Extracting and sorting data in a column

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

[Alex Mcaulley]

Can you show the expected result in your example?

[Olu B]

The expected result would be

NewMx = [22 23 54 23 67 34 21 43 56 32]

Cheers

Answer 1

[madhan ravi]

Works for odd as well as even number of elements:

NewMx = max(reshape([Mx(:);...
    -Inf(mod(numel(Mx),2))],2,[])) % thank you Guillaume

[Jos (10584)]

nice, but only works when the number of elements in Mx is even

[madhan ravi]

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]

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]

Thank you Guillame :)

[Guillaume]

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)]

My apologies Madhan!

[madhan ravi]

No problem Jos :)

Answer 2

[Jos (10584)]

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)