# Making Choices

## Overview

Teaching: 30 min
Exercises: 0 min
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

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:

``````num = 37;

if num > 100
disp('greater')
else
disp('not greater')
end

disp('done')
``````
``````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:

``````num = 53;
disp('before conditional...')

if num > 100
disp('53 is greater than 100')
end

disp('...after conditional')
``````
``````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:

``````%CONDITIONAL_DEMO   Demo script to illustrate use of conditionals

num = 53;

if num > 0
disp([num2str(num), ' is positive'])
elseif num == 0
disp([num2str(num), ' is zero'])
else
disp([num2str(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.

``````% Demo script to illustrate use of conditionals
num = 53;

if num > 0
disp([num2str(num), ' is positive'])
elseif num == 0
disp([num2str(num), ' is zero'])
elseif num > 50
# This block will never be executed
disp([num2str(num), ' is greater than 50'])
else
disp([num2str(num), ' is negative'])
end
``````

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

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

`||` is true if either test is true:

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

In this case, “either” means “either or both”, not “either one or the other but not both”.

## 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?

## Solution

``````%NEAR   Display 1 if variable a is within 10% of variable b
%       and display 0 otherwise
a = 1.1;
b = 1.2;

if a/b >= 0.9 && a/b <= 1.1
disp(1)
else
disp(0)
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:

``````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)])
``````
``````sum of positive values: 20
``````

With a little extra effort, we can calculate the positive and negative sums in a loop:

``````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)])
``````
``````sum of positive values: 26
sum of negative values: -6
``````

We can even put one loop inside another:

``````for consonant = 'bcd'
for vowel = 'ae'
disp (strcat(consonant, vowel))
end
end
``````
``````ba
be
ca
ce
da
de
``````

## 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.

## Solution

``````for vowel = 'ae'
for consonant = 'bcd'
disp (strcat(consonant, vowel))
end
end
``````

Reordering the nested loops changes the output. In the new code, the consonants loop happens within the vowel loop, so while vowel = a, consonant takes the values b, c, and d in turn.

Currently, our script `analyze.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:

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

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

``````figure('visible', 'off')
``````

to:

``````figure('visible', 'on')
% or equivalently: figure()
``````

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:

``````
%ANALYZE   Print statistics for three patients.
%          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;

for idx = 1:3

% Generate strings for file and image names:
file_name = sprintf('data/inflammation-%02d.csv', idx);
img_name = sprintf ('data/patient_data-%02d.png', idx);

ave_inflammation = mean(patient_data, 1);

if plot_switch == 1
figure('visible', 'off')
else
figure('visible', 'on')
end

subplot(2, 2, 1)
plot(ave_inflammation)
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('-dpng', img_name)
close()
end

end
``````

## Key Points

• Use `if` and `else` to make choices based on values in your program.