MATLAB: Lookup into array given a matrix (just replacing the values of matrix by vector)

fastest way to search in lookup tableimage processingImage Processing Toolboxlookup tablereplace the values of matrix by vector values

Hi everyone,

I have a very big matrix R of uint8. For every element (0-255) I have an other value in array A, where A is 256 length vector.

Now How can I replace each element in R by their lookup value in A. I want to do it as fast as it can.

Best Answer

The function you want to use is called intlut() and it's in the Image Processing Toolbox. It's fast and is just a single line of code.

new_R = intlut(R, A);

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See sub2ind.

index = sub2ind(size(R), A(:), B(:));
C = reshape(R(index), size(A));

MATLAB: How to join text in a table cell

Your whole process of adding a new column and detecting your excess threshold is overly complicated. There is certainly never any need to convert to a matrix.

%demo/test data
Table = array2table(randi([0 150], 69, 4), 'VariableNames', {'something', 'W', 'X', 'N'})
colnames = Table.Properties.VariableNames(2:4);  %asumming you only want to test column 2:4
warnings = rowfun(@(val) {strjoin(colnames(val > 100), ',')}, ...
                  Table, ...
                  'InputVariables', 2:4, ...
                  'OutputVariableNames', 'Warning', ...
                  'SeparateInputs', false);
Table = [warnings, Table]

I've separated the rowfun call onto several lines for readability.

In the above, I call rowfun to process each row one at a time. 'InputVariables' tells it which columns to process (2 to 4), 'OutputVariableNames' tells it which name to give to the output column, 'SeparateInputs', false tells it to keep the values of columns 2:4 together as a vector (easier to process).

The processing function I'm giving to rowfun is an anonymous function which one input val. That val will receive columns 2:4 of each row as a vector. I compare the values of this vector to your threshold. This gives me a logical vector that is used to filter the columns names. I then join these filtered column names together into one char array with strjoin and finally wrap that into a cell array so that it can be put into a table column.

P.S: Please give a better name to your table and to columns M, X, N. Something that actually explain what's in there. Yearlysomething would be a better name for the table, whatever that something is.

Best Answer

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MATLAB: Lookup table as a matrix instead of an array

MATLAB: How to join text in a table cell

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