Use elements from 2d matrix as row number in a function

Hello!!

I have a question about a function, I hope somebody could help me.

I have a 2d matrix --> MATR= (2160 rows, 4320 columns) --> MATR possible values: [1,2,3,4,5,6,7,8,9,10]

I have a function that must read the value in each MATR's cell, read numbers from a table where rows corresponds to MATR values, and assign new values to another 2d matrix.

EXAMPLE:

MATR(500,500)=7;

from row=7 of Table I read "18" --> new_MATR(500,500)=18.

All this must be done in a single step, avoiding loops or "if", for all the cells.

Thank you very much for your help

M

Answer 1

[Adam Danz]

If I understand correctly, you're describing a lookup table where you want to match the values in MATR with the values in column 1 of a table and return the values in column 2. If that's incorrect, please explain again.

Here's a demo.

% Create "fake" data for MATR and the lookup table T
MATR = randi(10,2160,4320);
T = table((1:10)', randi(20,10,1), 'VariableNames', {'A','B'}); 

% Match values from MATR to the first column of T (T.A)
% and return the values from the 2nd column (T.B).
[~, rowNum] =  ismember(MATR, T.A);
MATR2 = T.B(rowNum); 

[Matteo Rolle]

thank you for the help, but this is not my case...

I have a matrix of data (2160 rows, 4320 columns) and those data values can be integers between 1 and 10. This matrix is not generated, is downloaded data.

I mast read the value in each cell and create another matrix with the same dimension, where in each cell i want the value taken from an array of 10 rows.

EXAMPLE: original matrix -->M_data, result matrix-->M_results, array--->A (A dimensions: 10rows, 1colums)

suppose that in the i-cell of M_data the value is 6. I want that the function reads the value of the 6-th element in A and assign this value to M_results, in the same position of the data from M_data

[Adam Danz]

Please provide an example because it's still not clear.

How is your goal different from my answer?

[Stephen23]

Matteo Rolle's "Answer" moved here:

I'm sorry, it was exactly what I needed!! Thank you very much

I only have a problem with the last command (MATR2 = T.B(rowNum); )... this is the error message in the command:

Array indices must be positive integers or logical values.

Error in tabular/dotParenReference (line 108)

b = b(rowIndices);

[Adam Danz]

That means one of the values in MATR is not listed in T.A (variable names from my answer). The ismember function returns a 0 for the unmatched values and 0 cannot be used as a row index.

There are two ways to fix that. First, make sure T.A contains all possible vables in MATR. You could use the unique function to help you with that. Second, you could ignore the non-matches and return some default value such as NaN in their place.

[isMatch, rowNum] =  ismember(MATR, T.A);
MATR2 = nan(size(MATR));
MATR2(isMatch) = rowNum(isMatch);