How to extract columns of a matrix into seperate vectors?

Hallo,

Let's assume a matrix (100x3). I want to extract out each of the columns, and store them in 3 separate vectors.

The code for the loop is clear, but what's the right code for storing each column?

----------------------------

for i = 1 : 3    
    vector = matrix(:, i);       
end

----------------------------

I guess it's probably simple, but I am still a beginner and couldn't find much useful information in the documentation so far.

Thank you. Dominik

[Oleg Komarov]

Why do you want to do that? And, don't do that. Storing your data has to be kept separate from the design you have in mind.

Answer 1

[Image Analyst]

Try this:

column1 = your2DMatrix(:, 1);
column2 = your2DMatrix(:, 2);
column3 = your2DMatrix(:, 3);

Unlike the others, I don't find any problem with doing this if referencing these 3 column vectors individually by name will make your code easier to understand and follow. It will certainly make your subsequent code more compact. I wouldn't do it if you had dozens and dozens of columns, but for only 3 I don't see a problem.

[Oleg Komarov]

Yes, I would agree with you IA if only the question was posed without the loop, and we all know where it leads.

[Sean de Wolski]

I wouldn't recommend against the mx3 column case. But how about the mx200 column case?

[Image Analyst]

OK, so we're all in agreement then - good.

[Doina Gumeniuc]

Hi! I went through this conversation but could not find the answer. What if there are more than 3 columns? In my case 50 columns, so taking it one by one is a bit painful. Thank you very much!

[Stephen23]

@Doina Gumeniuc: if you "went through this conversation" then you must have read the comments where very experienced MATLAB users and TMW staff recommend not doing this for a larger number of vectors.

Read this to know why they all agreed that this is a bad idea:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

"so taking it one by one is a bit painful."

Yes, it is. And it is almost invariably a slow, buggy, complex way to write code, no matter how many beginners want to do this. What do you hope to achieve with 50 vectors in your workspace? How do you imagine passing 50 variables to SIN, or any other function? How would you search for those variables in the editor?

[Doina Gumeniuc]

Hi Stephen, thank you for your reply, my bad for not noticing the information you mentioned. It is nice to have the possibility to ask for advice the more experienced matlab users. I have some operations to do column by column, I wrote a post for it, please see it here https://se.mathworks.com/matlabcentral/answers/368884-operation-with-big-files

Thank you!

Answer 2

[Sean de Wolski]

x = mat(:,1);
y = mat(:,2);
%etc.

But like Oleg said - don't do it!

Answer 3

[Dominik]

The matrix example is actually just a simple way, I wanted to figure out how to store data in automatically created vectors/columns.

In my actual problem I want to fetch data of different securities, what I do via a loop, and then want to save every set in a single (automatic) created vector or all together in a new column of a matrix.

for i = 2:nassets
    security = tickers{i};
    data = fetch(connection,security,'close','2011-01-01','2012-08-01');
end
%%%how can I save the results from each round of the loop?

Thank you

[Image Analyst]

How about using save() or fopen(), fprintf(), and fclose()? Or you could use dlmwrite() or csvwrite(). it just depends on what form you want the saved column to take. Whatever you do, you'd create a new filename to save the "data" variable. See the FAQ: http://matlab.wikia.com/wiki/FAQ#How_can_I_process_a_sequence_of_files.3F

Also see this in the FAQ: http://matlab.wikia.com/wiki/FAQ#How_can_I_create_variables_A1.2C_A2.2C....2CA10_in_a_loop.3F

[Dominik]

That looks good, thank you guys!

[Oleg Komarov]

Please, don't use eval.

Answer 4

[Korosh Agha Mohammad Ghasemi]

To extract each column of a matrix and store them in separate vectors, you can use a loop in MATLAB. You can store each vector in a cell array or directly assign them to separate variables if you know the number of columns in advance. Below are two methods to achieve this.Method 1: Using a Cell Array

This method is useful if the number of columns is not known in advance or if you prefer dynamic storage.

% Example matrix (100x3)
matrix = rand(100, 3);

% Preallocate a cell array to store each column
numCols = size(matrix, 2);
vectors = cell(1, numCols);

% Loop to extract each column and store it in the cell array
for i = 1:numCols
    vectors{i} = matrix(:, i);
end

% Display the vectors
for i = 1:numCols
    fprintf('Vector %d:\n', i);
    disp(vectors{i});
end

Method 2: Assigning to Separate Variables

This method is suitable if you know the number of columns beforehand and prefer direct variable assignment.

% Example matrix (100x3)
matrix = rand(100, 3);

% Preallocate vectors (if necessary)
vector1 = matrix(:, 1);
vector2 = matrix(:, 2);
vector3 = matrix(:, 3);

% Display the vectors
disp('Vector 1:');
disp(vector1);

disp('Vector 2:');
disp(vector2);

disp('Vector 3:');
disp(vector3);

Loop Method with Dynamic Variable Names (Not Recommended)

Using dynamic variable names in a loop is generally discouraged because it makes the code harder to read and maintain. However, for completeness, here’s how you might do it:

% Example matrix (100x3)
matrix = rand(100, 3);

% Loop to extract each column and assign to dynamically named variables
for i = 1:3
    eval(sprintf('vector%d = matrix(:, i);', i));
end

% Display the vectors
disp('Vector 1:');
disp(vector1);

disp('Vector 2:');
disp(vector2);

disp('Vector 3:');
disp(vector3);

Summary

• Cell Array Method: Flexible and scalable for an unknown number of columns.
• Direct Assignment: Simple and straightforward for a known number of columns.
• Dynamic Variable Names: Generally not recommended due to readability and maintainability issues.

Choose the method that best suits your needs and coding style.

[Voss]

An alternative method using a cell array is to use num2cell with the dimension argument specified.

Example:

matrix = rand(100, 3);
vectors = num2cell(matrix,1)

vectors = 1x3 cell array
    {100x1 double}    {100x1 double}    {100x1 double}