Working with Variables
+Last updated on 2024-10-29 | + + Edit this page
+ + + +Estimated time: 12 minutes
+ +Overview
+Questions
+- How can I fetch data out of a variable? +
- How can I change the values of parts of variables? +
- What are operations? +
Objectives
+- Explain how to use markdown with the new lesson template +
- Demonstrate how to include pieces of code, figures, and nested +challenge blocks +
Introduction
+So far we have learnt how to create variables of various sizes with +different methods. This episode will now focus on how can we use +variables? ## Extracting Variables
+Now is a good time to clear and clc +your workspace and command window!
+First lets start by making a dummy variable that we can use as our +stand-in dataset.
+ +Subsets
+Subsets of values or single values can be extracted from a variable
+with round brackets ()
For example to take the value on the third row and second column:
+ +We can use colon notation to extract values as well:
+ +A single colon with no numbers will select all
+ +Some more examples bringing together the tools we’ve seen so far:
+MATLAB +
+% Extract the first 4 rows and all columns and save it in the variable subset1
+subset1 = data(1:4,:)
+
+% Select every other column in the first row, save in subset2
+subset2 = data(1,1:2:6)
+
+% Select the first, third and forth rows for all columns, save in subset3
+subset3 = data([1 3 4], :)Altering Variables
+As we have seen so far, the object on the left hand side of the equal +sign ‘=’ is set to what is on the right hand side. So to change the +value of a variable we simply put it on the left:
+ +If you look at data in your workspace now you will see that value has +been changed!
+One use case of altering data may be when you find an erroneous +value. For example if you were looking at a table of reviews out of 5, +you may wish to change a rating that was somehow set to above 5 to NAN, +to show it was invalid.
+ +Transpose
+One useful tool in manipulating matrices (plural of matrix) in MATLAB +is the transpose. This will effectively pivot the matrix so each row +becomes a column and each column becomes a row. This is done in MATLAB +by adding an apostrophe after a variable:
+ +You should see that data_t has a flipped size compared to data
+Concatenation
+Concatenation (also called concat or cat) is a common operation in +data handling. Concatenating means to link or put together, it allows to +to take two matricies or variables and add them into a single variable. +This is useful for if example your dataset is saved across multiple +files.
+First let’s clear our workspace again, create a new data variable and +some subsets of the data to work with.
+ +Both our subsets are column vectors, if we wanted to concaternate +them together into a larger column vector there are 2 ways
+ +Callout
+Don’t forget if you find a function you aren’t familiar with you can
+use help or doc to learn more!
This is a good point to use help cat and explain how
+they can learn that the parameters to it are and why you need a 1 at the
+front. Getting them used to looking up functions and not getting stuck
+is important!
If we wanted to concat the subsets into 1 variable as separate +columns we could do
+ +One advantage of using cat is that it can work for arrays of larger +dimensions, where the square bracket shortcut only works for up to 2D +data.
+Challenge 1
+Extract every other row from Data assign it to the varibale name +subset_a
+Extract the first four rows from the 2nd column of Data. Call it +subset_b
+Transpose subset_b, call this variable subset_t
+Concatenate subset_a and subset_t along the first +dimension
+
MATLAB +
+% Extract every other row from Data assign it to the varibale name subset_a
+subset_a = Data(1:2:6,:)
+
+% Extract the first four rows from the 2nd column of Data. Call it subset_b
+subset_b = Data(1:4,2)
+
+% Transpose subset_b, call this varibale subset_t
+subset_t = subset_b'
+
+% Concatenate subset_a and subset_t along the first dimension
+newdata = cat(1, subset_a, subset_t)newdata should be of size 4x4
+Operations
+We’re now going to look at some mathematical operations we can +perform on our variables.
+Before continuing this is a good point to clear your workspace again +and make a new dummy data variable. This time we will round each data +point to the nearest whole number with the function round
+MATLAB +
+
+data = 100 * rand(10,10)
+% Round to nearest whole number and overwrite data variable
+data = round(data)You may be familiar with the syntax (syntax is a word typically used +in programming to mean format) of the most basic operators from other +computer programs.
+MATLAB +
+
+data_add = data + 10;
+data_subtract = data - 10;
+data_divide = data / 10;
+data_multiply = data * 10;One common mistake made by users of MATLAB is with the multiply +operator. When multiplying by a single number like above the behaviour +may be as you except.
+Challenge 2
+- make a row vector called row with values 1, 2 & 3 +
- make a column vector called column with values 4, 5 & 6 +
- before trying to multiply them, guess the size of the result of +row*column +
- multiply row and column and see if the result is as you expect +
As seen in the challenge above, by default MATLAB will attempt to +perform something called matrix +multiplcation, you don’t need to know about matrix multiplication +but it is worth knowing it is the default behaviour.
+What you may expect is something called dot +multiplication
+ +OUTPUT +
+ans =
+
+ 4 10 18As the example above shows, dot multiplication multiplies each +element of both variables with each other 1 to 1. This is why it is also +sometimes called element-wise multiplication.
+Functions
+Next we will look at some key functions that you may want to use in +data analysis and processing
+MATLAB +
+
+% Add together the rows in each column
+column_totals = sum(data, 1)
+
+% Take the mean of the rows in each row
+row_means = mean(data, 2)
+
+% Find the maximum value in each column
+data_max = max(data)
+
+% Find the minimum value in each row
+data_min = min(data, [], 2)
+
+% Find the maximum value of the entire matrix
+data_max_all = max(data, [], "all")It is worth exploring why the square brackets exist here, demoing +using the help or doc command to find why that exists and use it as a +learning example.
+challenge 3
+In this challenge we will combine the tools we have learnt so far to +compare rainfall data between Sheffield and India
+- Load the datasets from files +
MATLAB +
+% Import rainfall data.
+Sheffield_Rain = load('Sheffield_Rain.csv');
+India_Rain = load('SouthIndia_Rainfall.csv');- Investigate the data, open it up in the workspace and understand +what is in each column +
3
+Key Points
+- Use
.mdfiles for episodes when you want static +content
+ - Use
.Rmdfiles for episodes when you need to generate +output
+ - Run
sandpaper::check_lesson()to identify any issues +with your lesson
+ - Run
sandpaper::build_lesson()to preview your lesson +locally
+