3 matlab training

3. MATLAB Dr. Kashif Mahmood Rajpoot

SEECS, NUST Islamabad [email protected]

NUST Science Society (NSS) Workshop

Workshop Contents: Session 3

• Conditions: if, else

• Loops: for, while

• Vectorization

• Function: input/output arguments, saving, opening file, naming, calling function

Selection Statements

• Program execution is sequential by default – i.e. top-to-bottom execution of statements

– Statements are executed one after another in a fixed sequential order

– Each statement is executed only once

• Controlling the order in which the instructions are executed – Jump/selection in the order of statements under

some condition (if-else)

If-else • First, the condition is evaluated

– True, action1 is executed – False, action2 is executed

% This script calculates the area of a circle % It error-checks the user’s radius radius = input(‘Please enter the radius: ’); if radius <= 0 fprintf(‘Sorry; %.2f is not a valid radius\n’,radius) else area = 2*pi*(radius^2); fprintf(‘For a circle with a radius of %.2f,’,radius) fprintf(‘the area is %.2f\n’,area) end

If-else

if ( grade >= 60 )

disp(‘Passed’)

else

disp(‘Failed’)

Relational operators • Relational operators – test the “relation”

between two expressions/numbers/letters – Important, note the == symbol for testing the equality – Don’t mix/confuse it with = symbol, which is the

assignment operator

• Relational operators produce logical result of either 1 or 0 – 1 – true – 0 – false

• Can be used with numbers, letters, variables, scalars, vectors, and matrices – For vectors/matrices, the shape and size must match

Logical operators

• Often, to test multiple conditions we can use logical operators like:

– AND (&&), OR (ІІ), NOT (~)

– x>=0 && y>=0

– X>=0 ІІ y >=0

– ~(x>=0)

Operator Precedence

If-elseif • If a condition is false, instead of jumping to an

else we can test alternative conditions by elseif

marks ≥ 80 A

60 ≤ marks < 80 B

marks < 60 F

Logical Functions

• MATLAB built-in functions that perform some logical test

Loops

• Default execution of programs is:

– Top-to-bottom

– Every statement is executed only once

• Loops permit us to execute a code block more than once under a specified condition

– Repeat a code block indefinite number of times, until some condition is met (while loop)

– Repeat a code block definite number of times, i.e. number of repetitions are known in advance (for loop)

for loop

• for loop executes a code block a specified number of times

– index is called the loop variable or loop index

– expr is the loop control expression

– Columns in expr are stored in index and loop is executed once for each column in expr

– expr can also be of the form first:incr:last

for loop

• MATLAB generates control expression

• Program assigns first column to index

• Execute the loop body

• Assign the next column to index

• Again, execute the loop body

• Repeat above step as long as there are any columns left in the control expression vector

Example syntax: for loop

Example: for loop

• Factorial function

Loops – important points

• Loops vs. vector operations

• Since MATLAB is designed for array/vector/matrix operations, it is often possible to perform operations on the entire array

• Vector operations may be significantly faster than loop operations

while loop

• while loop continuously executes a set of statements until the specified condition is met

• Expression is non-zero (i.e., true)

– Code block is executed

– Control returns to while statement

– Re-check the expression

– Continue with execution of code block

– Repeat until expression becomes zero (false)

Functions

• Dividing a task into individual subtasks according to their ‘function’ – E.g., departments of an organization

• Division helps to tackle subtasks easily and efficiently

• Subtasks can then be utilized (& re-used)

• In MATLAB, functions permit us to write each ‘subtask’ as an individual unit that can be ‘called’ from some place in the code

Functions • Special type of m-file

• Runs in its own independent workspace

– This is important: functions can’t access the normal workspace of MATLAB

– Variables in MATLAB’s workspace are not directly accessible by functions

– Variables created by functions are not directly accessible by MATLAB

Functions

• Functions receive input data through input arguments, and return the results through output arguments

• General format of a function is:

function *outarg1, outarg2, …+ = fname(inarg1, inarg2, …)

% comments

Executable code

Functions

• MATLAB’s built-in functions – fprintf, disp, input, sin, cos, mean, std, exp, … – They take input arguments, perform some processing

using that input, and return the results as output arguments

• Note that passing the input arguments and returning the output arguments are not mandatory – i.e., we can write functions that do not accept input

arguments and/or do not return output arguments – E.g., tic, toc, clock, now, whos

Functions • Script vs Function • Script – collection of code placed together in an

m-file – Run by typing the filename – Running a script is same as writing the statements in

MATLAB’s command window – Shares MATLAB’s workspace

• Function – collection of code placed together as a function in an m-file – Run by typing the filename – On calling, we can provide input arguments and get

output arguments – Uses its own private workspace

• Calling a user-defined function – Can be called from MATLAB’s command window

or from a script file or from inside another function

– The function is called/invoked by writing its name

– Will need to provide any mandatory input arguments and receive any returned output arguments

• Example – dist2.m – distance between 2 points

– test_dist2.m – test the distance function

Functions

Variable passing: Pass-by-value

• MATLAB programs communicate with functions using a pass-by-value scheme

• On a function call, the input arguments are passed by copying their value into new variables inside the function

– i.e., any modification inside the function does not affect the variables outside the function

Example: coordinate system