how to compute blocks of ones in a row

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imola
imola 2015-3-16
评论： imola 2015-3-18
Dear All,
I have this matrix.
P=[1 0 1 1 0 0 1 1 1
0 0 1 0 1 1 0 0 0];
I need to accumulate the ones numbers but as blocks of ones with the length of the block, here I have in the first row 3 blocks 1 , 1 1 , 1 1 1 , so the sum is 1+1+1 for the blocks and 2 for the length 1 1 and 3 for 1 1 1 so
sum=1+1+1+2+3
in the second row
sum=1+1+2
I wrote this
ind =strfind([0, P(1,:)],[0 1])
or
ind =find (diff([0 P(1,:)])==1)
it gives me the number of blocks but not the length of every block. I need to write the formula but is rather confuse. could anybody help me please
regards,
Imola
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Guillaume
Guillaume 2015-3-18
You've not really answered Sean question.
In your example, for the first row you've got three blocks (so first part of sum is 3*1). Three block would seem to imply three lengths: [1 2 3]. Hence we would expect the complete sum to be 3*1 + 1 + 2 + 3. Why are you not accounting for the blocks of length 1?
Guillaume
Guillaume 2015-3-18
And by the way, don't call your variable sum

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Guillaume
Guillaume 2015-3-18
A simple way to get the lengths and number of blocks:
P = [1 0 1 1 0 0 1 1 1];
transitions = diff([0 P 0]);
%transitions is 1 when going from 0 to 1 and -1 when going from 1 to 0.
%because 0 is added before and after, the first transition is always 0 to 1 and last always 1 to 0
%so there's always the same numbers of 1 and -1 in the diff
numblocks = sum(transitions == 1) %or sum(transitions == -1)
blocklengths = find(transitions == -1) - find(transitions == 1)
To operate on a whole matrix at once is a bit trickier:
P = [1 0 1 1 0 0 1 1 1
0 0 1 0 1 1 0 0 0];
transitions = diff([zeros(size(P, 1), 1) P zeros(size(P, 1), 1)], [], 2);
[starts, rows] = find(transitions' == 1);
[ends, ~] = find(transitions' == -1);
blocklengths = ends - starts;
sumlengthsbyrow = accumarray(rows, blocklengths, [size(P, 1) 1])
numblocksbyrow = accumarray(rows, 1, [size(P, 1) 1])
totalsumbyrow = sumlengthsbyrow + numblocksbyrow
If you don't want to include the blocks of length 1 in sumlengthsbyrow:
sumlengthsbyrow = accumarray(rows(blocklengths > 1), blocklengths(blocklengths > 1), [size(P, 1) 1])
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Guillaume
Guillaume 2015-3-18
As I wrote at the end of my answer, if you don't want to include the lengths of one, it's:
sumlengthsbyrow = accumarray(rows(blocklengths > 1), blocklengths(blocklengths > 1), [size(P, 1) 1])
numblocksbyrow = accumarray(rows, 1, [size(P, 1) 1])
totalsumbyrow = sumlengthsbyrow + numblocksbyrow
imola
imola 2015-3-18
Dear Guillaume,
Thanks a lot. I really appreciate your help.
regards

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