Creating indivdual column vectors based off a location vector

Hello all,

I am trying to figure out how to do this and can't come up with a good way to do it. What I am trying to do is break up this long column vector up, based on a location vector. So, I have a long column vector of data ( roughly) 10000 x 1. I then have a location vector that contains locations where I want to break up the column vector. It looks something like this:

 location=[2;200;360;500;700;850;900]

I am trying to create individual column vectors of the data based on the location vector. So for example I will end up with column vectors such as:

a1=data(2:200)
a2=data(200:360)
a3=data(360:500)
etcc.... Based off the location vector

The problem is, I need a way to automate this, because the location vector actually contains many locations. I was trying to do this with a for loop, but I keep getting stuck.

Answer 1

[Yannick]

Hi, here's a way to do it using ARRAYFUN:

    initialIndices = location(1:end-1);
    finalIndices   = location(2:end);
    C = arrayfun(@(i,j)data(i:j),initialIndices,finalIndices,'UniformOutput',false)

Then, C{1} would correspond to your first matrix ( a1 in the description), C{2} to the next chunk, and so on. Maybe there are better ways, but this is one. A FOR loop is certainly another way of doing it.

Here are some useful doc pages:

• ARRAYFUN
• Function handles
• Cell Arrays

[Christopher]

I like that, but I still have to manually enter C{1}, C{2}, etcc.. I was trying to have them output automatically, because there is around 50 different a's that I would have to type out.

I couldn't figure out how to get it work in a for loop. I would think it would be easy, but it's not obvious.

[Yannick]

This piece of code fully automates the process of breaking the original array up into chunks, and has the benefit of collecting the chunks into a single data structure, the cell array C. It is preferable to have one data structure you can index into, as opposed to 50 different variables. So I don't see any benefit to doing a1=C{1}, a2=C{2}, and so on for all chunks.

Here's an example to illustrate this. Say you have your data in the form of a cell array, and you need to do some computation on chunks 10 through 20. You can easily do the following:

    for k = 10:20
        performComputationOnChunk(C{k});
    end

Easy. Now imagine that instead, you have a bunch of variables a1, a2, etc. How do you do this easily in a loop?

Now, there is a way, but it is NOT recommended at all. If you read that link carefully, you should take away two things:

1. eval should be avoided (luckily this is possible in the vast majority of cases);
2. for your particular case, cell arrays (or perhaps structure arrays) seem like the way to go.

If you decide to ever use eval, you didn't hear it from me.

I hope this helps clarify my suggestion!