Example of matlab

Matlab program

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show you how to create and run a basic MATLAB program so if we open up MATLAB
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we'll see a big panel in the middle this is called the command window the command
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window is used to execute commands one by one I'll show you what I mean we can
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create a vector X that goes from 1 to 10 and when I hit enter this vector is
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created in our workspace or I can create a vector Y which is equal to the square
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root of x and when I hit enter this vector is also created or I can even
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create a plot from these two vectors and when I hit enter this figure pops up on
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the front of my screen so as we can see the command window is great if you want
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to execute commands line by line but what if you want to execute multiple
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commands at a time well you can do this by creating a live script let's close
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out of this figure and I'll show you two ways to accomplish this the first way is
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to click new and then live script and when you do this you'll see the live
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editor window pop up on top of the command window here you can type
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multiple commands without executing them right away
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but let's say you already typed a bunch of code in the command window and you
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want to preserve it for when you create your live script how do you do that
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first go to your command history and highlight all the commands you want to
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include in your script I'll pick the 3 that we just typed then right click
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these commands and hit create live script you'll notice that just like
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before the live editor pops up on top of the command window but this time it
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already has the commands that we highlighted now to run this script I
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first have to save my file by going to save save as and giving it a relevant
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name then I'll go to the top here where it says run all and hit that button when
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I do this all of the commands in the script are executed we can see in our
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workspace that 2 vector X&Y were created and right next to our
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code we see the figure that was generated from the plot command based on
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your preference you can choose whether to have these outputs on the side of
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your code or in line with your code these two buttons up here will let you
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switch between the modes I'll switch to in line mode so you can see what it
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looks like okay so the program we wrote so far is pretty simple I want to go
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through a few tasks that you may use while writing something more complex
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task number one is adding comments comments will help make your code
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understandable typically they'll describe what your code is doing you can
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add a comment by typing a percent sign and then typing your comment like this
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another way to organize your code is to add sections to it what do I mean by
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this well when using live scripts in MATLAB
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you don't have to run the entire file you have the option to run your code
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section by section sections will also visually split up your code making it
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easier to read and comprehend to add a section I'll type two percent signs
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followed by a space and the name of my section when I hit enter we'll see a
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visual break in the code that separates the section I just created let's put a
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couple of commands here to show you how to run a specific section of code I'll
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create a vector X with values 1 to 100 and I'll create another vector Y with
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values 101 to 200 and then I'll plot X comma Y and make the plot red now
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instead of hitting run all like I did before
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I'll highlight the section I want to run and hit run section we can see the
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figure that corresponds to the section pop-up next to the code we just type and
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if we click on one of these plot commands we can see that the
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corresponding figure is highlighted this is a really useful feature for
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navigating through your results now we've talked a little bit about
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formatting our code but let's talk about some common tasks and constructs used in
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programming the first thing that we'll talk about is called indexing indexing
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is used to take a subset of or matrix that we already created for
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example let's look at one of the vectors we created earlier why if we open the
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variable editor we can see that each element in Y takes on a different value
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now let's say we want to access the fifth element in this vector we can type
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Y and then in parentheses five to get this number or if we want the seventh
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element we'll type Y parentheses seven close parentheses here's some practical
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examples of tasks that we can do with indexing we can create a new variable a
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which is equal to the fifth element in Y a equals y of five now our workspace
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shows that a is equal to the same value as Y of five or we can change the fifth
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element of Y to be 100 we'll type Y of five equals 100 enter and we can see
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that after executing this command the fifth value of Y has changed okay one
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last example let's say we want to change the first five values of Y to be the
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numbers six through ten we can type Y of 1 colon 5 equals and then in square
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brackets 6 7 8 9 10 when we hit enter we can see that the first 5 values of Y
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have changed accordingly so this covers some of the basics of
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indexing and the next thing I want to talk about is something called the
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if-else statement you'll use an if-else statement if you want to execute a set
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of commands only if a certain expression is true this will be more clear with an
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example let's start by writing a statement that takes in a user input the
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line of code I just typed is going to ask for input from the user and convert
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their answer to a string before proceeding I'll show you what I mean if
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we run this section it asks us did you learn something in the command window
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whatever we type as our answer will be stored in the variable user in as a
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string now that we have our user input will create an if-else statement that
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determines what to do based on that user's input so I'll type it all out and
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then we can go through it starting from the top if the user types Y then the
