# Finding maximums in matrix columns

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Christopher on 16 Nov 2013
Commented: Azzi Abdelmalek on 16 Nov 2013
Hello all,
I have a little bit of a tricky problem that I am trying to solve. I am still rather new to Matlab, so I am a little bit stuck on how to approach this problem. I will try to explain with a simple example. I have a matrix and I am trying to find the maximums of the 2nd and 3rd columns between every two values of the last column. Here is an example:
Matrix=[1 2;2 2;1 2;3 3;2 3;2 3;2 4;3 4;4 4]
I then want to find the maximums for column 1 for every i and i+1, based off the last column. So the first maximum would be found for 2:3 of the last column, which would be the first 6 rows. The first maximum would then be 3.
The next maximum would then be between 3:4 based off the values in the last column. The maximum would then be 4. I would also like to find the indices of these maximums that way I can pull the location of the maximum values.
I hope this makes sense, it's rather hard to explain.

Azzi Abdelmalek on 16 Nov 2013
Edited: Azzi Abdelmalek on 16 Nov 2013
Edit
n=2; % first column
m=7 % last column
c1=Matrix(:,n);
c2=Matrix(:,m);
k=1;
max_val=[];
idx_val=[];
index=[];
while k<numel(c2)
idx=find(c2==c2(k) | c2==c2(k)+1);
index{end+1}=idx;
a1=c1(idx);
[ii,jj]=max(a1);
max_val(end+1)=ii;
idx_val(end+1)=jj+k-1;
k=k+numel(idx);
end
disp(max_val)
disp(idx_val)

Christopher on 16 Nov 2013
How would I do this if I had a matrix that was say 1000 x 7. So it has 7 columns and I want to base it off of the last column and find the maximum values for column 2?
thank you
Azzi Abdelmalek on 16 Nov 2013
Look at Edit

Roger Stafford on 16 Nov 2013
Edited: Roger Stafford on 16 Nov 2013
Here's another way. Let A be your two-columned array. (Corrected)
f = find([true;diff(A(:,2))~=0;true]);
n = length(f)-2;
if n > 0
M = zeros(n,1);
I = zeros(n,1);
for k = 1:n
[mx,ix] = max(A(f(k):f(k+2)-1,1));
M(k) = mx;
I(k) = ix;
end
else
[M,I] = max(A(:,1));
end
M is a column vector of the maximum values and I their corresponding indices in A.