How to get list of matrices with rows of data frame as input?

Question

I have a data frame (2 columns, some rows) and a function that takes a vector of 2 numbers and builds a matrix out of it. My goal is to apply that function on each row of the data frame such that in the end I have a list of matrices. I tried the following

df <- data.frame(c(1, 2, 3), c(4, 5, 6))
gen_mat <- function(x) matrix(c(x[1], x[1] + 1, x[2], x[2] +1), nrow = 2)
bad <- apply(df, 1, gen_mat)

Unfortunately, bad is not a list but a 4 by 3 matrix where each column contains the elements of one of the 3 generated matrices.

Rui Barradas · Accepted Answer · 2017-09-02 15:27:47Z

1

Is this it?

bad2 <- lapply(as.data.frame(t(df)), gen_mat)
bad2

answered Sep 2, 2017 at 15:27

Rui Barradas

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2 Comments

Joe Over a year ago

Could you please elaborate on why your solution works and bad3 <- lapply(df, gen_mat) does not?

Rui Barradas Over a year ago

Because lapply is meant for objects of class list. Since a data.frameis a list where all elements, the columns, are of the same size, lapply will process one column at a time. Now, you need to process rows. So, transpose the df. But this makes of it a matrix. Then you have to coerce the matrix to data.frame.

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