Conditionals

Learning Objectives

Understand what is meant by a conditional statement
When to apply conditional statements in your code
Use conditional statements for indexing and if, elseif and else commands.

Conditional statements

Sometimes we may have instructions or code that only act when a criterion is met. We can construct conditional statements that allow us to check whether a criterion is met or not. In python, this is commonly known as True-False or Boolean statements. In MATLAB, it returns a binary option, either 1 for True or 0 for False. An easy example to try is greater than and less than:

5 < 7  % is 5 less than 7

which will return 1 for True. Or,

8 >= 10 % is 8 greater or equal to 10

will return 0 for False.

For equals, we must use two equal signs ==:

5 == 10/2 % is 5 equal to 10/2

Sometimes we may want to check if is not equal, in which case we use the tilde and equals together:

5 ~= 2

We can use conditional statements for indexing, which tells where in an array the statement is true.

x = 1:10  
x(x<4)      % selects elements that are less than 4

y = [3,8,4,1,7,0,2]
y(y>3)      % selects elements that are more than 3

We can even use this to remove elements from an array:

y(y>3) = []; % removes elements that are more than 3 from the array
y

Combining conditions

Sometimes we require multiple conditions to be met at the same time. Here we use two ampersands &&:

5 < 8 && 2 > 5 % is 5 less than 8 and is 5 greater than 2

This requires all statements to be correct to return 1 or True. In contrast, we may only need one condition to be True. In which case we can use an or statement, where we use two pipes ||.

5 < 7 || 7 < 5 % is 5 less than 7 or is 7 less than 5.

Clearly in the above one of these statements is true and therefore would return 1.

If statements

To use conditional statements in our code, we can use an if function. This checks if a criterion is met, and executes the code between the if statement and end if True, or otherwise passes. Try this:

x = 5;
if x < 6
  disp([num2str(x), ' is smaller than 6'])
end

Change the number and see what the output is.

If-else statements

We may also want something to happen if the statement is not True. This is one option:

x = 8;
if x < 6
  disp([num2str(x), ' is smaller than 6'])
end

if x > 6
  disp([num2str(x), ' is greater than 6'])
end

Alternatively, we can use an else statement:

x = 8;
if x < 6
  disp([num2str(x), ' is smaller than 6'])
else
  disp([num2str(x), ' is greater than 6'])
end

For many lines of code, this is much more readable and also more efficient. With the above example, there is an edge case that is not considered. This is when x == 6. We can also include an elseif statement, which is another conditional we can add:

x = 8;
if x < 6
  disp([num2str(x), ' is smaller than 6'])
elseif x == 6
  disp([num2str(x), ' is equal to 6'])
else
  disp([num2str(x), ' is greater than 6'])
end

Task 5a

Create a function that inputs a whole number and displays whether the number is even or odd. Use the MATLAB function mod(number, base) with base = 2:

mod(5,2)   % returns 1 as odd
mode(4,2)  % returns 0 as even 

Task 5b

Fizz-Buzz is a game for young students whereby numbers are called in ascending order, “1, 2, 3…” except when the number is either a multiple of 3 or 5. If the number is a multiple of 3, we replace the number with fizz, and for a multiple of 5, we replace with buzz. If the number is both a multiple of 3 and 5, they would call out fizz buzz. Using if statements, reproduce the output of this game given an input number:

function fizzbuzz(number)
% Add your code here.
end