R help - Sep 2020 - Function that finds elements from matrix, and saves the indices of the elements to another matrix

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_numbercolnames(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

	[[alternative HTML version deleted]]

Is this a homework problem? We try not to do others' homework here.

Incidentally, this can easily be done much more efficiently without any
for() loops.

Bert Gunter

"The trouble with having an open mind is that people keep coming along and
sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Mon, Sep 14, 2020 at 11:50 AM Tuomas Koponen <tuomas.o.koponen at
gmail.com>
wrote:
> 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
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
	[[alternative HTML version deleted]]

Hello,

This is simple:


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


Hope this helps,

Rui Barradas

?s 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
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>