MATLAB Files -- Scripts and Functions Video Lecture | MATLAB Programming for Numerical Computation - Software Development

hello and welcome to matlab programming for
numerical computations course we are in module
1 introduction to matlab programming this
is lecture 1 dash 4 where we are going to
cover ah using files in matlab
so matlab files come in 2 types scripts and
functions and that is what we are going to
cover in this particular lecture okay
ah we have done this already ah a few times
in the few in the previous lecture in order
to start editor in matlab we need to type
edit followed by the file name
when we do that the editor opens a file called
file name dot m dot m is ah an extension
to save state that these are maa ah matlab
co code files
matlab code files are return in ascii ah so
they are basically readable by any text editor
matlab provides an excellent editor which
is what you should use for typing any matlab
file ah indeed you can use say notepad or
any other editor as well which is something
that i will not recommend you to do okay
so do use matlab editor as we have done before
in the previous lectures for more help you
can go to the matlabs own website for writing
a matlab program the link for which is given
over here okay
so as i said there are 2 types of matlab files
script files and function files so what we
have done in the the ah previous 2 lectures
is we have written small snippets of codes
in order to calculate multiple things for
example fibonacci series or the ba height
of the ball at various times so on and so
forth okay all that we have done in those
2 lectures ah we have done it using what is
known as matlabs script files
so what are the script files
so script files are nothing but files that
contain sequence of matlab commands and these
sequence ah these commands sorry are are executed
1 after other as a they were typed on the
command prompt itself
so let us look at the files that we had created
so this was the the file that we had created
for ah ah the trajectory of a ball thrown
vertically upwards so these commands are executed
1 after the other when we click on run or
we type the name of the file and press enter
in the command prompt
likewise we can look at fibo using while or
fibo using for this these are what are known
as script files the difference between script
file and a function file is that function
file starts with the first line starts with
a command called function so that the syntax
for a function file is going to be the function
space followed by square bracket separated
output variables so say out 1 out 2 and so
on equal to the name of the function myfunctionname
or any name that you choose followed by input
variables in 1 in 2 so on and so forth okay
keep in mind that the input variables are
in circular brackets the output variables
are in square brackets having an output variable
or an input variable is optional so if you
do not have output variable the function is
just going to look like function myfuncfunctionname
with aah ah input variables which are comma
separated input variables are also optional
if you have output variables but no input
variables you can just say aah out 1 out 2
equal to myfunctionname and let us say you
do not have either input variables or output
variables you can say myfunc function equal
to myfunctionname okay the way matlab functions
and scripts work are something that i am going
to cover in this particular lecture but at
any point of time if you feel the need to
write a function of this type you should really
not be using a function but what you really
instead need is script okay when you save
this function okay matlab will ask you to
save it with the exact same name as the name
of the function so keep this in mind as for
as this particular course is concerned the
name of your function file will be the same
as the name of the function that you are using
in that function file okay this is going to
work for all the functions that we are going
to use in this particular lecture okay
i am going to close this particular function
file go back to matlab and del delete that
file name myfunctionfile i will just click
right click over here and click on delete
and that will delete that particular file
okay
so function files are files that taken certain
inputs execute a sequence of steps and return
the output at the end okay the matlab statements
are executed in functions own variable space
okay and once we exit that function all the
variables used are no longer accessible in
matlab
let us go back to matlab and take the example
of fibonacci using for loop okay and let us
execute that particular command i will c clc
and i will clear this screen and i will type
fibo using for and where i press enter let
us look at all the variables that were there
we had declared variable n declared variable
fibo and variable i when i will run that particular
comma va ahh run the particular file all the
3 variables that were used in the script file
are also available in the matlab workspace
as as can be seen over here
the reason is because a script file shares
the same workspace as the thing that was used
to call that script file because we call that
script file from the command prompt the ve
variables used in the script file are also
available in the workspace okay so script
file is nothing but the commands as if they
were executed on of the other in the command
prompt okay it will do the exact same thing
okay let me clear this okay and what i will
do this do now is convert this into a function
file in order to do that all i need to do
is say function fibo using for and save this
okay i am just doing this to show you how
a function differs from a script this is something
