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, test2_tp1, test2_tp2,...).

I am now trying to use dplyr to add a new column to a data frame that calculates the row wise mean over a selection of these columns (e.g., mean over all time points for test1).

1. I struggle even with the syntax for calculating the mean over explicitly named columns. What I tried without success was:

data %>% ... %>% mutate(test1_mean = mean(test1_tp1, test1_tp2, test1_tp3, na.rm = TRUE)

2. I would further like to use regex/wildcards to select the column names, so something like

data %>% ... %>% mutate(test1_mean = mean(matches("test1_.*"), na.rm = TRUE)

Answer 1

You can use starts_with inside select to find all columns starting with a certain string.

data %>%
  mutate(test1 = select(., starts_with("test1_")) %>%
           rowMeans(na.rm = TRUE))