Parsing an array A based on trigger data in array B ?

2013 7 3
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Hi everyone!
I have what should be a simply question but I cannot seem to find an effective solution. I have two columns of data in Excel files that I read into Matlab, at the moment as two separate arrays (column A and B).
Column A contains numeric data and Column B is a trigger stamp for the data in A. Example:
What I need to do is to parse A and split it into a number of (30) arrays depending on the trigger point in B i.e. the very large A would become A1, A2, A3 ... A30, split according to when there is a data-stamp in B.

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Jan
Jan 2013-7-3
Please post how you "read the Excel file". Then creating a matching answer is easier.
Katie
Katie 2013-7-3
编辑：Katie 2013-7-3
Hi Jan,
I was just crudely reading from the excel file and writing the columns to vectors, but I have switched to reading a cdv instead as Chad suggested, much neater. My trouble is how to split the matrix based on the second column (or B in my earlier version) that is causing the difficulty. Thanks.
[OLD version, reading from Excel] A=xlsread('file.xls','A:B'); rows=sizem(1); % Assigning the columns to vectors for i=1:1:rows if(~isnan(A(i,1))) signal(i)=A(i,1); end end etc.

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Chad Gilbert
Chad Gilbert 2013-7-3

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If I save similar data out as a .csv file:
A,B
1.303,1
1.61,
4,2
1.213,
and then read them in with:
>> a = csvread('a.csv',1,0)
I get a matrix the same shape as your data, with 0's filling all the empty spots:
a =
1.3030 1.0000
1.6100 0
4.0000 2.0000
1.2130 0
This will be pretty easy to parse into a cell array of arrays.

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Katie
Katie 2013-7-3
Thanks Chad. That might be more efficient than reading the excel file alright, but either way I could make the A and B arrays into a matrix. It's the next step of parsing or splicing A (or the A element of the matrix) based on B that doesn't seem to be working out.
E.g. based on your data - A1 [1.3030 1.6100] A2 [4.0000 1.2130] etc.
Thanks again
Chad Gilbert
Chad Gilbert 2013-7-3
编辑：Chad Gilbert 2013-7-3
Ah, I see. I thought your trouble was with reaading. To split it up, Rob's answer ought to work. Or a shorter version might be:
ind = [find(a(:,2)>0); size(a,1)+1];
b = arrayfun(@(i)a(ind(i):ind(i+1)-1),1:length(ind)-1,'uniformoutput',false)
Katie
Katie 2013-7-3
Hi Chad, and Rob!
I tried Chad's first as I had already switched to using a matrix 'a'. This works brilliantly, thank you both. The only thing that is wrong at the moment with it is that it populates b with 29 vectors, not 30. It misses the first one every time. I might just be missing something simple.
I can see in the workspace that 'b' is a 1 x 29 matrix now. If I want to write this out to a new file I'm guessing I have to do more with it?
Thank you both again, much appreciated!

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Rob
Rob 2013-7-3
编辑：Rob 2013-7-3

1 个投票

I'm sure there are more elegant solutions, but here's one way:
idx1 = find(~isnan(B));
idx2 = [idx1(2:end)-1; length(A)];
C = cell(length(idx1), 1);
for k = 1:length(idx1)
C{k} = A(idx1(k):idx2(k));
end
If the non trigger values in B are zeros then:
idx1 = find(B>0);
-Rob

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