如何将R apply函数的结果保存到矩阵中？

Question

Suppose I have a data set X, that has dimension 100 x 10. I want to apply a set of functions to each column, and save the results in a new data frame with the same column headers.

Example

M <- matrix(rexp(200, rate=.1), ncol=20)
M_df <- as.data.frame(M)
row1 <- apply(M_df,2,mean)
row2 <- apply(M_df,2,var)
row3 <- apply(M_df,2,sum)

I want to make a new matrix that is comprised of row1, row2, and row3 which has the same column headers as M_df.

Thanks! Max

Answer 1

Try this:

apply(M_df,2,function(x) c(mean = mean(x),var = var(x),sum = sum(x)))

Although, keep in mind that the first thing that apply does is convert M_df back to a matrix, so there's not much point in converting it to a data frame before you really have to.