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feat: tweak conditionals lesson
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cjfullerton committed Nov 13, 2023
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2 changes: 1 addition & 1 deletion episodes/01-intro.md
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Expand Up @@ -125,7 +125,7 @@ where we used the caret symbol `^` to take the third power of y.

### Logical operations

In programming, there is another type of opperation that becomes very important: comparison.
In programming, there is another type of operation that becomes very important: comparison.
We can compare two numbers (or variables) to see which one is smaller, for example
```matlab
>> mass = 20
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272 changes: 107 additions & 165 deletions episodes/06-cond.md
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Expand Up @@ -22,16 +22,15 @@ objectives:
::::::::::::::::::::::::::::::::::::::::::::::::::


Our previous lessons have shown us how to manipulate
data and repeat things.
However, the programs we have written so far always do
the same things, regardless of what data they're given.
We want programs to make choices based on the values
they are manipulating.
In the last lesson we began experimenting with scripts, allowing us to re-use code for analysing data and plotting
figures over and over again. To make our scripts even more useful, it would be nice if they did different things
in different situations - either depending on the data they're given or on different options that we specify. We
want a way for our scripts to make choices.

The tool that MATLAB gives us for doing this is called
a [conditional statement]({{ page.root }}/reference.html#conditional-statement),
and it looks like this:
The tool that MATLAB gives us for doing this is called
a [conditional statement]({{ page.root }}/reference.html#conditional-statement). We will use conditional statements
together with the logical operations we encountered back in [lesson 01]({{ page.root }}/01-intro.hmtl#logical-operations).
Together they work like this:

```matlab
num = 37;
Expand All @@ -51,14 +50,13 @@ done
```

The second line of this code uses the keyword `if` to tell MATLAB
that we want to make a choice. If the test that follows is true,
the body of the `if` (i.e., the lines between `if` and `else`) are
executed. If the test is false, the body of the `else` (i.e.,
the lines between `else` and `end`) are executed instead. Only one
or the other is ever executed.
that we want to make a choice. If the logical operation that follows is true,
the body of the `if` statement (i.e., the lines between `if` and `else`) is
executed. If the logical operation returns false, the body of the `else` statement (i.e.,
the lines between `else` and `end`) is executed instead. Only one of these statement bodies is ever executed, never both.

Conditional statements don't have to have an `else` block. If there
isn't one, MATLAB simply doesn't do anything if the test is false:
isn't one, MATLAB simply doesn't do anything if the logical operation returns false:

```matlab
num = 53;
Expand Down Expand Up @@ -93,12 +91,8 @@ else
end
```

One important thing to notice in the code above is that we use
a double equals sign `==` to test for equality rather than a
single equals sign. This is because the latter is used to mean
assignment. In our test, we want to check for the equality of `num`
and `0`, not *assign* 0 to `num`. This convention was inherited
from C, and it does take a bit of getting used to...
Recall that we use a double equals sign `==` to test for equality rather than a
single equals sign (which assigns a value to a variable).

During a conditional statement, if one of the conditions is true,
this marks the end of the test: no subsequent conditions will be tested
Expand All @@ -122,26 +116,26 @@ else
end
```

We can also combine tests, using `&&` (and) and `||` (or). `&&`
We can also combine logical operations, using `&&` (and) and `||` (or). `&&`
is true if both tests are true:

```matlab
if ((1 > 0) && (-1 > 0))
disp('both parts are true')
else
disp('one part is not true')
disp('At least one part is not true')
end
```

```output
one part is not true
At least one part is not true
```

`||` is true if either test is true:
`||` is true if either logical operation returns true:

```matlab
if (1 < 0) || (3 < 4)
disp('at least one part is true')
disp('At least one part is true')
end
```

Expand All @@ -156,7 +150,7 @@ In this case, "either" means "either or both", not

## True and False Statements

The conditions we have tested above evaluate to a logical value:
The operations we tested above evaluate to a logical value:
`true` or `false`.
However these numerical comparison tests aren't the only values
which are `true` or `false` in MATLAB.
Expand Down Expand Up @@ -236,181 +230,129 @@ end

::::::::::::::::::::::::::::::::::::::::::::::::::

Another thing to realize is that `if` statements can
also be combined with loops. For example, if we want
to sum the positive numbers in a list, we can write
this:
## Scripts with choices

```matlab
numbers = [-5, 3, 2, -1, 9, 6];
total = 0;
for n = numbers
if n >= 0
total = total + n;
end
end
disp(['sum of positive values: ', num2str(total)])
```

```output
sum of positive values: 20
```
In the last lesson, we wrote a script that saved several plots to disk. It would nice if our script could be more
flexible. Could we modify it so that it either saved the plots to disk *or* displayed them on screen? Could we do this
in such a way to make it easy to change between the two behaviours? This is something that conditional statements allow
us to do.

