z


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Exercises

Write a script to ask the user to input the scalarvalues for a, b, c, and x and then returns thevalue of ax+ bx+ c. The program repeats thisprocess until the user enters zero values for all

four variables.

Write a user-defined function to get the maximumvalue of three values.

Write a user-defined function that searches amatrix input argument for the element with theminimum value and returns the indices of that

element. Eng. Al-Hasan Al-Kuhlani26


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Exercises

x=input('Enter x in meter: ')disp('ft--> feet')

disp('in--> inch')

disp('me--> meter')

disp('cm--> centimeter')

disp('mm--> millimeter')

unit=input('Enter 2 char for unit you want to convert to: ')

Complete the previous script using switch control

structure.

Lecture6 Eng.alhassan

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