Control

Last updated on 2025-04-28 | Edit this page

Overview

Questions

  • What are the different types of loop?
  • How can you control loops to exit early or skip iterations?
  • How can you loop over multiple objects at the same time?

Objectives

  • Understand the main types of loop and when they should be used
  • Learn about loop controls such as break and continue

Loops in coding allow for blocks of code to be executed multiple times depending on some logic. This can save you from having redundant copy and pasted code, making your code shorter and easier to read.

There are two main loop types in MATLAB, for loops and while loops. A for loop repeats a specified number of times, a while loop repeats until a logical condition is met.

For Loops

For loops have the following syntax

for index = values
  code
end

values is typically a vector where the length will define the number of repetitions

index will be the current value of values for this iteration. In the exaple below,

MATLAB 

A = 1:5;
for ii = A
  disp(ii)
end

``` OUTPUT
1
2
3
4
5

In this example our values is 1:5 which if you remember from indexing produces a vector [1 2 3 4 5]

So our for loop, loops 5 times, the first loop our index ii will be 1, then 2, etc.

Challenge

Create a for loop that sums the even numbers in the range 0 to 20.

Try creating another variable called total equal to 0, for each iteration of the loop add the current index

Also think back to previous episodes where we covered indexes and counting in steps

MATLAB 

total = 0
for ii = 0:2:20
  total = total + ii;
end

While Loops

As previously mentioned, while loops will loop while a logical condition is being met.

MATLAB 

ii = 0
while ii < 10
  ii = ii + 1;
end

Interestingly, as 1 is logic true in MATLAB, the following loop will run until cancelled or MATLAB runs out of resources

Cancel execution

While loops can get stuck in an infinite cycle, to stop this running without shutting MATLAB you can press Ctrl+C or on Mac Ctrl+Break

MATLAB 

ii = 0
while 1
  ii = ii + 1;
end

While loops are useful when you don’t know how many iterations are needed. Some common use cases are:

  • Iterative Algorithms: Gradient-descent in machine learning, power series
  • Event driven: Continuously reading from a sensor, waiting for user input

Loop Controls

break and continue are statements that can be used to control behavior in a loop

The break statement immediately exits the loop it’s in, skipping any remaining iterations. Execution continues with the next statement after the loop.

MATLAB 

for ii = 1:10
    if ii == 5
        break;
    end
    disp(ii);
end
disp('Loop finished');

OUTPUT 

1
2
3
4
Loop finished

The continue statement skips the remaining code in the current iteration and jumps to the next iteration of the loop.

MATLAB 

for ii = 1:8
    if mod(ii, 2) == 0 % Check for even numbers
        continue; 
    end
    disp(ii);
end

OUTPUT 

1
3
5
7

Nested Loops

You may come across a situation when programming where you need to need to process every element of one dataset against every element of another. This scenario is often tackled using a nested loop, where you have one for loop inside of another.

This example multiplies each number from two ranges against each other, displaying the current iterators and the resulting product for each loop.

MATLAB 

for ii=1:3
  for jj=10:10:30
    ii
    jj
    ii*jj
  end
end

Callout

Nested loops are powerful tools but avoid nesting unnecessarily. Code nested 3 or 4 loops deep can be hard to understand and read.

Key Points

  • Use for loops for when you know how many iterations to loop, while loops when you don’t
  • Break and continue allow you to exit or skip loops
  • Use indents to keep looped code clear