With a little extra effort, we can calculate the
positive and negative sums in a loop:
We introduce a variable `save_plots` that we can set to either `true` or `false` and modify our script so that when
`save_plots = true` the plots are saved to disk, and when `save_plots = false` the plots are printed to the screen.

```matlab
pos_total = 0;
neg_total = 0;
for n = numbers
if n >= 0
pos_total = pos_total + n;
else
neg_total = neg_total + n;
end
end
% PLOT_DAILY_AVERAGE_OPTION Plots daily average, max and min inflammation accross patients. If save_plots is set to
% true, the figures are saved to disk. If save_plots is set to false, the figures are displayed on the screen.
disp(['sum of positive values: ', num2str(pos_total)])
disp(['sum of negative values: ', num2str(neg_total)])
```
% Load patient data
patient_data = readmatrix('data/base/inflammation-01.csv');
```output
sum of positive values: 26
sum of negative values: -6
```
save_plots=true;
We can even put one loop inside another:

```matlab
for number = 1:3
for letter = 'ab'
disp([num2str(number), letter])
end
if save_plots == true
figure(visible='off')
else
figure
end
```
```output
1a
1b
2a
2b
3a
3b
```
% Define tiled layout and labels
tlo = tiledlayout(1,3);
xlabel(tlo,'Day of trial')
ylabel(tlo,'Inflammation')
::::::::::::::::::::::::::::::::::::::: challenge
% Plot average inflammation per day
nexttile
plot(mean(patient_data, 1))
title('Average')
## Nesting
% Plot max inflammation per day
nexttile
plot(max(patient_data, [], 1))
title('Max')
Will changing the order of nesting in the above loop change
the output? Why? Write down the output you might expect from
changing the order of the loops, then rewrite the code to test
your hypothesis.
% Plot min inflammation per day
nexttile
plot(min(patient_data, [], 1))
title('Min')
::::::::::::::: solution
if save_plots == true
% Save plot in 'results' folder as png image:
saveas(gcf,'results/daily_average_01.png')
close()
```matlab
for letter = 'ab'
for number = 1:3
disp([num2str(number), letter])
end
end
```

Reordering the nested loops changes the output. In the new code,
the number loop happens within the letter loop, so while letter
= a, number takes the values 1, 2, and 3 in turn.

:::::::::::::::::::::::::
Save the script in a file names `plot_daily_average_option.m` and investigate what setting the variable `save_plots`
to `true` and `false` does.

::::::::::::::::::::::::::::::::::::::::::::::::::

Currently, our script `plot_all.m` reads in data, analyzes it,
and saves plots of the results.
If we would rather display the plots interactively,
we would have to remove (or *comment out*) the following code:

```matlab
print(img_name,'-dpng')
close()
```

And, we'd also have to change this line of code, from:
::::::::::::::::::::::::::::::::::::::: challenge

```matlab
figure(visible='off')
```
## Changing behaviour based on patient data

to:
We'd like to improve our `patient_analysis` script from the previous lesson, specifially it's output. Currently the
script displaus `0` or `1` to indicate whether or not the patient has a high mean, has a maximum equivalent to the
highest in the dataset, and has a minimum equivalent to the lowest in the dataset. Instead, we'd like the script to
print a line of descriptive text when each of these is true:
1. The meam inflammation for the patient is higher than the global mean.
2. The maximum inflammation for the patient is the same as the global maximum.
3. The minimum inflammation for the patient is the same as the gloabel minimum.

```matlab
figure(visible='on')
% or equivalently: figure()
```
If none of the above is the case, then the script should print a line informing us that the patient's mean, maximum
and minimum inflammation are not remarkable.

This is not a lot of code to change every time,
but it's still work that's easily avoided using conditionals.
Here's our script re-written to use *conditionals*
to switch between saving plots as images and plotting them interactively:
Using the `patient_analysis` script from the previous lesson as a starting point, can you use conditional statements
to make a script that does this? There are several different ways to do this, so cpmpare your finished script with
your neighbour and see if you did it the same way.

::::::::::::::: solution
```matlab
%PLOT_ALL Save plots of statistics to disk.
% Use variable plot_switch to control interactive plotting
% vs saving images to disk.
% plot_switch = 0: show plots interactively
% plot_switch = 1: save plots to disk
plot_switch = 0;
files = dir('data/inflammation-*.csv');
% Load patient data
patient_data = readmatrix('data/base/inflammation-01.csv');
% Process each file in turn
for i = 1:length(files)
file_name = files(i).name;
% Compute global statistics
g_mean = mean(patient_data(:));
g_max = max(patient_data(:));
g_min = min(patient_data(:));
% Generate strings for image names:
img_name = replace(file_name, '.csv', '.png');
patient_number = 8;
% Generate path to data file and image file
file_name = fullfile('data', filename);
img_name = fullfile('results', img_name);
% Compute patient statistics
p_mean = mean(patient_data(patient_number,:));
p_max = max(patient_data(patient_number,:));
p_min = min(patient_data(patient_number,:));
patient_data = readmatrix(file_name);
% Compare patient vs global
disp('Patient:')
disp(patient_number)
% Create figures
if plot_switch == 1
figure(visible='off')
else
figure(visible='on')
end
printed_something = false;
tlo = tiledlayout(1,3);
xlabel(tlo,'Day of trial')
ylabel(tlo,'Inflammation')
nexttile
plot(mean(patient_data, 1))
title('Average')
nexttile
plot(max(patient_data, [], 1))
title('Max')
if p_mean > g_mean
disp('Patient''s mean inflammation is higher than the global mean inflammation.')
printed_something = true;
end
nexttile
plot(min(patient_data, [], 1))
title('Min')
if p_max == g_max
disp('Patient''s maximum inflammation is the same as the global maximum.')
printed_something = true;
end
if plot_switch == 1
print(img_name, '-dpng')
close()
end
if p_min == g_min
disp('Patient''s minimum inflammation is the same as the global minimum.')
printed_something = true;
end
if printed_something == false
disp('Patient''s mean, maximum and minimum inflammation are not of interest.')
end
```

::::::::::::::::::::::::::::::::::::::::::::::::::


:::::::::::::::::::::::::::::::::::::::: keypoints

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