how to use gather() in a for loop in R - r

Question

I have a big data set of 72 columns and I want to gather each 3 of columns into a new column and thus getting 24 columns in the end.
I tried using gather() function but it works good for one time only t=i.e., it gather only 3 columns at a time.
Can I use this function in a for loop?
I tried this:
j=0
k=1
l=2
for (i in 2:24){
neww <- gather(columns, "KEy", "Proteins H/L", c((i+j), (i+k), (i+l)), na.rm = TRUE)
j=j+2;
k=k+2;
l=l+2;
}
I need to gather first 3 columns in a single column and then next 3 in another column and so on.

Answer 1

You can use the to_long function from the sjmisc-package for this purpose. This function is a convenient for-loop, which calls multiple gather() calls.
# create sample
mydat <- data.frame(age = c(20, 30, 40),
sex = c("Female", "Male", "Male"),
score_t1 = c(30, 35, 32),
score_t2 = c(33, 34, 37),
score_t3 = c(36, 35, 38),
speed_t1 = c(2, 3, 1),
speed_t2 = c(3, 4, 5),
speed_t3 = c(1, 8, 6))
# check tidyr. score is gathered, however, speed is not
tidyr::gather(mydat, "time", "score", score_t1, score_t2, score_t3)
> age sex speed_t1 speed_t2 speed_t3 time score
> 1 20 Female 2 3 1 score_t1 30
> 2 30 Male 3 4 8 score_t1 35
> 3 40 Male 1 5 6 score_t1 32
> 4 20 Female 2 3 1 score_t2 33
> 5 30 Male 3 4 8 score_t2 34
> 6 40 Male 1 5 6 score_t2 37
> 7 20 Female 2 3 1 score_t3 36
> 8 30 Male 3 4 8 score_t3 35
> 9 40 Male 1 5 6 score_t3 38
# gather multiple columns. both time and speed are gathered.
to_long(mydat, "time", c("score", "speed"),
c("score_t1", "score_t2", "score_t3"),
c("speed_t1", "speed_t2", "speed_t3"))
> age sex time score speed
> (dbl) (fctr) (chr) (dbl) (dbl)
> 1 20 Female score_t1 30 2
> 2 30 Male score_t1 35 3
> 3 40 Male score_t1 32 1
> 4 20 Female score_t2 33 3
> 5 30 Male score_t2 34 4
> 6 40 Male score_t2 37 5
> 7 20 Female score_t3 36 1
> 8 30 Male score_t3 35 8
> 9 40 Male score_t3 38 6
In this case, the time vector (indicating the gathered groups) just takes one of the multiple gathered column name. If this is too confusing, you can also just number the ID variable:
to_long(mydat, "time", c("score", "speed"),
c("score_t1", "score_t2", "score_t3"),
c("speed_t1", "speed_t2", "speed_t3"),
recode.key = TRUE)
> age sex time score speed
> (dbl) (fctr) (dbl) (dbl) (dbl)
> 1 20 Female 1 30 2
> 2 30 Male 1 35 3
> 3 40 Male 1 32 1
> 4 20 Female 2 33 3
> 5 30 Male 2 34 4
> 6 40 Male 2 37 5
> 7 20 Female 3 36 1
> 8 30 Male 3 35 8
> 9 40 Male 3 38 6
See ?to_long for more examples.
I'm not sure, but I think I read something on GitHub that "multiple column gathering" is also planned for tidyr somewhen...