for multiple columns in data frame reshaping from wide to long in R

Question

I need to reshape my data (a subset as below) from wide to long format. I have tried reshape() and pivot_longer(), but I could not make what I need. I have added the part of output of long format I would like to make. I really appreciate your help.

structure(list(id = c(1, 2, 3, 4), g6m = c("12", "11", "16.9", 
"17.1"), g12m = c("18", "11", "11.1", "19.5"), g18m = c("29", 
"11.6", "12.3", "17"), g24m = c("12", "13.6", "10", "11"), age = c(45, 
37, 51, 47), gender = c(0, 1, 0, 1), ms = c("11.136", "12.18", 
"19.01", "10.01")), class = c("tbl_df", "tbl", "data.frame"), row.names = c(NA, 
-4L))

Answer 1

In principle same as @Allan Cameron. Just adapted to your desired output:

library(tidyr)
library(dplyr)
df %>% 
  pivot_longer(
    cols = -c(id, age, gender, ms),
    names_to = "x",
    values_to = "g"
  ) %>% 
  group_by(id) %>% 
  mutate(time = row_number(), .before=2) %>% 
  select(id, time, g, age, gender, ms)

    id  time g       age gender ms    
   <dbl> <int> <chr> <dbl>  <dbl> <chr> 
 1     1     1 12       45      0 11.136
 2     1     2 18       45      0 11.136
 3     1     3 29       45      0 11.136
 4     1     4 12       45      0 11.136
 5     2     1 11       37      1 12.18 
 6     2     2 11       37      1 12.18 
 7     2     3 11.6     37      1 12.18 
 8     2     4 13.6     37      1 12.18 
 9     3     1 16.9     51      0 19.01 
10     3     2 11.1     51      0 19.01 
11     3     3 12.3     51      0 19.01 
12     3     4 10       51      0 19.01 
13     4     1 17.1     47      1 10.01 
14     4     2 19.5     47      1 10.01 
15     4     3 17       47      1 10.01 
16     4     4 11       47      1 10.01