Control Statements in Matlab

Learning Objectives

How is program control implemented in Matlab.?

What are the simple output statements?

Are there any special values?

• • • • • • •

Topics

IF statement and Logical Operators Switch-Case Disp() vs fprintf() Input() Statement Display Format Special Values Summary

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for loop

for j=1:10 done computations for j=1:10 % computations; end • Repeats for specified number of times • ALWAYS executes computation loop at least once!!!

• Can use + or – increments • Can escape ( BREAK ) out of computational loop

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while loop

initialize k while k<10 done computations change k k=0; while k<10 % computations; k=k+1; end • Will do computational loop ONLY if while condition is met • Be careful to initialize while variable • Can loop forever if while variable is not updated within loop!!!

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if statements

if condition true statements false if condition true statements (1) false statements (2) if A>10 % computations; end if A>10 % computations; else % computations; end • Can include multiple statements • Statements can also include other if statements (can nest if statements inside if statements) • Be careful not to overlap (crossover) if statements!

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if-elseif statement

if condition true statements (1) false elseif condition true statements (2) false

…

if A>10 % computations; elseif A<10 % computations; else % computations end elseif condition false else statements (n) statements (n+1) • Can have several elseif conditions… • Else is optional and executes if all other tests fail

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If Statement and Logical Operators

Relational Operators

< <= > >= == ~= less than less than or equal to Greater than greater than or equal to equality not equal

What is the value of K?

K=5 Interpret the following in words K>10 K*0 ~= 6

What if K is an array?

K=ones(5,5)

All elements in K are tested

if K>10

will fail, but

K(2,3)=20; if K>10

will also fail becase ALL elements must be >10.

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if Statement and Logical Operators (Cont.)

Logical Operators OP not and or xor Symbol ~ & | Note : 0 is false Anything else is true

A 0 0 1 1

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B 0 1 0 1 ~A 1 1 0 0 A|B 0 1 1 1 7 A&B 0 0 0 1 A xor B 0 1 1 0

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Relational Operators

• When relational operators are present: –

All arithmetic operations are performed first (in their particular order)

–

The relational operators are evaluated after.

• Example 1 –

(2*3) > (4+1); % solve by hand, then type this into MATLAB

-

The multiplication and addition are first:

-

6 > 5

The relational operator is evaluated:

-

6 is greater than 5, so this returns 1 (true)

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Examples

a=7; b=4; c=3; ~(a==3*b)

– – –

Evaluates 3*b = 12 Reads: is (a==12) not (from the ~) true?

Returns ans = 1 (true) a > 5 & b > 5

– – –

Evaluates (a>5) and (b>5) separately. One returns true, the other returns false.

Since both are not true, the expression returns false.

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Using Logicals in Assignments

• True/False values can be assigned to variables.

• The variables will be assigned the value that returns from relational and/or logical operators.

• The variables will have a value of 1 or 0.

• Example: –

X = a > 2;

– •

Then x = 1; Y = b==5;

•

Y will be equal to 0.

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More Examples

a=6; b=10; c=-2;

• Try the following examples without the use of Matlab: –

X1 = abs(c)>3 & c<(b-a) & b+a > 3

–

X2 = (b==10) | (a< 4)

–

X3 = a.*5 >= b.*3 & c < a

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Operator precedence

1. transpose (.'), power (.^), complex conjugate, transpose ('), matrix power (^) 2. unary plus (+), unary minus (-), logical negation (~) 3. multiplication (.*), right division (./), left division (.\), matrix multiplication (*), matrix right division (/), matrix left division (\) 4. addition (+), subtraction (-) 5. colon operator (:) 6. less than (<), less than or equal to (<=), greater than (>), greater than or equal to (>=), equal to (==), not equal to (~=) 7. logical AND (&) 8. logical OR ( | )

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Practice

• Evaluate the following without Matlab: –

Practice without the help of Matlab because you will not be able to use Matlab in the midterm.

– – – – –

a = 4; b = 20; c = 12; d = 5; One = a>4 & b==20 Two = b<40 | c>10 Three = d.*c > a.*b Four = b.*3<= 100 & d<10 & a.*d==b

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More practice

• When comparing vectors, the operator (>, <=, ~, &, etc.) is applied element-by-element:

a = [0,2,4,2]; b = [4,1,-2,3];

• What is: –

C = a .* b;

– –

C = b.^2-a.*b C = a >= b;

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If statement example

%DEMO %Header function output = DEMO(input) %put help info here!

