How to unpivot specific columns in R dataframe

Question

I have a dataframe in R formatted as shown in the first table below. I would like to merge columns "M1.1," "M1.2," and "M1.3" into a single column "M1" so that entries are on their own rows (id and values in other columns would be repeated) as shown in the second table. What function(s) could I use to accomplish this?

id	M1.1	M1.2	M1.3	M2	M3	M4	M5	M6
test a	test t	test a	test y	test	test t	test y	test u	test w
test s	test r	test a	test h	test r	test j	test j	test w	test d

id	M1	M2	M3	M4	M5	M6
test a	test t	test	test t	test y	test u	test w
test a	test a	test	test t	test y	test u	test w
test a	test y	test	test t	test y	test u	test w
test s	test r	test r	test j	test j	test w	test d
test s	test a	test r	test j	test j	test w	test d
test s	test h	test r	test j	test j	test w	test d

Answer 1

We could use pivot_longer:

library(dplyr)
library(tidyr)
df %>% 
    pivot_longer(
        cols = c(M1.1, M1.2, M1.3),
        names_to = "names",
        values_to = "M1"
    ) %>% 
    select(id, M1, M2:M6)

 A tibble: 6 x 7
  id     M1     M2     M3     M4     M5     M6    
  <chr>  <chr>  <chr>  <chr>  <chr>  <chr>  <chr> 
1 test a test t test   test t test y test u test w
2 test a test a test   test t test y test u test w
3 test a test y test   test t test y test u test w
4 test s test r test r test j test j test w test d
5 test s test a test r test j test j test w test d
6 test s test h test r test j test j test w test d

data:

structure(list(id = c("test a", "test s"), M1.1 = c("test t", 
"test r"), M1.2 = c("test a", "test a"), M1.3 = c("test y", "test h"
), M2 = c("test", "test r"), M3 = c("test t", "test j"), M4 = c("test y", 
"test j"), M5 = c("test u", "test w"), M6 = c("test w", "test d"
)), row.names = c(NA, -2L), class = c("tbl_df", "tbl", "data.frame"
))