MATLAB Tutorial 3

7 minute read

Published:

These notes recap my third MATLAB tutorial which further explore matrices.

2-Dimensional Matrices

Let’s start by creating a matrix of random numbers:

>> myMatrix = rand(4)

This will create a 4 by 4 matrix of randomly generated numbers between 0 and 1 and save it as myMatrix.

  • Indexing

We can call a specific value from a matrix in two different ways. Firstly, we can use linear indexing:

>> myMatrix(5)

This will output the 5th element of myMatrix. Remember that MATLAB ‘counts’ down the columns from left to right. This means that in our 4 by 4 matrix, the 5th element will be the first element in the second column. A more nuanced way of indexing is subscript indexing:

>> myMatrix(1,3)

This will output the element that is in the first row, third column of myMatrix. Remember that the first argument corresponds to which row, and the second argument corresponds to which column.

An example of the way we used matrices in cognitive research is to record participant’s data. Let’s imagine myMatrix corresponds to the percentage correct of each participant in each block. Each row is a different participant, each column is a different block.

>> myMatrix = myMatrix*100

We have just converted the numbers in myMatrix to percentages out of

  1. Imagine if we wanted to plot only the second participant’s data. A useful way to pull out that data as a vector is using the colon operator (:):
>> participantTwo = myMatrix(2,:)

This will output all the values of the second row of myMatrix. In our example, this would be the percentage correct for all four blocks (columns) for our second participant. Similarly, we can pull out a column vector from our matrix:

>> thirdBlock = myMatrix(:,3)

The corresponding column vector is the third column of myMatrix. In our example, this would be the percetange correct for all our participants on the third block.

Matrices as arguments for functions

Let’s look at some useful functions and how they handle vectors and matrices. Earlier, I’ve saved participantTwo and thirdBlock as a row vector and column vector respectively, as well as myMatrix, which is serving as our data matrix.

  • max function

The max function returns the largest value. Let’s find out what the highest percentage correct our second participant got:

>> max(participantTwo)

This should return the largest value in that vector! What if we wanted to find the highest percentage correct by any participant in any block?

>> max(myMatrix)

Notice here that this will output a vector of four values. This is because this function (and most other basic functions) examine the matrix by columns rather than all values. The resulting output is the largest value for each of our (four) columns from myMatrix. To get the largest of these values, we can just run the max function again,

>> max(max(myMatrix))

The min function works similarly. Can you find what the smallest percentage correct of all our participants was?

  • mean function

The mean function returns the average value and work similarly to the max function we just used. Let’s say we wanted to find mean performance in the third block.

>> mean(thirdBlock)

This will return the average performance of all our participants in the third block! Now, what if we input the matrix as the argument rather than a vector. It works similarly to the max function, such that it will give the mean of each column in the matrix (the mean percentage correct in each block!):

>> mean(myMatrix)

What if we wanted to find the average performance of each participant, rather than mean performance in each block? We can take advantage of the fact that these functions evaluate columns of matrices and transpose our matrix:

>> mean(myMatrix')

Our matrix is transposed and now our participants are the columns, and the blocks are the rows!

Another useful function is the sum function, which adds up all the values. Can you figure out how to find the block with the largest sum of percentage correct? You could do it like this:

>> max(sum(myMatrix))

Some special matrices

There are some special functions that are useful in generating matrices:

  • The zeros matrix
>> zeros(4)

The zeros matrix is commonly used to create “empty” data structures to save to. Values can be added to the matrix in each cell, or a value can be rewritten into the matrix. The above line of code creates a 4 by 4 matrix of zeros.

  • The ones matrix
>> ones(4)

ones is another useful function in generating matrices. The above line of code creates a 4 by 4 matrix of ones.

  • The NaN matrix
>> NaN(4)

NaN stands for not a number and the above line of code creates a 4 by 4 matrix of NaNs. This kind of matrix is extremely useful for preallocating values for something like the trials of a block, or recording data. It’s preferred over a zero matrix because a zero may be a valid value for the matrix (such as a participant response on a scale or a participant getting every trial response wrong). Also, it is easier to spot a missing value in a NaN matrix. For example, if a participant doesn’t complete a block, the remaining unfilled matrix will be kept as NaNs, rather than as zeros.

Matrix operations

Above, we converted randomly generated decimals to percentage correct by multiplying the matrix by 100. Similarly, we can divide the elements in our data matrix by integers (scalars). Multiplying and dividing matrices by scalars is fairly straightforward in MATLAB:

>> myMatrix*3
>> myMatrix/40

However, multiplying matrices by each other is different. Try the following:

>> myMatrix*ones(4)

Remember that ones(4) produces a 4 x 4 matrix of ones. The result you get when multiplying a matrix of ones by myMatrix might be surprising to you if you were expecting it to produce exactly the same matrix (where each element is multiplied by one). This doesn’t happen because of matrix algebra, and is a potential pitfall in data analysis because it spits output and doesn’t look like an error! As long as the number of rows in your matrix (or vector) matches the number of columns in your second matrix (or vector), the operation will function. You should be wary of this when working with square matrices like we are here.

When you want to multiply corresponding elements in two of the same-sized matrices, use a . in front of the operation.

>> myMatrix.*ones(4)
>> myMatrix./ones(4)

This will multiply and divide the corresponding elements. Since we multiply and divide by one, we get the same matrix out.

Try executing these lines to see this similarly with vectors:

>> vectorOne = 2:5
>> vectorTwo = randperm(20,4)
>> vectorOne*vectorTwo
>> vectorOne.*vectorTwo
>> vectorOne./vectorTwo

Dividing and multiplying vectors and matrices is useful in experimental psychology when you might have random conditions with different number of trials and conditions in each, such that you might have to divide corresponding elements in vectors or matrices.

3-D Matrices

Matrices can go beyond the two-dimensions of rows and columns. For 3 dimensional matrices, I like to refer to the third dimension as ‘layers’.

>> my3Dmatrix = randi(100,2,3,4)

This will produce a 3-D matrix of 2 rows, 3 columns and 4 “layers” (a 2 x 3 x 4 matrix) containing random integers from 1 to 100. This might occur if you have for example, two conditions with three different stimuli and four blocks.

A useful function is size, which will give you the length of all the dimension of your vector or matrix, no matter how many dimensions it contains.

>> size(myMatrix)
>> size(my3Dmatrix)

The same operations apply to 3-D matrices as they do to 2-D matrices.