Dan Lewer questions

• R: adding rows in a data frame depending on another variable
• Index a matrix using two equal-length vectors for row and column indices
• Is there a function in R that splits a matrix along a margin using a factor or categorical vector?
• R: calculating time-at-risk by age
• Is there an R function "parallel sum"? [duplicate]
• More efficient ways to use R than 'for' loops

Dan Lewer answers