Any way to find which row of a matrix a value is in.

Hi,

So say I have an AxB matrix which is 100x100 containing data.

I need to perform various calculations on the values based upon which position each value is in.

So is there anyway I can easily find which row of the matrix I am currently in?

i.e. a function within MATLAB that will return which row of the matrix the current variable is in, so if I was currently looking at say a value in row 40, I could do something along the lines of function(matrix()), and it would return the value of 40?

Thanks.

[Guillaume]

There is no concept of current variable nor of a variable being in a row, column of a matrix. A variable is just a container for some value.

Similarly which row of the matrix I am currently in doesn't mean anything.

Maybe show a concrete example of what you want to do. For example, starting with

 mymatrix = randi(256, 100);

what sort of commands are you planning to use.

[David]

So I have a matrix that contains data, and I need to multiple each value within the matrix by a constant, yet this constant should vary based upon the current 'y position' in the matrix

i.e. the y position in the matrix represents a radius, and the constant that needs to be mutlipled to each value depends on said radius.

This matrix is created from a set of data loaded in from a separate file, not created within the code itself

Basically:

matrix=load(....)
newmatrix = constant * current y position within matrix * matrix()

Answer 1

[Star Strider]

The find function (with both row and column outputs) could do what you want.

[David]

How? I have looked at find and as far as I can tell it will be able to return various positions based upon if the values meet a certain condition (non zero, certain value etc), and not just return what the row and column positions of any given (if not all) value(s).

[Star Strider]

When you say ‘matrix’, do you mean ‘vector’? You will likely have to loop through your matrix (or vector) and keep track of where you are as you do your calculations. The find function will tell you the index positions of the values matching the one you are using.

Answer 2

[Guillaume]

So your constant is not actually constant.

You would have to either

a) use loops to perform your calculation on each column of the matrix

matrix=load(...)
newmatrix = zeros(size(matrix);
for col=1:size(matrix, 2)
   newmatrix(:, col) = matrix(:, col) * constant * y;
end

b) create a matrix where your constant is already multiplied by the column and use that to multiply your original value

matrix=load(...)
colmultiplier = constant * (1:size(matrix, 2);
matmultiplier = repmat(constant, size(matrix, 1), 1);
newmatrix = matrix .* matmultiplier;

c) nearly the same as b) but just create a vector of the constant multiplied by the column and use bsxfun to do the multiplication for each row

matrix=load(...)
colmultiplier = constant * (1:size(matrix, 2);
newmatrix = bsxfun(@times, matrix, colmultiplier);