Repeating With Loops
OverviewTeaching: 30 min
Exercises: 0 minQuestions
How can I repeat the same operations on multiple values?Objectives
Explain what a for loop does.
Correctly write for loops that repeat simple commands.
Trace changes to a loop variable as the loops runs.
Use a for loop to process multiple files
Recall that we have to do this analysis for every one of our dozen datasets, and we need a better way than typing out commands for each one, because we’ll find ourselves writing a lot of duplicate code. Remember, code that is repeated in two or more places will eventually be wrong in at least one. Also, if we make changes in the way we analyze our datasets, we have to introduce that change in every copy of our code. To avoid all of this repetition, we have to teach MATLAB to repeat our commands, and to do that, we have to learn how to write loops.
Let’s start another script, that we will call
loop_practice.m to practice our loops.
%LOOP_PRACTICE Examples of loops
Suppose we want to print each character in the word “lead” on
a line of its own. One way is to use four
word = 'lead'; disp(word(1)) disp(word(2)) disp(word(3)) disp(word(4))
l e a d
But this is a bad approach for two reasons:
It doesn’t scale: if we want to print the characters in a string that’s hundreds of letters long, we’d be better off typing them in.
It’s fragile: if we change
wordto a longer string, it only prints part of the data, and if we change it to a shorter one, it produces an error, because we’re asking for characters that don’t exist.
word = 'tin'; disp(word(1)) disp(word(2)) disp(word(3)) disp(word(4))
error: A(I): index out of bounds; value 4 out of bound 3
There’s a better approach:
word = 'lead'; for letter = 1:4 disp(word(letter)) end
l e a d
This improved version uses a for loop to repeat an operation—in this case, printing to the screen—once for each element in an array.
The general form of a for loop is:
for variable = collection do things with variable end
The for loop executes the commands in the
for every value in the array
This value is called the loop variable,
and we can call it whatever we like.
In our example, we gave it the name
We have to terminate the loop body with the
and we can have as many commands as we like in the loop body.
But, we have to remember
that they will all be repeated as many times as
there are values in
Our for loop has made our code more scalable, and less fragile. There’s still one little thing about it that should bother us. For our loop to deal appropriately with shorter or longer words, we have to change the first line of our loop by hand:
word = 'tin'; for letter = 1:3 disp(word(letter)) end
t i n
Although this works, it’s not the best way to write our loop:
We might update
wordand forget to modify the loop to reflect that change.
We might make a mistake while counting the number of letters in
Fortunately, MATLAB provides us with a convenient function to write a better loop:
word = 'aluminum'; for letter = 1:length(word) disp(word(letter)) end
a l u m i n u m
This is much more robust code,
as it can deal identically with
words of arbitrary length.
Here’s another loop that
repeatedly updates the variable
len = 0 for vowel = 'aeiou' len = len + 1; end disp(['Number of vowels: ', num2str(len)])
It’s worth tracing the execution of this little program step by step.
We can use the MATLAB debugger to trace the execution of a program.
The first step is to set a break point by clicking just to the right of a line number on the
-symbol. A red circle will appear — this is the break point, and when we run the script, MATLAB will pause execution at that line.
A green arrow appears, pointing to the next line to be run. To continue running the program one line at a time, we use the
We can then inspect variables in the workspace or by hovering the cursor over where they appear in the code, or get MATLAB to evaluate expressions in the command window (notice the prompt changes to
This process is useful to check your understanding of a program, in order to correct mistakes.
This process is illustrated below:
Since there are five characters in “aeiou”,
the loop body will be executed five times.
When we enter the loop,
len is zero -
the value assigned to it beforehand.
The first time through,
the loop body adds 1 to the old value of
len to refer to that new value.
The next time around,
len is 1,
len is updated to 2.
After three more updates,
len is 5;
since there’s nothing left in
aeiou for MATLAB to process,
the loop finishes and the
disp statement tells us our final answer.
Note that a loop variable is just a variable that’s being used to record progress in a loop. It still exists after the loop is over, and we can re-use variables previously defined as loop variables as well:
MATLAB uses the caret (
^) to perform exponentiation:
You can also use a loop to perform exponentiation. Remember that
Let a variable
bbe the base of the number and
xthe exponent. Write a loop to compute
b^x. Check your result for
b = 4and
x = 5.
