Chapter 2 - Matlab Environment

Matlab for Engineers

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r Matlab Environment

Chapter 2

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In this chapter we’ll…

• Get started with Matlab• Explore the Matlab windows• Solve some problems using

Matlab• Learn how to save our work

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Section 2.1Getting Started

• In Windows or Apple operating systems click on the desktop icon

• In Unix type matlab

At the shell prompt

MATLAB 7.0.lnk

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Matlab opens to a default window configuration

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• Matlab uses a standard windows menu bar

• To exit Matlab use the close icon

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Command WindowEnter commands at the promptCurrent Directory

MATLAB WindowsCommand History

Workspace Window

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You can use the command window much like you’d use a calculator

The standard order of operation rules apply

Let’s try Practice Exercise 2.1

To watch this demonstration, click in the figure window. To move to the next slide use the down arrow

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Section 2.2Matlab Windows

• Matlab uses several different windows to display data, commands and results.

• They are not necessarily all open at once

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Let’s look at the windows one at a time

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Command Window

• Similar to a scratch pad• Once you hit enter, you can’t edit

any commands• You can retype them or use the

arrow keys to retrieve commands and edit them before hitting enter again

Command Window

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Command History

• Records the commands you issue in the command window

• When you exit the command window, or when you issue the clc command, the command window is cleared

• But the command history remains

Command History

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Command History

• You can transfer commands from the command history to the command window– Double click on a command– Click and drag

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Workspace Window

• Keeps track of the variables you’ve defined– Name– Value– Class– Size– Bytes

• You may need to click on the name bar and select size and bytes in order to see these parameters

Workspace Window

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When you define variables in the command window, they are listed in the workspace

window

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Scalar

Vector

2-D Matrix

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Current Directory

• The current directory window is a list of files

• When you try to load information from a file or try to save information – Matlab uses the current directory

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Document Window

• If you double click on any variable in the workspace window Matlab launches a document window containing the array editor

• You can edit variables in the array editor

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Document Window

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Figure Window

• When Figures are created a new window opens

• It’s extremely easy to create graphs in Matlab

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The semicolon suppresses the output from each command

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Editing Window

• This window allows you to type and save a series of commands without executing them

• There are several ways to open an editing window– From the file menu– With the new file icon

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Open an editing window from the file menu or with the new file icon

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Save and Run

Write your code in the editing window, then run it using the Save and Run icon

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Section 2.3Solving Problems with Matlab

• We’ve already solved some simple problems

• We need to understand how Matlab works to solve more complicated problems

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Variables

• MATLAB allows you to assign a value to a variable

• A=3• Should be read as A is assigned a

value of 3• Use the variables in subsequent

calculations

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Naming Variables

• All names must start with a letter• They may contain letters,

numbers and the underscore ( _ )• Names are case sensitive• There are certain keywords you

can’t use

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Use the iskeyword function for a list of keywords

iskeywordans =

'break' 'case' 'catch'

'continue' 'else'

'elseif' 'end' 'for'

'function'

'global' 'if' 'otherwise' 'persistent' 'return' 'switch' 'try' 'while'

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You can reassign function names

• Matlab will let you use built-in function names as variables – but it’s a really bad idea

• sin = 3 changes sin from a function to a

variable name

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Practice Exercise 2.2Which of these names are allowed

in Matlab?

• test• Test• if• my-book• my_book• Thisisoneverylongnamebutisitstillallowed?• 1stgroup• group_one• zzaAbc• z34wAwy?12#• sin• log

x xbad idea

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Matrices in MatlabThe basic data type

• Group of numbers arranged into rows and columns

• Single Value (Scalar)– Matrix with one row and one column

• Vector (One dimensional matrix)– One row or one column

• Matrix (Two dimensional)

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Scalar Calculations

• You can use MATLAB like you’d use a calculator

>> 9 + 10ans=19

Command Prompt

Result

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Assignment Operator

• To define a variable a we might typea=1+2which should be read as:a is assigned a value of 1+2

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How is the assignment operator different from an equality?

• In algebra the equationx=3+5means that both sides are the same

• In computers when we sayx=3+5we are telling the machine to store the value on the right hand side of the equation in a memory location, and to name that location x

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Is that really different?

• Yes!!!• In algebra this is not a true statement

x=x+1• In computers (assignment statements)

it means replace the value in the memory location named x, with a new value equal to x+1

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Order of Operation

• Same as you’ve learned in math class

• Same as your calculator– Parentheses first– Exponentiation– Multiplication / division– Addition / subtraction

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Order of Operation

5*(3+6) = 45

5*3+6 = 21

White space does not matter!!!

