Making Choices

Last updated on 2025-02-28 | Edit this page

Estimated time: 40 minutes

Overview

Questions

How can programs make choices depending on variable values?

Objectives

Introduce conditional statements.
Test for equality within a conditional statement.
Combine conditional tests using AND and OR.
Construct a conditional statement using if, elseif, and else.

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. We will use conditional statements together with the logical operations we encountered back in lesson 01. The simplest conditional statement consists starts with an if, and concludes with an end, like this:

MATLAB

% *Script* to illustrate use of conditionals
num = 127;
disp('before conditional...')

if num > 100
    disp('The number is greater than 100')
end

disp('...after conditional')

OUTPUT

before conditional...
The number is greater than 100
...after conditional

Now try changing the value of num to, say, 53:

OUTPUT

before conditional...
...after conditional

MATLAB skipped the code inside the conditional statement because the logical operation returned false.

The choice making is not quite complete yet. We have managed to “do” or “not do” something, but we have not managed to choose between to actions. For that, we need to introduce the keyword else in the conditional statement, like this:

MATLAB

% *Script* to illustrate use of conditionals
num = 53;
disp('before conditional...')

if num > 100
    disp('The number is greater than 100')
else
    disp('The number is not greater than 100')
end

disp('...after conditional')

OUTPUT

before conditional...
The number is not greater than 100
...after conditional

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.

We can also “nest” a conditional statements inside another conditional statement.

MATLAB

% *Script* to illustrate use of conditionals
num = 53;

disp('before conditional...')
if num > 100
    disp('The number is greater than 100')
else
    disp('The number is not greater than 100')
    if num > 50
        disp('But it is greater than 50...')
    end
end

disp('...after conditional')

OUTPUT

before conditional...
The number is not greater than 100
But it is greater than 50...
...after conditional

This “nesting” can be quite useful, so MATLAB has a special keyword for it. We can chain several tests together using elseif. This makes it simple to write a script that gives the sign of a number:

MATLAB

% *Script* to illustrate use of conditionals

num = 53;

if num > 0
    disp('num is positive')
elseif num == 0
    disp('num is zero')
else
    disp('num is negative')
end

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 and execution jumps to the end of the conditional.

Let’s demonstrate this by adding another condition which is true.

MATLAB

% *Script* to illustrate use of conditionals
num = 53;

if num > 0
    disp('num is positive')
elseif num == 0
    disp('num is zero')
elseif num > 50
    % This block will never be executed
    disp('num is greater than 50')
else
    disp('num is negative')
end

We can also combine logical operations, using && (and) and || (or), as we did before:

MATLAB

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

OUTPUT

At least one part is not true

MATLAB

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

OUTPUT

at least one part is true

Close Enough

Write a script called near that performs a test on two variables, and displays 1 when the first variable is within 10% of the other and 0 otherwise, that is, one is greater or equal than 90% and less or equal than 110% of the other.

Compare your implementation with your partner’s. Do you get the same answer for all possible pairs of numbers? Remember to try out positive and negative numbers!

Show me the solution

MATLAB

%NEAR   *Script* that displays 1 if variable a is within 10% of variable b and 0 otherwise.
a = 1.1;
b = 1.2;

if a >= 0.9*b && a <= 1.1*b
    disp(1)
else
    disp(0)
end

Scripts with choices

In the last lesson, we wrote a script that saved several plots to disk. It would nice if our script could be more flexible.

Can you modify it so that it either saves the plots to disk or displays them on screen, making it easy to change between the two behaviours using a conditional statement?

Show me the solution

We can 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

% PLOT_PATIENT_INFLAMMATION_OPTION   *Script* Plots daily average, max and min inflammation.
% 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.

save_plots = true;

patient_number = 5;
pn_string = num2str(patient_number);

% Load patient data
patient_data = readmatrix("data/base/inflammation-01.csv");
per_day_mean = mean(patient_data);
per_day_max = max(patient_data);
per_day_min = min(patient_data);
patient = patient_data(patient_number,:);
day_of_trial = 1:40;

if save_plots == true
    figure(visible='off')
else
    figure
end
clf;

% Define tiled layout and labels
tlo = tiledlayout(1,2);
xlabel(tlo,"Day of trial")
ylabel(tlo,"Inflammation")

% Plot average inflammation per day with the patient data
nexttile
title("Average")
hold on
plot(day_of_trial, per_day_mean, "DisplayName", "Mean")
plot(day_of_trial, patient, "DisplayName", "Patient " + pn_string)
legend
hold off

% Plot max and min inflammation per day with the patient data
nexttile
title("Max and Min")
hold on
plot(day_of_trial, per_day_max, "DisplayName", "Max")
plot(day_of_trial, patient, "DisplayName", "Patient " + pn_string)
plot(day_of_trial, per_day_min, "DisplayName", "Min")
legend
hold off

if save_plots == true 
    % Save plot in "results" folder as png image:
    saveas(fig,"results/patient_" + pn_string + ".png")

    close(fig)

Save the script in a file names plot_patient_inflammation_option.m and confirm that setting the variable save_plots to true and false do what we expect.

Key Points

Use conditional statements to make choices based on values in your program.
A conditional statement block starts with an if and finishes with end. It can also include an else.
Use elseif to nest conditional statements.
Use && (and), || (or) to combine logical operations.
Only one of the statement bodies is ever executed.