Learning Objectives

• Apply for loops into functions and scripts.
• Understand the usefulness of nested loops.
• Use while loops and the risks if not careful.

for Loops

Loops help to automate code, executing many similar actions. From the previous example, we may want to calculate the circumference of multiple circles. We could run the calculate_circumference function each time, however, this can be time-consuming if we want to do this many times:

calculate_circumference(1)
calculate_circumference(2)
calculate_circumference(3)
...
calculate_circumference(100)

Instead we can use a for loop:

for radius = 1:100
  calculate_circumference(radius)
end

Let’s dissect this code first:

1. We use for and end to let MATLAB know where the for loop begins and ends.
2. Code in-between will be run as many times as the loop is defined for. In the above example, this will be 100 times.
3. In the first loop, the radius will equal 1, and in the second two remember that 1:100 will create a list of numbers 1 to 100.
4. The loop will continue until radius = 100, then will continue to run code beyond the end.

To get a better idea, let’s try a simpler loop to get used to the idea:

for loop = [1,5,9,100]
  disp(['Loop number: ', num2str(loop)])
end

You don’t necessarily need to use the variable in the loop either:

for random_variable = 1:3
  disp('Loop')
end

Task 6a

Using what we have learnt already, create a loop which prints multiples of 3.

Nested Loops

Within a loop, we can also include other loops. This is called nested loops and these can be helpful when considering multiple variables of differing values:

for big_loop = ['A','B','C']
  for little_loop = 1:3
    disp([big_loop,num2str(little_loop)])
  end
end

What do you expect the output of the above code to be?

Task 6b

Try calculating the volume of a cylinder for radii between 0.1 and 1 in 0.1 steps, and lengths from 0.05 to 0.2 in 0.04 steps using nested for loops. What challenges can you see when loops get very large?

While Loops

Sometimes we may not know how many loops are required. Instead, we may want to break the loop when a condition has been met. Try this countdown:

number = 5;
while number >= 0
  disp(number)       
  number = number -1;
end

If we perhaps forgot to include number = number -1 this code would run infinitely.

You do have to be careful with these loops to make sure they stop. Otherwise, they will continue to run - if this happens, click the red square in the toolbar to stop the execution.

We can combine conditions to prevent this. A step counter is one popular way to limit the number of loops.

step = 0;
number = 10;
while number >=0 && step < 5
  disp(number)
  number = number -1;
  step = step + 1;
end

We will use a while loop in the next episode