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Author: Rajesh Bhaskaran, Cornell University [ Specification|MATLAB Intro Problem Specification] [Step {~}A~ R{~}B~|MATLAB Intro 1 Find Reactions] [Step 2: Calculate _σ{_}{_}{~}x{~}_ for _r{_}{_}{~}i{~}_ = 1 cm|MATLAB Intro 2 Calculations] [Step 3: Plot _σ{_}{_}{~}x{~}_ vs. _r{_}{_}{~}i{~}_|MATLAB Intro 3 First plot] [Step 4: Plot _σ{_}{_}{~}x{~}_ vs. _r{_}{_}{~}i{~}_: Take 2| MATLAB Intro 4 Second plot] [Step 5: Plot _σ{_}{_}{~}x{~}_ vs. _r{_}{_}{~}i{~}_, Take 3: File Input/Output|MATLAB Intro 5 Third plot] {color:#ff0000}{*}Step 6: Plot{*}{color} {color:#ff0000}{*}{_}σ{_}{*}{color}{color:#ff0000}{*}{_}{~}x{~}{_}{*}{color} {color:#ff0000}{*}vs.*{color} {color:#ff0000}{*}{_}r{_}{*}{color}{color:#ff0000}{*}{_}{~}i{~}{_}{*}{color}{color:#ff0000}*, Take 4: Functions{*}{color} {panel} h1. Step 6: Plot _σ{_}{_}{~}x{~}_ vs. _r{_}{_}{~}i{~}_ , Take 4: Functions Let's create a function to calculate the bending stress that outputs _σ{_}{_}{~}x{~}_ given (_M_, _r{_}{_}{~}i,~_ _r{_}{_}{~}o{~}_ ). Functions are really useful to break down your code into modules and also reuse parts of your code. Let's first pick MATLAB's brain on how to create functions in MATLAB. Bring up the following page in the Help navigator: *Help > Contents > MATLAB > Programming > M-File Programming > M-File Scripts and Functions > M-File Functions.* This page has a simple example: a function called _average_ that calculates the average of the elements in a vector. Take a couple of minutes to peruse this example. MATLAB has extensive documentation on the use of functions; however, one has to poke around a bit before finding the most useful information. I personally go for the examples first. The correct syntax for creating a function is: \\ {{{color:#0000ff}function{color} return_value =RB |
Step 6: Plot σx vs. ri , Take 4: Functions
Let's create a function to calculate the bending stress that outputs σx given (M, ri, ro ). Functions are really useful to break down your code into modules and also reuse parts of your code.
Let's first pick MATLAB's brain on how to create functions in MATLAB. Bring up the following page in the Help navigator:
Help > Contents > MATLAB > Programming > M-File Programming > M-File Scripts and Functions > M-File Functions.
This page has a simple example: a function called average that calculates the average of the elements in a vector. Take a couple of minutes to peruse this example. MATLAB has extensive documentation on the use of functions; however, one has to poke around a bit before finding the most useful information. I personally go for the examples first.
The correct syntax for creating a function is:
function return_value = function_name(parameter_1,
...
parameter_2,...)
...
%function description - MUST be in a comment
code...
...
return_value
...
=
...
value
A few noteworthy points to ponder:
- return_value
...
- is
...
- the
...
- only
...
- data
...
- that
...
- gets
...
- passed
...
- back
...
- to
...
- the
...
- main
...
- code.
...
- You
...
- do
...
- not
...
- need
...
- to
...
- have
...
- a
...
- function
...
- description
...
- but
...
- it
...
- is
...
- good
...
- programming
...
- practice
...
- to
...
- add
...
- a
...
- comment
...
- on
...
- each
...
- function
...
- that
...
- describes
...
- what
...
- the
...
- function
...
- does.
...
- Also,
...
- if
...
- you
...
- add
...
- a
...
- description
...
- of
...
- the
...
- function,
...
- MATLAB
...
- will
...
- be
...
- able
...
- to
...
- index
...
- it
...
- and
...
- return
...
- a
...
- description
...
- of
...
- your
...
- function
...
- if
...
- you
...
- type
...
help
...
function_name
...
- in
...
- the
...
- Command
...
- Window.
...
- Once
...
- you
...
- have
...
- created
...
- a
...
- function
...
- you
...
- MUST
...
- name
...
- the
...
- .m
...
- file
...
- with
...
- the
...
- same
...
- name
...
- the
...
- function
...
- has.
...
- Otherwise
...
- MATLAB
...
- will
...
- not
...
- be
...
- able
...
- to
...
- access
...
- your
...
- function
...
- when
...
- you
...
- call
...
- it.
...
We
...
will
...
start
...
by
...
creating
...
the
...
bending
...
stress
...
function
...
that
...
outputs
...
σ
...
x
...
given
...
(
...
M
...
,
...
r
...
i,
...
r
...
o
...
).
...
Create
...
a
...
new
...
page
...
in
...
the
...
editor.
...
Type
...
in
...
the
...
following
...
statements
...
into
...
the
...
new
...
page:
...
You can be lazy like me and copy-and-paste
...
the
...
last
...
two
...
statements
...
from
...
your
...
previous
...
code.
...
Save
...
this
...
file
...
as
...
bending_stress.m
...
,
...
which
...
is
...
the
...
name
...
that
...
MATLAB
...
automatically
...
assigns
...
the
...
file.
...
Thus,
...
the
...
function
...
name
...
and
...
the
...
.m
...
have
...
the
...
same
...
name.
...
Bring
...
up
...
beam3.m
...
in
...
the
...
MATLAB
...
editor.
...
Make
...
a
...
copy
...
of
...
beam3.m
...
using
...
Save
...
As
...
...
...
and call the new file beam5.m
...
.
...
In
...
this
...
file
...
comment
...
out
...
the
...
lines
...
below
...
since
...
this
...
calculation
...
is
...
now
...
done
...
within
...
the
...
function.
...
We'll
...
replace
...
these
...
statements
...
with
...
a
...
call
...
to
...
the
...
bending_stress
...
function.
...
The
...
following
...
statement
...
does
...
this:
...
Add this to beam5.m
...
.
...
Run
...
the
...
file
...
and
...
check
...
the output You should get the same plot you got with beam3.m
...
.
Subfunctions
Functions can be called within a function. As crazy as this sounds, this sometimes makes very complex code appear much more manageable by sectionalizing the code. To explain how to create a subfunction, let's look to the matlab help for an example:
In the above example, there are three functions: the main function is called "newsstats". Within this function, there are two subfunctions; one called "mean" and the other, "median". The subfunction is defined the exact same way as the main function; but the difference is the subfunction can only used when the main function is used - you can not call your subfunctions in the command window. As you can see, the main function calls both of the subfunctions, making the code much shorter and simpler. For longer, more complex code, this can save you a lot of debugging time by breaking down your code into smaller pieces.
That brings us to the end of this tour. Before we part, let's remind ourselves of some important programming guidelines that we have followed in this tour:
- Develop code incrementally, testing obsessively at each stage. Develop a plan for how you are going to build your code before you sit at the computer.
- Dig through the MATLAB help diligently to figure out how to use specific functions etc. Usually, the examples are the best place to start. This is a better strategy than desperately hunting for the TA every time you need help with your code.
- Comment your program liberally.