How to expand a list in R

July 24, 2023

I am trying to change a list into a dataframe. Each element of the list may have an unequal length of values. An example list is given here:

a = c(1:9)
b = c(10:14)
test_list = list(a, b)
print(test_list)

#> [[1]]
#> [1] 1 2 3 4 5 6 7 8 9
#> 
#> [[2]]
#> [1] 10 11 12 13 14

And the desired result is given here:

desired_result = data.frame(var1 = c(rep(1,9), rep(2,5)),
                            var2 = c(1:14))
print(desired_result)

#>    var1 var2
#> 1     1    1
#> 2     1    2
#> 3     1    3
#> 4     1    4
#> 5     1    5
#> 6     1    6
#> 7     1    7
#> 8     1    8
#> 9     1    9
#> 10    2   10
#> 11    2   11
#> 12    2   12
#> 13    2   13
#> 14    2   14

^{Created on 2023-07-24 with reprex v2.0.2}

>Solution :

Give names to the list and then stack():

stack(setNames(test_list, seq_along(test_list)))[2:1]

#    ind values
# 1    1      1
# 2    1      2
# 3    1      3
# 4    1      4
# 5    1      5
# 6    1      6
# 7    1      7
# 8    1      8
# 9    1      9
# 10   2     10
# 11   2     11
# 12   2     12
# 13   2     13
# 14   2     14