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first expression user in equals Y is true and will display hurray
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but if the user does not type Y and types n instead then the second
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expression is true and will display oh man finally if the user types anything
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that's not Y or n will display please reread the directions
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so let's actually run the program for the three cases
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I'll hit run section and answer the question with Y for yes the live editor
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executes the code and the if section and displays hurray I'll run it again and
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this time I'll answer the question with n for no this time the live editor
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executes the code in the else if statement and displays oh man I'll run
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it one last time and I'll type something different as my answer now the live
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editor executes the code in the else section and displays please reread the
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directions now one last thing it's important to note that you can only have
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one if and one else in each construct but you can have multiple elsif
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statements for example I can add in the following section now if I run this
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section and respond with Q the third expression is true and the live editor
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displays why did you type Q the last construct that we'll talk about is
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called loops specifically we're going to talk about four loops loops are used to
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repeat a set of commands within your code so I have a function that will help
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us illustrate when this comes in handy the function is called random walk and
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I'll give you a visual explanation of how it works there's a person who's
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standing a certain number of steps away from his home let's just call him LeBron
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in the function LeBron flips a coin and if it
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heads he takes one step towards his home and if it lands tails he takes one step
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away from his home the question is how many coin flips will it take LeBron to
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get back to his home so the input for the function is going to be his starting
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position and when we run the function it automatically simulates his coin flips
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until he reaches his house so the output for the function will be the number of
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coin flips that it took him to get home for this example let's say LeBron always
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starts 3 steps from his house so the input for the function will be 3 if I
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hit run we can see that it took 15 coin flips to get back home let's run it a
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couple more times and see what results we get okay notice how every time the
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result is different and at this point you might be curious if we ran this a
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hundred times what would be the average number of coin flips that it would take
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LeBron to get home or maybe you're just wondering what the entire distribution
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would look like instead of rewriting the same command 100 times we can create a
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loop in this case a for loop so let me show you what I mean I'll type it out
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and then I'll explain what's going on alright so what does all of this mean
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well our first statement is for I equals 1 : 100 in this case I is called an
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index variable it's responsible for counting the number of iterations we go
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through in our loop so in this case we'll iterate through the loop 100 times
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and I will take on values 1 2 3 4 5 6 etc all the way up until 100 everything
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contained between the 4 and the end is going to be repeated 100 times in this
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case our function is within our loop so this is what will be repeated
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I'll walk us through a couple of iterations when we first enter the loop
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I is equal to 1 and then we hit our statement num flips of I is equal to
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random walk of 3 this is going to create a variable
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called num flips remember that AI is equal to one so when we have I in
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parentheses we are indexing into the first element of num flips we're going
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to set that value equal to the output of random walk of three we reach the end of
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our loop so we go back to the top and this time I is equal to two then we'll
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hit our function again this time it will index into the second element of num
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flips and set that value equal to the output of our function we hit the end of
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the loop so we go back to the top and this time I is equal to three again will
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execute the random walk function and this time the result is stored in the
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third element of the vector we keep going through this loop until I is equal
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to 100 and each time we go through it we add one more element to our vector
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num flips so after 100 iterations we'll have a vector that has 100 elements in
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it so let's go ahead and run this and take a look at the results in the
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variable editor here's our vector with all 100 elements now let's do some basic
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analysis one thing we might want to do is to check the mean of the data we'll
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go back to the command window and use the mean command whoa that seems kind of
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high you might be curious to see why it's so high so let's plot the number of
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flips that it took on each trial just as a sanity check okay we can see that some
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trials took thousands of flips until LeBron got back home so that's
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definitely skewing our mean to the higher side since we have several large
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outliers in our distribution the median may give a better understanding of the
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data central tendency so let's type median of num flips and see what we get
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okay that's a little bit closer to what we want so once we know what kind of
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analysis we want to do we can then take those lines and add them to the live
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editor and then go ahead and save the program
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so that wraps up this video on creating a basic MATLAB program the examples in
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this video we're pretty simple to illustrate the logic but these basic
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principles are applicable for everything from analyzing big data all the way to
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creating a graphical user interface and the best way to get familiar with
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writing MATLAB programs is to practice if you get stuck there's plenty of
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resources on MathWorks comm and you can post your questions in MATLAB answers as
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well thanks for tuning in and I'll see you guys in another video

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