How do I seperate two columns of data into 12 different variables based on the number from the first column?

I have a variable with two columns, the first column are numbers that correspond to parameters for an experiment (there are 12 unique parameters total called "speed"), the second column is my raw data. I need to seperate the raw data into 12 different variables based on the numbers from the first column.

One of the parameters is a speed of 4... I pretty much want to tell matlab "If Column one = 4, the number next to 4 will go into a variable called speed4" Is there anyway to do this???

Answer 1

[Star Strider]

One approach that appears to work:

Data = [randi(12, 20, 1)  randi(99, 20, 2)];                        % Create Data

speed = accumarray(Data(:,1), Data(:,2), [], @(x) {cat(2,x)});      % Construct ‘speed’

[Marissa Holden]

This is beautiful! you've solved my problem :) Do you think you can explain a little bit about the string..? especially "[], @(x) {cat(2,x)});" I am VERY new to matlab and I really want to learn what I'm doing rather than just copy what people have done

[Star Strider]

Thank you!

My pleasure. The accumarray function is quite useful. It operates on the first argument as the index variable (indices must be integers greater than zero), then takes the second argument and either adds it to the other values (the default) or uses the function in the fourth argument to create the desired output. (The third argument is the size of the output, not used here, so is designated as the empty matrix. Arguments to functions have to appear in the specific order the function expects them.)

The anonymous function, ‘@(x) {cat(2,x)}’ creates a cell array of the elements in ‘Data(:,2)’ by concatenating them along the second (column) dimension. (See the documentation on Anonymous Functions (link) for more information on them.) The cat function performs this. The accumarray function, using the anonymous function, then stores them as a cell array (the curly brackets ‘{}’ denote a cell array, or indexing into a cell array) because cell arrays allow for empty elements and for different sizes of the columns (here), something a numeric array does not allow.

You can access the data in ‘speed’ by using cell array indexing. So you would access the values of the 4th element of ‘speed’ as ‘speed{4}’, the cell element being a double array. See the documentation on Cell Arrays (link) if you are not familiar with them. They are quite useful for storing data, and are usually easy to use in calculations.

If you want any further clarification or explanation, I will do my best to provide it.

[Marissa Holden]

Thank you so so much for your help :)!!

[Star Strider]

As always, my pleasure!