Find max and min of groups in columns of a matrix

I have a matrix with positive and negative values as:

a=[0 1;-3 3;-2 -7;0 5;4 -6;1 -2].

I would like to find by columns the maximum positive and minimum negative of each group of positive and negative values without any sorting. For the example given the result b would be:

b{1,1}= [-3;4]
b{1,2}= [3;-7;5;-6]

Thanks

[Walter Roberson]

How is a "group" to be determined ? Are the group boundaries always marked by 0 in the first column? If so is the value in the second column of that row to be included or excluded from the calculations? Would you not also be wanting to record the boundaries of the groups found?

[Dana]

Each column must be analyzed independently, they are not connected. The boundary between groups is the change of sign. In the example, the first column would have two groups: [0;-3;-2] and [0;4;1]. The aim is find the minimum of the negative values and the maximum of the positive ones. The first group is a negative one and the minimum of them is -3. The second group is positive and the maximum is 4. The answer for first column is therefore [-3;4]. For the second column there are four groups:[1;3] [-7] [5] and [-6;-2]. The answer for the second column would be then [3;-7;5;-6]. And yes, I would like to record the boundaries of the groups also... Thanks

Answer 1

[Andrei Bobrov]

a=[0 1;-3 3;-2 -7;0 5;4 -6;1 -2]

t = [true(1,size(a,2));diff(a<0)~=0];
ir = cumsum(t);
jj = max(ir);
ii = bsxfun(@plus,ir,[0,jj(1:end-1)]);
out1 = mat2cell(accumarray(ii(:),a(:),[],@(x)max(abs(x))*sign(sum(x))),jj,1);
out = cellfun(@(x)x(x~=0),out1,'un',0);

[Dana]

Great! The only problem comes when there are Nans in the source data but it works for the rest of cases! I've got another solution but it's really inefficient...problems of being new with matlab! Thanks a lot!

Answer 2

[Joseph Cheng]

Why not break it up into the groups and handle them in a for loop? you can search for groups with groupstart = find(a(:,1)==0)

groupstart = find(a(:,1)==0)

after that you can use a for loop that does the min and max for a(groupstart(i):groupstart(i+1)-1,:)

[Dana]

Thanks for that but the problem with this comes when there is a change of sign with no zero to be found by the find command.