MATLAB part II (control flow, scripts & functions)

• Control flow
  • for loops
  • if statements
  • switch/case statements
  • while statements
• Scripts
  • Writing & running a basic script
• Functions
  • Scripts vs. functions
  • Local vs. global variables
  • Writing & executing a basic function

We use for loops to iterate over something with multiple element (e.g. vector, array) and peform an action(s) during each iteration.

for each_element in my_vector
    do something to this element
end

We use if statements to evaluate one or more conditional statements and perform different action(s) based on the different conditions.

if condition 1 is true
    do action 1
elseif condition 2 is true
    do action 2
else
    do action 3
end

We use switch/case statements for the same purpose as if statements, but these are useful when we have many possible conditions to evaluate.

switch condition_of_interest
case condition_is_1
    do action 1
case condition_is_2
    do action 2
case condition_is_3
    do action 3
case condition_is_4
    do action 4
case condition_is_5
    do action 5
end

We use while statements to execute an action(s) only while a certain condition is met.

while condition_is_true
  do something
    %(at some point, condition becomes false, and loop ends)
end

Let's try writing the loops and statements we've reviewed in MATLAB. Open the script for this week's lesson in MATLAB.

Scripts are files containing a set of commands. When we save a script file, we can run the script and execute all the commands in that file.

Functions are more flexible versions of scripts. They include commands that are performed on a set of input and output variables. Like scripts, they are also saved as files. (Note: you can save functions within scripts but I don't recommend getting in the habit of this.)

Example: We create a short script that calculates the circumference of a circle:

radius = 3;
circumference = 2 * pi * radius

If we ran this script, we would find that radius would be set to 3 and circumference to 18.85 in our workspace. But what if we wanted to easily modify the value of radius, to test the effect of different values on the resulting circumference value? This is where functions come in handy. We could define a function find_circumference that computes the circumference of a circle with given radius, as follows:

[circumference] = function find_circumference(radius)
    circumference = 2 * pi * radius;
end

Then, we could run the function as follows, changing the input value of radius as we wish:

circumference = find_circumference(3);
circumference
18.84956

circumference = find_circumference(5);
circumference
31.42

• Each function has its won local variables
• If we want variables in our workspace to be "seen" by a function, we must declare those variables as global

Example: below, we create a function that takes a numeric input and adds the number 5 to that input. We save this function as a file my_function.m.

[output] = function my_function(input)
    x = 5;
    output = x + input;
end

If we then run this function as my_function(3) we should get output = 8.

What if we have a variable x in our workspace? Importantly, this won't affect the output of our function, because x as defined in our function is local to that function, separate from the x that exists in our workspace.

Perhaps, though, we would like to make the x variable a global variable, such that we wouldn't need to specify it as an input to our function. We could recreate our function my_function.m as follows (see next slide)

[output] = function my_function(input)
    output = x + input;
end

Then, we could use the following code to define x and make it global:

x = 5;
global x

we would get the same behavior of the function as we saw originally if we ran output = my_function(3).

Take home: be aware that variables created within your functions are not seen by your workspace, unless they are included as an output of the function. Now, let's try creating a script and function of our own. Return to this week's MATLAB script.

Topic: MATLAB part III (building a basic experiment in Psychtoolbox)

We will install Psychtoolbox together at the beginning of next week's session.