Breaking down a large array

2013 5 23
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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

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Roger Stafford
Roger Stafford 2013-5-23

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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);

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Senaasa
Senaasa 2013-5-23
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
Walter Roberson 2013-5-23
p = [1:200, 202:3:length(old_array)];
Cedric
Cedric 2013-5-23
编辑：Cedric 2013-5-23
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
Senaasa 2013-5-23
Ok, I've got it now. Thanks a lot guys

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