Extracting and sorting data in a column
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Hi For example I have a 19 x1 colunmn called Mx and I need to find the maximum value in every 2 rows till the end of the column.
Mx
20
22
23
21
34
54
23
14
67
56
34
32
21
12
43
23
56
34
32
回答（2 个）
madhan ravi
2019-8-8
编辑：madhan ravi
2019-8-8
Works for odd as well as even number of elements:
NewMx = max(reshape([Mx(:);...
-Inf(mod(numel(Mx),2))],2,[])) % thank you Guillaume
7 个评论
Jos (10584)
2019-8-8
nice, but only works when the number of elements in Mx is even
madhan ravi
2019-8-8
编辑：madhan ravi
2019-8-8
Hä? Jos did you even try the solution before claiming??
>> Mx = 1:11
Mx =
1 2 3 4 5 6 7 8 9 10 11
>> NewMx = max(reshape([Mx(:);...
zeros(mod(numel(Mx),2))],2,[]))
NewMx =
2 4 6 8 10 11
>>
Guillaume
2019-8-8
Since we're looking at the max, I'd recommend padding with -Inf rather than 0:
NewMx = max(reshape([Mx(:); -Inf(mod(numel(Mx),2), 1)],2,[]))
madhan ravi
2019-8-8
Thank you Guillame :)
Guillaume
2019-8-8
By the way, the mod expression should be:
mod(-numel(Mx), groupsize) %note the -
When groupsize is 2, - or + give the same result. For larger groupsize it's critical.
But the idea is there, when you want to group consecutive elements of a vector, the solution is to reshape the vector so that each group is row/column.The idea extends to higher dimensions as well. If you want to calculate something for every 3 consecutive columns of a matrix, reshape the matrix to 3D to have 3 columns and ncolumns/3 pages.
Jos (10584)
2019-8-8
My apologies Madhan!
madhan ravi
2019-8-8
No problem Jos :)
Jos (10584)
2019-8-8
This works for both an even or an odd number of elements:
N = 11 ; % odd
Mx = randi(10, N, 1)
M2 = accumarray(ceil((1:numel(Mx))/2).', Mx, [], @max)
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2019-8-8
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