Possibility to know the index of the max() value

Hello there,

I have a problem where I need to keep the maximum value and its position. In a while cycle I have one matrix that is created randomly, and I have to keep the maximum sum of the row in each iteration.

For example, I have one matrix (3 rows by 4 columns) that is p.e matrix a =

 4  5  6  2

 8  5  7  1

 2  3  5  2

And another matrix that is (3 by 1) that keeps the sum of the matrix in some iteration p.e. matrix C=

 17

 21

 12

Then I have a vector that keeps the max of the matrix C in order to build a graph of the evolution of the maximum value.

aux_vector=max(C);

I would like to keep the line that corresponds to the max of C, in this case the second line of the matrix a.

Thanks in advance.

Answer 1

[Walter Roberson]

[aux_vector, aux_index] = max(C):
max_a = a(aux_index, :);

[André Pacheco]

how does this work in an iteration? aux_vector will have is values changed continuously? In my case i use aux_vector(1,iteration)=max(C); I didn't understand how to put it when using [aux_vector, aux_index]

Thanks

[Walter Roberson]

[aux_vector(iteration), aux_index] = max(C):
max_a(iteration,:) = a(aux_index, :);

[André Pacheco]

It worked! thanks!

Answer 2

[Image Analyst]

Try this code:

  sumOfRows = sum(a, 2) % Your "C"
  [maxValue, rowWithTheMax] = max(sumOfRows)

Here it is in a demo where we generate lots of random matrices "a" in a loop:

format compact;  
numberOfRuns = 5;
% Initialize a matrix to keep track of the rows having
% the max sum at each iteration.
maxRows = zeros(numberOfRuns, 4); % One row for each run.
for k = 1 : 5
  % Generate a matrix
  if k == 1
    % For the first run, use 
    % the example matrix the poster supplied.
    a = [...
     4  5  6  2
     8  5  7  1
     2  3  5  2]
  else
    % Subsequent runs use a random matrix.
    a = randi(9, [3 4])
  end

  sumOfRows = sum(a, 2) % Your "C"
  [maxValue, rowWithTheMax] = max(sumOfRows)
  % Store this row in an array that depends on the run.
  % In other words, store the 4 values from that row of "a".
  maxRows(k, :) = a(rowWithTheMax, :)
end

Hopefully it's commented well enough to explain what's going on.

[Image Analyst]

Actually I noticed that this is virtually the same as Walter's. I didn't realize at first because I didn't see sum() in his code.

[André Pacheco]

I've tried and it worked! Although i used the first answers but it is the same as u made. Thanks !