[R] Function that finds elements from matrix, and saves the indices of the elements to another matrix

[Rui Barradas]

Hello,

This is simple:


which(A == 1, arr.ind = TRUE)


Hope this helps,

Rui Barradas

Às 12:03 de 14/09/20, Tuomas Koponen escreveu:
> Hi all dear R-list users,
> 
> This might sound a silly problem, but but for one reason or another it has
> proved unsolvable to me.
> 
> I need to solve the following task. I have tried to use two nested for
> loops as a solution, but have not been able to made it work. It would be
> great, if someone could formulate a code for me:
> 
> I need to create a function X that takes as its input a 10x10 matrix, which
> elements consists of ones and zeros. The function X must find the elements
> in the matrix A that are "ones", and save the indices of those elements in
> 2-column matrix, in a following way: the first column has a row number, and
> the second a column number.
> 
> The answer should be formulated in a way, that it uses two nested for
> loops. The matrix to be returned has a maximum of 100 rows, so one
> possibility exists for a 100x2 matrix filled with zeros in the index and
> finally separate those rows with zeros.
> 
> I have tried the following code:
> 
> A<-matrix(1:0, nrow = 10, ncol = 10)
> 
> X <- data.frame()
> for (i in seq(1, nrow(A))) {
> for (j in seq(1,ncol(A))) {
> if (A [i,j] > 0) {
> 
> new_row <- data.frame(Row_number= rownames(A)[i], Column_number=
> colnames(A)[j])
> 
> X <- rbind(X, new_row)
> 
>    }
> 
>    }
> }
> 
> I only manage to get "Error: object 'X' not found", as a result.
> 
> thanks in advance for your help.
> 
> Br. Tuomas
> 
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