add column with row wise mean over selected columns using dplyr

Question

I have a data frame which contains several variables which got measured at different time points (e.g., test1_tp1, test1_tp2, test1_tp3

Answer 1

Not a dplyr solution, but you can try:

cols_2sum <- grepl('test1',colnames(data))
rowMeans(data[,cols_2sum])