MATLAB: Create a new matrix with 0,-1 and 1 if and where in another matrix appears a max value

if statement

Hello, I try to well explain my problem… I have an array A(200×14) in which all the rows show 14 values for each observation period. I need to create another matrix B in which, for each row, the values need to be 1 if it is the maximum value, -1 if it is the min and 0 otherwise. I try to explain better, for example, if in A, in the first row, the values are: [0.5 1 3 4.6 2.8 3.4 6.7 0.01 2.3 8.4 5.1 7.3 1.8 4.3] in matrix B the values should be [0 0 0 0 0 0 0 -1 0 1 0 0 0 0] and I have to make this for each row (considering that the values in the A rows are not the same for each row). Is it possible?Can someone explain me how? I have tried with if,elseif else…but I think there is something that does not work. THANKS

Best Answer

Sounds like homework, but hopefully it isn't and then you can use this. It's longer than Roger's and it's a "for loop" approach, but thought you might like to see several different approaches. You forgot to share your ifelse approach though.

m = rand(200, 14); % Create sample data.
B = zeros(size(m)); % Initialize output to all 0's.
for row = 1 : size(m, 1)
  % Find max location in this row.
  [~, indexOfMax] = max(m(row, :));
  % Set B = 1 there
  B(row, indexOfMax) = 1;
  % Find min location in this row.
  [~, indexOfMin] = min(m(row, :));
  % Set B = -1 there
  B(row, indexOfMin) = -1;
end

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MATLAB: Finding maximum number location in a matrix

"Is there a reason why this line is wrong?"

Yes, because you nested max inside find. Take a look at the output of max: what is it? (hint: it will be a row vector of the maximum value from each column of your matrix). As its description states, find returns the indices of non-zero elements, so it will return the indices of whichever of those vector values is non-zero (which could be none). Note that find does NOT know anything about your matrix A (or its size or indices etc), because its input is a row vector (the output of max).

One easy alternative is to get rid of the find entirely:

>> A = zeros(8,6);
>> A(8,1) = 1;
>> A(6,2) = 1;
>> A(6,3) = 1;
>> A(6,4) = 1;
>> A(7,5) = 1;
>> A(6,6) = 1
A =
     0     0     0     0     0     0
     0     0     0     0     0     0
     0     0     0     0     0     0
     0     0     0     0     0     0
     0     0     0     0     0     0
     0     1     1     1     0     1
     0     0     0     0     1     0
     1     0     0     0     0     0
>> [~,idr] = max(A,[],1)
idr =
     8     6     6     6     7     6
>> idc = 1:size(A,2)
idc =
     1     2     3     4     5     6

If you really must use find then probably the simplest solution is to generate a logical matrix as the input to find (note if the maximum values occurs more than once in one column this will return all of their indices, use cumsum to return only the first one):

>> [idr,idc] = find(bsxfun(@eq,A,max(A,[],1)))
idr =
     8
     6
     6
     6
     7
     6
idc =
     1
     2
     3
     4
     5
     6

MATLAB: How to find maxima and minima points of intensity plot of one row of an image

Don't use i (the imaginary variable) as an image name or loop counter.

Try this:

oneRow = grayImage(myRow, :);
% Find the min and max
[minGL, indexOfMin] = min(oneRow)
[maxGL, indexOfMax] = max(oneRow)

See my demo, test.m, attached below in blue, which will generate the following figure.

Best Answer

Related Solutions

MATLAB: Finding maximum number location in a matrix

MATLAB: How to find maxima and minima points of intensity plot of one row of an image

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