Computer Programming with MATLAB

Selection

Control Flow

Sequential control

MATLAB Interpreter

Control Construct

• A method by which the interpreter selects the next command to execute
• Sequential control: default
• Selection or Branching

if-statement

Most common selection construct: if-statement, Example:

                
              function guess_my_number(x) 
               if x == 2 
                fprintf('Congrats! You guessed my number.\n');
               end
               
               

Schematic of an if-statement

Condition: true

Condition: false

if-else-statement

Executing a different set of statements based
on the condition:

            
              function guess_my_number(x)
              if x == 2 
               fprintf('Congrats! You guessed my number!\n');
              else
                fprintf('Not right, but a good guess.\n');
              end 
              
              

if-else-statement

Condition: true

Condition:false

if-else-statement

if-statement: if conditional block end

Relational Operators

Produces a result that depends on the relation between its two operands. It can appear outside if-statements!

Logical Operators

Logical values:
◦ Non-zero: true
◦ Zero: false
◦ MATLAB returns 1 for true
How to combine logical values? Logical operators:

Truth table

not:
◦ flips the value of its (single) operand and:
◦ true if and only if both of its operands are true or:
◦ false if and only if both of its operands are false

Truth table

not:
◦ flips the value of its (single) operand and:
◦ true if and only if both of its operands are true or:
◦ false if and only if both of its operands are false

Precedence revisited

Precedence revisited

>> help 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. element-wise logical AND (&)
8. element-wise logical OR (|)
9. short-circuit logical AND (&&)
10. short-circuit logical OR (||)

Nested if-statements

1. if-statements can contain other if-statements
2. Consider the example with a single if-elseif-
else statement:

function ultimate_question(x)
if x == 42
fprintf('Wow! You answered the question.\n');
elseif x < 42
fprintf('Too small. Try again.\n');
else
fprintf('Too big. Try again.\n');
end

Nested if-statements

Nested if-statements

Polymorphic functions

Functions that behave differently based on
◦ Number of input or output arguments
◦ Type of input or output arguments
Many built-in functions are polymorphic (sqrt, max, size, plot, etc.) How do we make our functions polymorphic?

Number of arguments

Two built-in functions:
◦ nargin: returns the number of actual input arguments that the function was called with
◦ nargout: returns the number of output arguments that the function caller requested

Example : multiplication table

function [table summa] = multable(n, m)
The function multable returns an n-by-m multiplication table in the output argument table
Optionally, it can return the sum of all elements in the output argument summa
If m is not provided, it returns and n-by-n matrix

Example

function [table summa] = multable(n, m)
if nargin < 2
m = n;
end
table = (1:n)' * (1:m);
if nargout == 2
summa = sum(table(:));
end

Robustness

A function declaration specifies:
◦ Name of the function,
◦ Number of input arguments, and
◦ Number of output arguments
Function code and documentation specify:
◦ What the function does, and
◦ The type of the arguments
◦ What the arguments represent
Robustness
◦ A function is robust if it handles erroneous input and output arguments, and
◦ Provides a meaningful error message

Example

function [table summa] = multable(n, m)

if nargin < 1
error('must have at least one input argument');
end
if nargin < 2
m = n;
elseif ~isscalar(m) || m < 1 || m ~= fix(m)
error('m needs to be a positive integer');
end
if ~isscalar(n) || n < 1 || n ~= fix(n)
error('n needs to be a positive integer');
end
table = (1:n)' * (1:m);
if nargout == 2
summa = sum(table(:));
end

Comments

Extra text that is not part of the code
MATLAB disregards it anything after a % is a comment until the end of the line Purpose:
◦ Extra information for human reader
◦ Explain important or complicated parts of the program
◦ Provide documentation of your functions
 Comments right after the function declaration are used by the built-in help function

Example

function [table summa] = multable(n, m)

%MULTABLE multiplication table.
% T = MULTABLE(N) returns an N-by-N matrix
% containing the multiplication table for
% the integers 1 through N.
% MULTABLE(N,M) returns an N-by-M matrix.
% Both input arguments must be positive
% integers.
% [T SM] = MULTABLE(...) returns the matrix
% containing the multiplication table in T
% and the sum of all its elements in SM.

if nargin < 1
error('must have at least one input argument');
end
…
>> help multable

Persistent variable

Variables:
◦ Local
◦ Global
◦ Persistent
Persistent variable:
◦ It’s a local variable, but its value persists from one
call of the function to the next.
◦ Relatively rarely used
◦ None of the bad side effects of global variables.