you should not be doing you should not be
using a function in this particular way because
we want in this particular example we want
to execute the commands 1 after the other
and for this purpose we need to be using scripts
okay
so let us go to matlab and now type fibo using
for again okay when we type this and press
enter you see none of the variables are accessible
anymore if i type fibo that variable is not
accessible because that variable has a scope
and that scope of that was in that particular
function so when this function file is called
n is defined fibo is defined i is defined
computations are done when we exit that function
n fibo and i are completely gone and no longer
available in the workspace if we want to access
that in the workspace we need to use input
and output arguments with the function okay
so that comes brings me to the next aspect
scoping of the variables is a scripts shares
the variables with the workspace okay where
as function uses the variables in its own
workspace so the scoping of the variables
used in the function they have a local scope
the scoping of the variables in the script
is ah the scope is the same as the function
on the workspace calling that particular script
okay
and this functions are going to talk through
the thee the workspace to the input and the
output variable so as i shown earlier the
syntax for a function is basically going to
be output aah equal to function name and inputs
that there is an error over here the function
should come before this and not after the
equal to sign okay
so we wanted to calculate let us say this
calculate the factorial ve ve because this
is a single purpose thing we are going to
write a script in order to calculate the factorial
but if you wanted to write a function in order
to calculate a function of this sort we are
going to write a function so let us go ahead
and write a script for calculate calculating
the factorial
so let us say edit myfact and that is going
to be a script okay calculate factorial of
n so n equal to let us say 6 fact value equal
to 1 thats what we are going to start with
for i equal to 1 colon 6 fact value equal
to fact value multiplied by i end okay and
this is the script because this does not start
with aah function call i am going to go to
matlab
and type myfact okay and all the variables
will be accessible in the workspace as we
are ah did shown before and fact value is
going to be the factorial value 70 okay the
other thing that i would want to do is want
to make this a bit more the code to be bore
more more better i do not want to use for
loop in doing this instead what i am going
to do is use the array aah function called
product so if i give prod 1 colon 6 that is
going to calculate the factorial forming okay
so i will go over what this particular command
does in a bit that is let us execute myfact
in matlab and see what let us clear the vartiables
first clear all and clc okay so as you can
see in the workspace they are no longer any
variables i give the aaah script name my fact
and press enter i now have the 2 variables
fact value and n the fact values value is
720 as we had seen before okay
so let us see what the command product does
prod let us say 1 to ff 6 is what it is going
to do is hm calculate the product of this
particular vector the vector 1 to 6 is going
to be nothing but 1 2 3 4 5 6 so lets just
type that particular thing here that is nothing
but 1 2 3 4 5 6 prod which is operated on
this is going to do 1 multiplied by 2 multiplied
by 3 multiplied by 4 and so on upto 6 so thats
what exactly i am going to get as 720 and
thats what the script myfact is doing over
here okay
so thats the script that was the purpose of
this script is to run or execute certain commands
in matlab command prompt okay
now instead if i want to calculate ah a given
function and
the function that i wanted to calculate was
0 plus c1x plus c2x square upto cnx to the
power n for that
i am going to u use ah a function file i will
call this edit myfunc okay and function result
equal to myfunc n okay and what i wanted to
do let us go back over here
i wanted to calculate c0 plus c1x plus c2x
square and so on for c0 equal to 1 and c1
equal to 1 by 1 c2 equal to 1 by 2 c3 equal
to 1 by 3 so on and so forth okay
so what i will do is i will create a vector
of aah the coefficients call lets call this
as a vector capital c equal to 1 comma i will
put this in square brackets 1 divided by and
i have element by element division 1 to n
okay so let us go over this once again okay
so c i will just say equal to c not which
is 1 okay
and after that i need to append 1 by 1 1 by
2 1 by 3 and so on so lets define a vector
vec equal to 1 to n so our c is just going
to be equal to c comma 1 dot slash vec you
recall in the 2nd lecture of this module we
had introduced the aah element by element
operation dot slash so what this is going
to do this is going to be do 1 divided by
the 1st element of vec 1 divided by 2nd element
of vec 1 divided by 3rd element of vec so
on and so forth that is going to create a
vector so we are going to have a vector 1
followed by 1 by 2 followed by 1 by 3 upto
1 by n so that gives a vector of coefficients
so now i have c0 i have c1 i have c2 and so
on what are the multiplicands over here is
going to be x to the power 0 x to the power
1 x to the power 2 and so on upto x to the
power n
so for that let us define a vector a so a
is just going to be equal to x to the power
and for that power i am going use dot caret
vec okay so that is what i am going to get
over here what i need is the 1st guy is nothing
but 1 x to the power 0 is nothing but 1 so
i will append that over here so 1 comma x
to the power vec okay and my result is going
to be result equal to c dot star a okay so
