Appending Values to an Empty Array

I am unable to run this simple chunk of code. I am trying to break down over a million points of data in chunks of 6000. I then want to calculate the number of peaks and then return it to an array so that I can calculate the average peaks per 6000 data points.

I am however receiving this error:

Unable to perform assignment because the indices on the left side are not

compatible with the size of the right side.

Error in filter (line 104)

rate(end+1) = peaks;

Why is appending not possible?

rate = [];
for k = 1:size(x):6000
    section = sg_filt(k:i,:);  
    i = k+6000;
    [pks] = findpeaks(section, 'Threshold', 500);
    peaks = size(pks);
    rate(end+1) = peaks;
end

Answer 1

[Walter Roberson]

size() always returns a vector. Consider numel()

Answer 2

[dpb]

Because as written size() returns a two-vector and rate(end+1) is an address for a single array element -- you're trying to store two values into one location.

I believe w/o testing that since you're passing a column vector that findpeaks will return the pks array as a column vector as well in which case you're looking for

rate(end+1)=size(pks,1);

If it is a column vector instead, then size(pks,2) or just use numel(pks) instead of size()

It would be better to preallocate and store instead of appending -- while this is a small enough of a case that the extra copy operations won't show up, in general try to avoid doing this when can.

Try something more like

N=6000;                             % use variables, don't bury magic numbers in code
nSeg=ceil(numel(sg_filt)/N);        % compute number of segments
rate = zeros(nSeg,1);               % preallocate output
for k = 1:N:numel(sg_filt)
    section = sg_filt(k:i,1);  
    i = k+6000;
    [pks] = findpeaks(section, 'Threshold', 500);
    peaks = size(pks);
    rate(end+1) = peaks;
end

I fixed up a few other things based on what I'd guess you want/have as well...