Cfrgtkky

The problem:

I have a list of matrices. I would like to convert all the rows, unless the last row, to zero. I tried the lapply function but it does not work as expected.

Example:

x <- matrix(3,4,4)

y <- matrix(5,4,5)

z <- list(x,y)

NewZ <- lapply(1:2, function(i) z[[i]][-nrow(z[[i]]), ] <- 0)

The lapply returns me this:

> NewZ

[[1]]

[1] 0



[[2]]

[1] 0

I would like to get matrices like this:

> z[[1]][-nrow(z[[1]]),] <- 0

> z[[1]]

     [,1] [,2] [,3] [,4]

[1,]    0    0    0    0

[2,]    0    0    0    0

[3,]    0    0    0    0

[4,]    3    3    3    3

> z[[2]][-nrow(z[[2]]),] <- 0



> z[[2]]

     [,1] [,2] [,3] [,4] [,5]

[1,]    0    0    0    0    0

[2,]    0    0    0    0    0

[3,]    0    0    0    0    0

[4,]    5    5    5    5    5

Where is my mistake? any idea, please?

You are using lapply() like a for loop. lapply loops through the list on its own, no need to use indexing:

NewZ <- lapply(z, function(x){

  x[-nrow(x),] <- 0

  return(x)

})



> NewZ

[[1]]

     [,1] [,2] [,3] [,4]

[1,]    0    0    0    0

[2,]    0    0    0    0

[3,]    0    0    0    0

[4,]    3    3    3    3



[[2]]

     [,1] [,2] [,3] [,4] [,5]

[1,]    0    0    0    0    0

[2,]    0    0    0    0    0

[3,]    0    0    0    0    0

[4,]    5    5    5    5    5

Also, when you change specific parts of a list element with lapply (meaning you are using the <- operator within the call), you need to return the element.