1 week 1: variables, assignment, expressions reading: 1.2 – 1.4

Variables

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• To store a value in a MATLAB program, a variable is used

• To create a variable, we use an assignment statement:

variablename = expression

The variable is always on the left, followed by the = symbol, followed by an expression

• A variable stores a value that can be changed at any time

EECS 1541 -- Introduction to Computing for the Physical Sciences

Variables

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• Example:

• The variable “z” is on the left, followed by the = symbol

>> z = 6

z =

6

• This means a value of 6 is assigned to the variable “z”


Variables

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• Putting a semicolon at the end of a statement suppresses the output

• Example: >> z = 6;>>

• This would assign a value of 6 to the variable z, but the result is not shown.


Variables

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1. a variable name must start with a letter

2. the rest of the name can include letters, digits, or underscores

3. names are case sensitive, so “A” and “a” are two different variables

4. MATLAB has some reserved words called keywords that cannot be used as variable names

• use the command iskeyword to get a list of keywords


Variables

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• use short, meaningful names

• a name that conveys the purpose of the variable is often useful for others who need to read your code, e.g., use

massEarth instead of mEmassSun instead of mS

• exceptions to the rule:• if you are solving a problem that contains variable names, you should try to

use the same names, e.g., in physics the following would likely be common:

g, c, v0 (g = gravity, c = speed of light, v0 = initial velocity)


Variables

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• Examples:

valid variable names

invalid variable names

reason invalid

x $ • does not begin with a letter• $ is not allowed in variable names

x6 6x • does not begin with a letter

lastValue If • if is a keyword

pi_over_2 pi/2 • punctuation marks are not allowed in variable names


Operators

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• Numerical expressions are created using values, variables, operators, built-in functions and parentheses.

+ addition- negative, subtraction* Multiplication/ division ^ exponentiation ()parentheses

• Common operators used with numerical expressions:


Operator Precedence Rules

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• Just like normal mathematical computations, some operators have precedence over others in MATLAB.

operator name precedence

( ) Parentheses • Highest

^ Exponentiation

- Negation

*, /, \ Multiplication and division

+, - Addition and subtraction • Lowest


Operators

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• Example: >> y = 6 - 5 * 8 + 9

y =

• First 5 is multiplied by 8, then the result is subtracted by 6 and added to 9

• What about: >> y = (6 – 5 * 8) + 9

y =

-25


-25

Operators

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• What about: >> y = 6 - 5 * 8 + 9;

>> y = y + 1

y =

• The above computation where y = y + 1 is called incrementing, it increases the previous value by 1

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Constants

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• Recall that variables are used to store values that might change

• Constants are values that cannot be changed at any time. Some constants that are pre-defined in MATLAB are:


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• Example: >> pi_over_2 = pi / 2

pi_over_2 =

1.5708

Constants


Built-in Functions

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• most MATLAB is provided through functions

• a function in MATLAB accepts a set of inputs and (usually) calculates a set of outputs• there can be 0 or more inputs• there can be 0 or more outputs

• the user of the function provides the inputs• the input values are called arguments to the function

• the function provides the outputs• the user uses the name of the function to use the function

• we say that the user calls the function


Built-in Functions

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• A list of elementary math functions from MATLAB can be found using the following command:

>> help elfun


Built-in Functions

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• This list introduces a long list of functions:


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• What does the following expression mean?

• We can use help to search for the meaning of exp

>> exp(1)

>> help exp

Built-in Functions: Exponential functions


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>> help exp

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• Since exp is the exponential function, exp(1)means evaluate the exp function with an input argument of 1 (i.e. = e1)

>> exp(1)

ans =

2.7183

• Note: MATLAB uses a default variable named “ans” if an expression is typed at prompt and it is not assigned to a variable



Built-in Functions: Trigonometric functions

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>> help sin• Example:


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• Consider the following expressions:

y = 2 * sin(pi/2);z = yz = pi

• What will the output values of the above program?



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y = 2 * sin(pi/2);z = yz = pi



NOTE: “ ; ” is added after the statement, so the final value “2” will not be displayed

• The first expression will evaluate the sin function with an input argument of pi/2, which is equal to 1, then multiply by 2

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y = 2 * sin(pi/2);z = yz = pi

• The second expression simply means

whatever is in y is now also assigned to z





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y = 2 * sin(pi/2);z = yz = pi

• The second expression simply means

whatever is in y is now also assigned to z





• The last expression means to assign the constant pi (or 3.1416) to variable z

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• NOTE: the new value assigned to z will overwrite the previous value


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>> help sin


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>> help sind

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• Example: >> y = sind(90)

y =

1

• What do you expect to see from:

>> y = cos(sind(360))

y =

1


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Built-in Functions

• If you are interested in other built-in functions in MATLAB:


http://www.mathworks.com/help/matlab/functionlist.html

1 week 1: variables, assignment, expressions reading: 1.2 – 1.4

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