MATLAB: Dividing a year-length varying-resolution time vector into months - matlab

Question

I have a time series in the following format:
time data value
733408.33 x1
733409.21 x2
733409.56 x3
etc..
The data runs from approximately 01-Jan-2008 to 31-Dec-2010.
I want to separate the data into columns of monthly length.
For example the first column (January 2008) will comprise of the corresponding data values:
(first 01-Jan-2008 data value):(data value immediately preceding the first 01-Feb-2008 value)
Then the second column (February 2008):
(first 01-Feb-2008 data value):(data value immediately preceding the first 01-Mar-2008 value)
et cetera...
Some ideas I've been thinking of but don't know how to put together:
Convert all serial time numbers (e.g. 733408.33) to character strings with datestr
Use strmatch('01-January-2008',DatesInChars) to find the indices of the rows corresponding to 01-January-2008
Tricky part (?): TransformedData(:,i) = OriginalData(start:end) ? end = strmatch(1) - 1 and start = 1. Then change start at the end of the loop to strmatch(1) and then run step 2 again to find the next "starting index" and change end to the "new" strmatch(1)-1 ?
Having it speed optimized would be nice; I am going to apply it on data sampled ~2 million times.
Thanks!

Answer 1

I would use histc with a list a list of last days of the month as the second parameter (Note: use histc with the two return functions).
The edge list can easily be created with datenum or datevec.
This way you don't have operation on string and you that should be fast.
EDIT:
Example with result in a simple data structure (including some code from #Rody):
% Generate some test times/data
tstart = datenum('01-Jan-2008');
tend = datenum('31-Dec-2010');
tspan = tstart : tend;
tspan = tspan(:) + randn(size(tspan(:))); % add some noise so it's non-uniform
data = randn(size(tspan));
% Generate list of edge
edge = [];
for y = 2008:2010
for m = 1:12
edge = [edge datenum(y, m, 1)];
end
end
% Histogram
[number, bin] = histc(tspan, edge);
% Setup of result
result = {};
for n = 1:length(edge)
result{n} = [tspan(bin == n), data(bin == n)];
end
% Test
% 04-Aug-2008 17:25:20
datestr(result{8}(4,1))
tspan(data == result{8}(4,2))
datestr(tspan(data == result{8}(4,2)))

Answer 2

Assuming you have sorted, non-equally-spaced date numbers, the way to go here is to put the relevant data in a cell array, so that each entry corresponds to the next month, and can hold a different amount of elements.
Here's how to do that quite efficiently:
% generate some test times/data
tstart = datenum('01-Jan-2008');
tend = datenum('31-Dec-2010');
tspan = tstart : tend;
tspan = tspan(:) + randn(size(tspan(:))); % add some noise so it's non-uniform
data = randn(size(tspan));
% find month numbers
[~,M] = datevec(tspan);
% find indices where the month changes
inds = find(diff([0; M]));
% extract data in columns
sz = numel(inds)-1;
cols = cell(sz,1);
for ii = 1:sz-1
cols{ii} = data( inds(ii) : inds(ii+1)-1 );
end
Note that it can be difficult to determine which entry in cols belongs to which month, year, so here's how to do it in a more human-readable way:
% change this line:
[y,M] = datevec(tspan);
% and change these lines:
cols = cell(sz,3);
for ii = 1:sz-1
cols{ii,1} = data( inds(ii) : inds(ii+1)-1 );
% also store the year and month
cols{ii,2} = y(inds(ii));
cols{ii,3} = M(inds(ii));
end

Answer 3

I'll assume you have a timeVals an Nx1 double vector holding the time value of each datum. Assuming data is also an Nx1 array. I also assume data and timeVals are sorted according to time: that is, the samples you have are ordered according to the time they were taken.
How about:
subs = #(x,i) x(:,i);
months = subs( datevec(timeVals), 2 ); % extract the month of year as a number from the time
r = find( months ~= [months(2:end), months(end)+1] );
monthOfCell = months( r );
r( 2:end ) = r( 2:end ) - r( 1:end-1 );
dataByMonth = mat2cell( data', r ); % might need to transpose data or r here...
timeByMonth = mat2cell( timeVal', r );
After running this code, you have a cell array dataByMonth each cell contains all data relevant to a specific month. The corresponding cell of timeByMonth holds the sampling times of the data of the respective month. Finally, monthOfCell tells you what is the month's number (1-12) of each cell.