Control Statements in Matlab
Learning
Objectives
How is program
control
implemented in
Matlab.?
What are the simple
output statements?
Are there any
special values?
AE6382 Design Computing
Topics
• IF statement and Logical
Operators
• Switch-Case
• Disp() vs fprintf()
• Input()
• Statement Display Format
• Special Values
• Summary
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Fall 2006
for loop
done
for
j=1:10
for j=1:10
% computations;
end
computations
•
•
•
•
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
done
while
k<10
k=0;
while k<10
% computations;
k=k+1;
end
computations
change k
• 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
if
condition
false
false
true
true
statements
statements (1)
statements (2)
if A>10
% computations;
else
% computations;
end
if A>10
% 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
false
true
statements (1)
elseif
condition
false
…
elseif
condition
false
else
true
statements (2)
statements (n)
statements (n+1)
if A>10
% computations;
elseif A<10
% computations;
else
% computations
end
• 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 if K is an array?
K=ones(5,5)
All elements in K are tested
if K>10
What is the value of K?
K=5
will fail, but
K(2,3)=20;
if K>10
Interpret the following in words
K>10
K*0 ~= 6
AE6382 Design Computing
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
B
~A
A|B
A&B
A xor B
0
0
1
0
0
0
0
1
1
1
0
1
1
0
0
1
0
1
1
1
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?
Fall 2006
Switch-Case Statement
switch expression
case 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 units
%convert to centimeters
case {‘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 +
AE6382 Design Computing
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 \n,\r,\t,\b,\f can be used to produce linefeed,
carriage return, tab, backspace, and formfeed characters
respectively.
• 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 inf -
the value 3.1416 (How would I see more values?)
sqrt(-1) (How is this represented?)
infinity (How can you prove this represents
infinity?)
NaN- “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
AE6382 Design Computing
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 circles
• allow the radius to be an input variable
• output radius, area and circumference IF the area is greater
than 20 square units.
• output the number of circles with area  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 the area is greater
than 20 square units.
• output the number of circles with area  20.
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