Question

I was trying to get the min values of a matrix before the max values of the matrix occurred. I have two matrices: matrix data and matrix a. Matrix a is a subset of matrix data and is composed of the max values of matrix data. I have the following code but obviously doing something wrong.

edit:

Matrix a are the max values of matrix data. I derived it from:

for x=1:size(data,1)
a(x)=max(data(x,:));
end
a=a'
clear x 

matrix b code:

for x=1:size(data,1)
b(x)=min(data(x,(x<data==a)));
end
b=b'
clear x

matrix data   matrix a   matrix b
1  2   3  4      4        1
6  5   4  7      7        4
9  6  12  5     12        6

I need all the min values that occurred before to matrix a occurred in matrix data

Answer 1

Short and simple:

[a,idxmax] = max(data,[],2);
b = arrayfun(@(ii) min(data(ii,1:idxmax(ii))), 1:size(data,1));

which is the same as

b=NaN(1,size(data,1)); % preallocation!
for ii=1:size(data,1)
    b(ii) = min(data(ii,1:idxmax(ii)));
end

Ignore maximum itself

If you want minimum of everything really before (and not including the maximum), it’s possible that the maximum is the first number, and you try taking minimum of an empty matrix. Solution then is to use cell output, which can be empty:

b = arrayfun(@(ii) min(data(ii,1:idxmax(ii)-1)), 1:size(data,1),'uni',false);

Replace empty cells with NaN

If you want to replace empty cells to Nan and then back to a matrix use this:

b(cellfun(@isempty,b))={NaN};
b=cell2mat(b);

or simply use the earlier version and replace b(ii) with NaN when it is equal to a(ii) same outcome:

b = arrayfun(@(ii) min(data(ii,1:idxmax(ii))), 1:size(data,1));
b(b'==a) = NaN

Example:

data=magic(4)

    16     2     3    13
     5    11    10     8
     9     7     6    12
     4    14    15     1

outputs:

a' = 16    11    12    15

b =
    16     5     6     4

and

b =[1x0 double]    [5]    [6]    [4]

for the 2nd solution using cell output and ignoring the maximum itself also.

And btw:

for x=1:size(data,1)
    a(x)=max(data(x,:));
end
a=a'
clear x

can be replaced with

a=max(data,[],2);