Breaking down a large array

Hi,

I have a large array (nx1) of values that I'd like to break up into a new array. This new array takes the first 200 or so values from the original array and then only selects every 3rd value (or even a logarithmic selection) of the original array to construct the new array. I was thinking about doing this with a loop, but I was wondering if matlab had any built-in functions that could do this.

Thanks, Charles

Answer 1

[Roger Stafford]

For spacing every third element

 p = a:3:b;

For logarithmic spacing

 p = round(k*a.^(1:n));

for appropriate a, k, and n. Then do this:

 new_array = old_array(p);

[Senaasa]

Those would work if I wanted to split up the entire array. What I'm trying to do is split up a portion of the array while keeping the beginning of the array (first 200 points) the same. After these first 200 points, I then want to go by some incremental amount (say 3) and splice together that result with the first 200 points to form the new array.

[Walter Roberson]

p = [1:200, 202:3:length(old_array)];

[Cedric]

What Roger is generating is an array of IDs/positions (that he then uses for indexing operations); you can work with these as you work with any other array; in particular, you can concatenate them, e.g.

 p = [1:200, 203:3:1000] ;

or

 p = [1:200, 200+3^(1:5)] ;

etc. Experiment with smaller numbers so you can easily visualize p.

[Senaasa]

Ok, I've got it now. Thanks a lot guys