COMP 116: Introduction to
Scientific Programming
Lecture 11: Functions
So far

Script files
◦ All code inside one big file
◦ Perhaps structured into cells

Used built-in matlab functions
◦ sin, cos, zeros etc.
How do we structure more complex
code?
 How do we write our own functions?

Calling Functions

How does MATLAB call its own functions?
% MyScript.m
x = [4 3 9 2 9 1 2 7 4];
maxX = max(x);
...
...

input
max.m
output
Matlab loads it’s own function files and runs
them through the interpreter
◦ Input variables map onto function inputs
◦ Function outputs get stored in specified variables
Calling Functions

How does MATLAB call its own functions?
% MyScript.m
x = [4 3 9 2 9 1 2 7 4];
maxX = max(x);
...
...

input
max.m
output
In MATLAB, each function should go into a
separate m-file
Syntax vs. Semantics

What is syntax?
◦ Grammar
◦ Rules that let you write in the language
◦ Punctuation, etc.

Why do we need syntax rules?
◦ Syntax rules allow compilers and interpreters
to correctly convert our source code into
something the computer understands.
Semantics

What are semantics?
◦ Meaning
◦ What does your function actually do?
◦ What problem(s) does it solve?
Writing a function: Syntax
function [outputs] = funcName( inputs )
% Function Comments
…
% Body (implementation)
end %optional
Note: The name of the function and the name of the m-file
should be the same
Function Syntax

Must start with function keyword
◦ Otherwise, it’s a script
Function Syntax

Function name
◦ Again: remember that this must be the same as the name of the
m-file
Function Syntax

Function return values/output
◦ Potentially multiple values may be returned from the function
◦ [r, c] = size(A)
Function Syntax

Function input values/parameters
◦ Potentially multiple arguments may be passed into a function
◦ s = sum(A, 2)
Function Syntax

Comment block, just below the first line
◦ Searched by lookfor
◦ Displayed when you type help
Function Syntax

Function implementation
◦ Where you do all the ‘work’
◦ Has comments, expression, function calls…
Jargon

Parameters
◦ The variables declared in the function interface

Arguments
◦ The actual values supplied when the function is
called.
These are function parameters
When calling the function:
c = DiceToss(num_throws, desired_value);
These are function arguments
A summary of function rules

Most important: function name and its
corresponding .m file name should match.

Functions can have several inputs
◦ common in most languages

Functions can also have several outputs
◦ This is different from most other languages.

Input and output are optional

Comments are optional
◦ But a good programming practice
More rules …

One function per file
◦ Exception: helper functions
 Meant to only be used internally by the main function
function [avg, med] = newstats(u)
% NEWSTATS Find mean w/ subfuctions.
n = length(u);
avg = helper_mean(u, n);
function a = helper_mean(v, n)
% Subfunction: calculate average.
a = sum(v)/n;
All in a single m file
More rules …

Function Names are case sensitive
◦ DiceToss is different from dicetoss is
different from diceToss…
More rules …
function [avg, med] = newstats(u)
% NEWSTATS Find mean w/ subfuctions.
n = length(u);
avg = helper_mean(u, n);
function a = helper_mean(v, n)
% Subfunction: calculate average.
a = sum(v)/n;
More rules …
function [avg, med] = newstats(u)
% NEWSTATS Find mean w/ subfuctions.
n = length(u);
avg = mean(u, n);
function a = mean(v, n)
% Subfunction: calculate average.
a = sum(v)/n;

Gotcha: you can accidently hide system
functions, constants, and workspace variables
by creating your own function with the exact
same name.
More rules …

Be careful with parentheses: [] vs ()
◦ [r, c] = size(A)
◦ (r, c) = size(A)
◦ [r, c] = size[A]

Incorrect
Think:
◦ Difference between
 myfunc([1, 2, 3]) and myfunc(1, 2, 3)
Function examples
Multiple inputs
No inputs
Multiple outputs
No outputs
Exercise 1

Write an absolute value function
◦ Assume the input is just a scalar

Convert your guess-the-number script to
a function
◦ What is the input?
◦ What is the output?
Scope

Functions run in their own ‘workspaces’
MATLAB
foo =4 x2 =5
bar =16
sq.m
x =4
x2 =16
Scope: Global Variables (Workspace)

Global MATLAB workspace
◦ Variables belonging to script files and command
window

Workspace Variables
◦ come into existence after they are created by
assignment.
◦ exist until MATLAB quits or clear command is
used on variables to remove them.
◦ Accessible from command window and scripts
◦ NOT accessible from inside functions
Scope: Local Variables (Functions)

Function workspaces
◦ Local scope

Variables
◦
◦
◦
◦
◦
Parameter variables live from function entry
Local variables live from assignment
Until function finishes (or clear)
Local workspace is cleared at end of function
Output copied/assigned to variables in calling
workspace
Scripts vs. Functions
Why use Functions?
Top-down design
 Encapsulation
 More flexible, resuable code
 Testing strategy

Top-down design

Break a complex problem into simpler
manageable problems

Solve simpler problems

Connect simple solutions to solve original
problem
Functions give your code structure
Encapsulation

A function is isolated from the rest of the
system, and interacts only through its input
and output arguments.
◦ A function can't mess up the variables in your
workspace
◦ Likewise, you can't mess up a function by
changing values

Much more powerful, and fewer ‘sideeffects’ than scripts
Flexible, reusable code


A script only solves one instance of a problem
A function can solve all instances
◦ You can call hypotenuse with any values of a and b

Since functions are encapsulated, this means you
only need to know its interface (what it does), not
its implementation (how it does it)

Share your solution to a problem with others.
Collaboration

◦ Team, organization, world
Easier testing

If you write your program as a 500-line script,
and it gives the wrong answer. . .
◦ Good luck with that!

If you write your program as a small function
that calls other functions that call other
functions. . .
◦ Test the simplest functions first
◦ Check that functions are connected correctly
Variable number of inputs

How does a function like min() work?
◦ It can take a variable number of inputs
 min(x);
 min(x, 1)
 min(x, [], 1)

varargin, nargin
◦ varargin is a cell array – we’ll talk about cell
arrays later
◦ The variable nargin is automatically set in the
local workspace of each function, and tells you
how many input variables were actually supplied
to the function.
Variable number of outputs

How does size() work?
◦ Can return variable number of outputs

varargout, nargout
◦ nargout returns the number of output
arguments specified for a function.
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COMP 116 - Introduction to Scientific Programming