%Do stuff if input > 0 fprintf(‘Greater than 0’) elseif input < 0 fprintf(‘Less than 0’) else fprintf(‘Equals Zero’) end %Set return value if needed outvar = 1;

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What’s an alternative to the if statement?

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Switch-Case Statement

switch case

expression condition_1

%Do Stuff #1 case {condition_2a, condition_2b,…} %Do Stuff #2 … otherwise %Do Other Stuff end How does this relate to the ‘if’ statement?

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Switch-case statement example

x=2.5; units=‘m’; switch case units %convert to centimeters {‘inch’,’in’} y=x.*2.54; case {‘feet’,’ft’} y=x.*2.54.*12; case {‘meter’,’m’} y=x./100; case {‘centimeter’,’cm’} y=x; otherwise disp([‘Unknown units: ‘ units]) y=NaN; end

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Display Formats

Enter the following Matrix into Matlab… M = [55.3 -22 12; 10 23.4324 30.42]

Let’s explore each format: COMMAND format short format long format short e format long e format bank format + FUNCTION default 14 decimals 4 decimals 15 decimals 2 decimals +,-,blank

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Disp() and fprintf()

• disp(X) – prints elements of an array X • disp(‘hello world’) – prints the string • fprintf(fid, format, A) – – does the following:

Write A to file fid using format ( omitting fid prints to screen)

–

format is a string containing output string and format instructions for each variable in A

–

Variables of all printable data types:

Conversion specifications involve the character %, optional flags, optional width and precision fields, optional subtype specifier, and conversion characters: d, i, o, u, x, X, f, e, E, g, G, c, and s.

–

The special formats carriage return, tab, backspace, and formfeed characters respectively.

\n,\r,\t,\b,\f can be used to produce linefeed,

• Let’s use DEMO to explore these differences.

• We will discuss I/O in further depth in a later lecture

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Demonstration Problem

% This program will calculate the % area and circumference of ten circles, % allowing the radius as an input, % but will only provide output for circles % with an area that exceeds 20.

N = 0; R = 0.0; AREA = 0.0; CIRC = 0.0; for J = 1:1:10 R = input('Please enter the radius: '); AREA = pi * R^2; CIRC = 2.0 * pi * R; if AREA > 20.0

fprintf('\n Radius = %f units',R) fprintf('\n Area = %f units squared', AREA) fprintf('\n Circumference = %f units\n', CIRC) else N = N + 1; end end fprintf('\n Number of circles that do not have area > 20: %.0f \n', N)

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Getting User Input

• How do I prompt user for input?

Myvariable = input (‘Some String’);

• How can I format this better for the user?

Myvariable = input (‘Another String’, ‘s’);

• What’s the difference between the two input lines?

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Special Values

These objects have special meanings in Matlab: pi i,j the value 3.1416 (How would I see more values?) sqrt(-1) (How is this represented?) inf NaN infinity (How can you prove this represents infinity?) “Not a number” (When do we get this message?) clock - matrix with date and time date – Current date in string form eps – “Epsilon” the smallest amount by which two values can differ on the current computer ans – just computed value

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Summary

• • • • • • •

Topics

IF statement and Logical Operators Switch-Case Disp() vs fprintf() Input() Statement Display Format Special Values Summary

Action Items

•

Review the lecture

•

Work out the simple control statement examples

•

How do these compare to other languages you have used?

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Problem Statements

Sequential computation 1 Draw a flowchart and write the MatLab code that • • •

will calculate the area and the circumference of a circle allow the radius to be an input variable output radius, area and circumference.

Introduce ‘if’ structure 2 Draw a flowchart and write the MatLab code that • • •

will calculate the area and the circumference of a circle allow the radius to be an input variable output radius, area and circumference IF the area is greater than 20 square units.

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Problem Statements … contd.

Introduce ‘for’ loop 3 Draw a flowchart and write the MatLab code that • • • •

will calculate the area and the circumference of TEN allow the radius to be an input variable circles output radius, area and circumference IF than 20 square units.

output the number of circles with area



the area is greater 20.

Introduce ‘while’ loop 4 Draw a flowchart and write the MatLab code that •

will calculate the area and the circumference of ANY NUMBER of circles

• • •

allow the radius to be an input variable output radius, area and circumference IF than 20 square units.

output the number of circles with area



the area is greater 20.

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