Rowwise median for multiple columns using dplyr

Question

Given the following dataset, I want to compute for each row the median of the columns M1,M2 and M3. I am looking for a solution where the final column is added to the dataframe under the name 'Median'. The column names (M1:M3) should not be used directly (in the original dataset, there are many more columns, not just 3).

# A tibble: 8 x 5
 I1    M1    M2    I2    M3
<int> <int> <int> <int> <int>
1     3     4     5     3     5
2     2     2     2     2     1
3     2     2     2     2     2
4     3     1     3     3     1
5     2     1     3     3     1
6     3     2     4     4     3
7     3     1     3     4     1
8     2     1     3     2     3

You can load the dataset using:

df = structure(list(I1 = c(3L, 2L, 2L, 3L, 2L, 3L, 3L, 2L), M1 = c(4L, 
2L, 2L, 1L, 1L, 2L, 1L, 1L), M2 = c(5L, 2L, 2L, 3L, 3L, 4L, 3L, 
3L), I2 = c(3L, 2L, 2L, 3L, 3L, 4L, 4L, 2L), M3 = c(5L, 1L, 2L, 
1L, 1L, 3L, 1L, 3L)), class = c("tbl_df", "tbl", "data.frame"
), row.names = c(NA, -8L), .Names = c("I1", "M1", "M2", "I2", 
"M3"))

I know that several similar questions have already been asked. However, most solutions posted use rowMeans or rowSums. I'm looking for a solution where:

1. no 'row-function' can be used.
2. the solution is a simple dplyr solution

The reason for (2) is that I am teaching the 'tidyverse' to total beginners.

Answer 1

We could use rowMedians

library(matrixStats)
library(dplyr)
df %>% 
    mutate(Median = rowMedians(as.matrix(.[grep('M\\d+', names(.))])))

Or if we need to use only tidyverse functions, convert it to 'long' format with gather, summarize by row and get the median of the 'value' column

df %>% 
    rownames_to_column('rn') %>%
    gather(key, value, starts_with('M')) %>%
    group_by(rn) %>% 
    summarise(Median = median(value)) %>%
    ungroup %>% 
    select(-rn) %>%
    bind_cols(df, .)

Or another option is rowwise() from dplyr (hope the row is not a problem)

df %>% 
   rowwise() %>% 
   mutate(Median =  median(c(!!! rlang::syms(grep('M', names(.), value=TRUE)))))