5*3 + 6 = 21

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Parentheses

• Use only ( )• { } and [ ] mean something

different • MATLAB does not assume

operators

5 * (3+4) not 5(3+4)

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Compute from left to right

5*6/6*5 = 25

5*6/(6*5) = 1

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Here’s an exampleFind the surface area of a cylinder

)(222 2 hrrrhrSA

r = radiusr = 5

h = heighth = 10

2π r * h

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Array Operations

• Using Matlab as a glorified calculator is OK, but its real strength is in matrix manipulations

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To create a row vector, enclose a list of values in brackets

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You may use either a space or a comma as a “delimiter” in a row vector

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Use a semicolon as a delimiter to create a new row

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Use a semicolon as a delimiter to create a new row

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Hint: It’s easier to keep track of how many values you’ve entered into a matrix, if you enter each row on a separate line. The semicolons are optional

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Shortcuts

• While a complicated matrix might have to be entered by hand, evenly spaced matrices can be entered much more readily. The command

b= 1:5 or the command

b = [1:5] both return a row matrix

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The default increment is 1, but if you want to use a different increment put it between the first and final values

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To calculate spacing between elements use…

• linspace• logspace

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Initial value in the array

Final value in the array

number of elements in the array

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Initial value in the array expressed as a power of 10

Final value in the array expressed as a power of 10

number of elements in the array

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• You can include mathematical operations inside a matrix definition statement.

• For example

a = [0: pi/10: pi]

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Mixed calculations between scalars and arrays

• Matrices can be used in many calculations with scalars

• There is no confusion when we perform addition and subtraction

• Multiplication and division are a little different

• In matrix mathematics the multiplication operator (*) has a very specific meaning

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Addition between arrays is performed on corresponding elements

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Multiplication between arrays is performed on corresponding elements if the .* operator is used

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Matlab interprets * to mean matrix multiplication. The arrays a and b are not the correct size for matrix multiplication in this example

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Array Operations

• Array multiplication .*• Array division ./• Array exponentiation .^

In each case the size of the arrays must match

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The matrix capability of Matlab makes it easy to do repetitive calculations

• For example, assume you have a list of angles in degrees that you would like to convert to radians. – First put the values into a matrix. – Perform the calculation

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Either the * or the .* operator can be used for this problem, because it is composed of scalars and a single matrix

The value of pi is built into Matlab as a floating point number, called pi

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More about pi

• Because pi is an irrational number, it can not be expressed exactly with a floating point representation

• The Matlab constant , pi, is really an approximation.

• If you find sin(pi) Matlab returns a very small number.

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Transpose

• The transpose operator changes rows to columns or vice versa.

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The transpose operator makes it easy to create tables

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table =[degrees;radians]’ would have given the same result

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The transpose operator works on both one dimensional and two dimensional arrays

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Number Display

• Scientific Notation– Although you can enter any number in

decimal notation, it isn’t always the best way to represent very large or very small numbers

– In Matlab, values in scientific notation are designated with an e between the decimal number and exponent. (Your calculator probably uses similar notation.)

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It is important to omit blanks between the decimal number and the exponent. For example, Matlab will interpret

6.022 e23as two values (6.022 and 1023 )

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Display Format

• Multiple display formats are available• No matter what display format you

choose, Matlab uses double precision floating point numbers in its calculations

• Matlab handles both integers and decimal numbers as floating point numbers

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Default

• The default format is called short• If an integer is entered it is

displayed without trailing zeros• If a floating point number is

entered four decimal digits are displayed

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Other formats

• Changing the format affects all subsequent displays– format long results in 14 decimal

digits– format bank results in 2 decimal

digits– format short returns the display to

the default 4 decimal digits

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Really Big and Really Small

• When numbers become too large or too small for Matlab to display using the default format, it automatically expresses them in scientific notation

• You can force scientific notation with – format short e– format long e

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Common Scale Factor

• For long and short formats, a common scale factor is applied to the entire matrix if some of the elements become very large, or very small. This scale factor is printed along with the scaled values.

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Common Scale Factor

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Two other formats

• format +• format rat

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Spacing in the command window

• The format command also allows us to control how tightly information is spaced in the command window – format compact– format loose – (default)

• Most of the examples in this presentation use format compact

Notice that the value of A is still being displayed using the rat format, because we haven’t changed it back to format short

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Section 2.4Saving Your Work

• If you save a MATLAB session, all that is saved are the values of the variables you have named

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Variables are saved, not the work space

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Save either by using the file menu or...

Save with a command in the command window

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Matlab automatically saves to a .mat file

• If you want to save to another format, such as .dat, you need to explicitly tell the program

save <file_name> <variable_list> -ascii

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Script M-files

• If you want to save your work, you need to create an M-file

• File->New->M-file• Type your commands in the edit

window that opens

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• The file is saved into the current directory

• It runs in the command window

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Comments

• Be sure to comment your code– Add your name– Date– Section #– Assignment #– Descriptions of what you are doing

and why

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The % sign identifies comments You need one on each line

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Summary

• Introduced the MATLAB Windows• Basic matrix definition• Save and retrieve MATLAB data• Create and use script M-files

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