the vector c multiplied by the vector a so
that is going to be my result and i will end
this with end the command end is an optional
in a function okay and i am s going to save
this as the file myfunc okay
you can see over here that this myfunc is
actually highlighted the reason is because
i have made an error in naming this the file
name is my capital func where as name of the
function is small func okay
aah recall that in matlab the ah all the commands
are hmm a case sensitive so i want to have
the function name same as my file name so
if click on fix it will change the function
name to capital f
i will just save this and go to matlab and
run myfunc myfunc n and lets say result equal
to myfunc n where n was equal to 6 okay okay
i get error over here that is because i have
not defined my x so i need myfunc n comma
x over here and i need to call it with 2 arguments
n and x so n was 6 and lets say x was 0 point
1 and i call this particular function and
i am going to get this result equal to sum
result and that is going to be sum of so it
is going to be 1 plus ah ah 0 point 1 divided
by 1 0 point 1 square divided by 2 0 point
1 cube divided by 3 so on and so forth and
that is going to be my result okay so see
what have what has happened is while doing
this particular function i have made an error
that error was
i am calculating c0 i am calculating c1x i
am calculating c2x square and so on upto cnx
to the power of n but i have not summed at
up in order to calculate the function f okay
i will go in over here and just change this
to say sum c star dot a save this and now
i will call this clear clc okay and call result
equal to myfunc 6 comma 0 point 1 and i am
going to get the result over here okay inadvertently
what i ended up the show ended up showing
over here also is the way we will do debugging
you recall that we got 2 errors 1 was an actual
error that stem because i had not defined
my input variable x at all which basically
meant when it came across this particular
comman command what happened was it was not
able to calculate because x was not defined
let me re go back to how i had return earlier
and run this again
okay i will run this again and i got that
error undefined function or variable x on
line number 6 i went to that function on this
particular line and realize that i had not
defined my x so i went ahead and made that
change defined at x and i call that particular
function using 2 arguments as shown over here
the other error that i had done was i was
calculating this particular array c star a
but not summing that up when i saw that result
which resulted not in a single value which
was the sum of that uh aah elements of that
array but which was just the 6 elements of
that array i realized that i needed to add
that sum command over here okay so this is
also how we do debugging in matlab
okay so lets recap when to use functions and
when to use scripts and for beginners that
is for most of you guys these are the general
guidelines that i have for you okay
so you want to use scripts when you wanted
to make small calculations such as factorial
or basic computing as we did in the marks
mark list example in the previous ah in a
previous lecture or when we wanted to do plotting
of some certain functions which we are going
to cover in the next lecture
that s when we want to use scripts when do
you want to use functions let us say we want
to calculate the values r a result as a function
of variables t y so on and so forth when we
want to calculate r as a function of t comma
y and so on
that is when we want to use functions the
second example when we want to use functions
is when we want to pa pass them as ah to matlab
functions for example in ode solving example
that we saw in dhoni hitting the 6
we are calculating ah function using ah ode
45 in matlab and we wanted to pass that f
of t comma y as a function to ode 45 that
is another example where we are going to use
functions this is something that we are going
to cover in module 3 and thereafter we wont
even cover this in module 2 just giving you
a brief overview of when to use functions
third example ah when to use function is lets
say we want to calculate ah something as a
function of temperature let us say we want
to calculate for example the saturation pressure
of steam at various temperatures or we want
to calculate the a let us say diffusivity
of something as a function of concentration
or lets say we want to calculate something
as a function of voltage and current so those
properties which we want to calculate as a
function of lets say temperature or concentration
current or current so on and so forth
that is when we will use functions so these
are the 3 cases when we use functions for
all other examples for all other purposes
we are more likely to use scripts rather than
function okay so with that i come to the end
of that lect of this lecture
what we covered in this lecture is the examples
of when to use scripts and when to use functions
in order to computations scripts is nothing
but sequential execution of certain commands
as if you wanted to the execute them ah in
the calling function or in the calling workspace
ah functions are to be used for achieving
a specific goal when you want to calculate
for example r as a function of t y and so
on and so forth okay so these are the main
uses of functions and script in matlab we
are going to rely heavily on using functions
and scripts in this particular set of ah lectures
from module 2 onwards okay so with that i
come to the end of this particular lecture
ah the next lecture is going to be the last
lecture in the introduction module the next
lecture is going to covered plotting and program
output thank and see you in the next lecture