Making Choices
Last updated on 2023-08-21 | Edit this page
Estimated time: 40 minutes
Overview
Questions
- “How can programs do different things for different data values?” objectives:
- “Construct a conditional statement using if, elseif, and else”
- “Test for equality within a conditional statement”
- “Combine conditional tests using AND and OR”
- “Build a nested loop”
Objectives
- xxx
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.
The tool that MATLAB gives us for doing this is called a conditional statement, and it looks like this:
OUTPUT
not greater
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.
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:
MATLAB
num = 53;
disp('before conditional...')
if num > 100
disp('53 is greater than 100')
end
disp('...after conditional')
OUTPUT
before conditional...
...after conditional
We can also chain several tests together using elseif
.
This makes it simple to write a script that gives the sign of a
number:
MATLAB
%CONDITIONAL_DEMO Demo 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
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…
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
% Demo 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 tests, using &&
(and) and
||
(or). &&
is true if both tests are
true:
OUTPUT
one part is not true
||
is true if either test is true:
OUTPUT
at least one part is true
In this case, “either” means “either or both”, not “either one or the other but not both”.
True and False Statements
The conditions we have 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. For example, 1
is considered
true
and 0
is considered false
.
In fact, any value can be used in a conditional statement.
Run the code below in order to discover which values are considered
true
and which are considered false
.
MATLAB
if ''
disp('empty string is true')
else
disp('empty string is false')
end
if 'foo'
disp('non empty string is true')
else
disp('non empty string is false')
end
if []
disp('empty array is true')
else
disp('empty array is false')
end
if [22.5, 1.0]
disp('non empty array is true')
else
disp('non empty array is false')
end
if [0, 0]
disp('array of zeros is true')
else
disp('array of zeros is false')
end
if true
disp('true is true')
else
disp('true is false')
end
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. Compare your
implementation with your partner’s: do you get the same answer for all
possible pairs of numbers?
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:
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
With a little extra effort, we can calculate the positive and negative sums in a loop:
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
disp(['sum of positive values: ', num2str(pos_total)])
disp(['sum of negative values: ', num2str(neg_total)])
OUTPUT
sum of positive values: 26
sum of negative values: -6
We can even put one loop inside another:
OUTPUT
1a
1b
2a
2b
3a
3b
Nesting
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.
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:
And, we’d also have to change this line of code, from:
to:
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:
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');
% Process each file in turn
for i = 1:length(files)
file_name = files(i).name;
% Generate strings for image names:
img_name = replace(file_name, '.csv', '.png');
% Generate path to data file and image file
file_name = fullfile('data', filename);
img_name = fullfile('results', img_name);
patient_data = readmatrix(file_name);
% Create figures
if plot_switch == 1
figure('visible', 'off')
else
figure('visible', 'on')
end
subplot(2, 2, 1)
plot(mean(patient_data, 1))
title('Average')
ylabel('Inflammation')
xlabel('Day')
subplot(2, 2, 2)
plot(max(patient_data, [], 1))
title('Max')
ylabel('Inflammation')
xlabel('Day')
subplot(2, 2, 3)
plot(min(patient_data, [], 1))
title('Min')
ylabel('Inflammation')
xlabel('Day')
if plot_switch == 1
print(img_name, '-dpng')
close()
end
end
Key Points
- “Use
if
andelse
to make choices based on values in your program.”