% Loop to perform exponentiation b = 4; % base x = 5; % exponent result=1; for i = 1:x result = result * b; end disp([num2str(b), '^', num2str(x), ' = ', num2str(result)])
Incrementing with Loops
Write a loop that spells the word “aluminum,” adding one letter at a time:
a al alu alum alumi alumin aluminu aluminum
% spell a string adding one letter at a time using a loop word = 'aluminium'; for letter = 1:length(word) disp(word(1:letter)) end
Looping in Reverse
In MATLAB, the colon operator (
:) accepts a stride or skip argument between the start and stop:
1 4 7 10
11 8 5 2
Using this, write a loop to print the letters of “aluminum” in reverse order, one letter per line.
m u n i m u l a
% Spell a string in reverse using a loop word = 'aluminium'; for letter = length(word):-1:1 disp(word(letter)) end
We now have almost everything we need to process
multiple data files with our
We need to generate a list of data files to process, and then we can use a loop to repeat the analysis for each file.
We can use the
dir command to return a structure array containing
the names of the files in the
Each element in this structure array is a structure, containing
information about a single file in the form of named fields.
A structure array is a data type that groups related data using data containers called fields. Each field can contain any type of data. To access information on a structure we use a dot notation of the form StructName.fieldName.
files = dir('data/inflammation-inflammation-*.csv')
files = 12×1 struct array with fields: name folder date bytes isdir datenum
To access the name field of the first file, we can use the following syntax:
file_name = files(1).name; disp(filename)
To get the modification date of the third file, we can do:
mod_date = files(3).date; disp(mod_date)
A good first step towards processing multiple files is to write a loop which prints the name of each of our files:
files = dir('data/inflammation-*.csv'); for idx = 1:length(files) data_file = files(idx).name; disp(data_file) end
inflammation-01.csv inflammation-02.csv inflammation-03.csv inflammation-04.csv inflammation-05.csv inflammation-06.csv inflammation-07.csv inflammation-08.csv inflammation-09.csv inflammation-10.csv inflammation-11.csv inflammation-12.csv
The final task is to generate the file names for the figures we’re going to save.
Let’s name the output file after the data file used to generate the figure.
So for the data set
inflammation-01.csv we will call the figure
We can use the
replace command for this purpose.
The syntax for the
replace command is like this:
NEWSTR = replace(STR, OLD, NEW)
So for example if we have the string
big_shark and want to get the string
terror_shark, we can execute the following command:
new_string = replace('big_shark', 'big', 'terror'); disp(new_string)
Recall that we’re saving our figures to the
The best way to generate a path to a file in MATLAB is by using the
This generates a file path with the correct separators for the platform you’re using
(i.e. forward slash for Linux and macOS, and backslash for Windows).
This makes your code more portable which is great for collaboration.
We’re now ready to modify
analyze.m to process multiple data files:
%ANALYZE Print statistics for first three patients. % Save plots of statistics to disk. files = dir('data/inflammation-*.csv'); % Process first three files only for idx = 1:3 file_name = files(idx).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', file_name); img_name = fullfile('results', img_name); patient_data = csvread(file_name); disp(['Maximum inflammation: ', num2str(max(patient_data(:)))]) disp(['Minimum inflammation: ', num2str(min(patient_data(:)))]) disp(['Standard deviation: ', num2str(std(patient_data(:)))]) ave_inflammation = mean(patient_data, 1); % Create figures figure('visible', 'off') 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') print('-dpng', img_name) close() end
We run the modified script using its name in the Command Window:
The figures output to the
results directory are as shown below:
Sure enough, the maxima of these data sets show exactly the same ramp as the first, and their minima show the same staircase structure.
We’ve now automated the analysis and have confirmed that all the data files show the same artifact. This is what we set out to test, and now we can just call one script to do it. With minor modifications, this script could be re-used to check all our future data files.
Lastly, in the above trick using
lswith the wildcard
*, another small Octave/MATLAB difference shows up. In Octave, the value returned by
filestr = ls('path/to/data/*.csv')is an array of strings, so we can loop over
filestrdirectly without the need to split it with
forto create a loop that repeats one or more operations.