R: Repeating row of dataframe with respect to multiple count columns - r

Question

I have a R DataFrame that has a structure similar to the following:
df <- data.frame(var1 = c(1, 1), var2 = c(0, 2), var3 = c(3, 0), f1 = c('a', 'b'), f2=c('c', 'd') )
So visually the DataFrame would look like
> df
var1 var2 var3 f1 f2
1 1 0 3 a c
2 1 2 0 b d
What I want to do is the following:
(1) Treat the first C=3 columns as counts for three different classes. (C is the number of classes, given as an input variable.) Add a new column called "class".
(2) For each row, duplicate the last two entries of the row according to the count of each class (separately); and append the class number to the new "class" column.
For example, the output for the above dataset would be
> df_updated
f1 f2 class
1 a c 1
2 a c 3
3 a c 3
4 a c 3
5 b d 1
6 b d 2
7 b d 2
where row (a c) is duplicated 4 times, 1 time with respect to class 1, and 3 times with respect to class 3; row (b d) is duplicated 3 times, 1 time with respect to class 1 and 2 times with respect to class 2.
I tried looking at previous posts on duplicating rows based on counts (e.g. this link), and I could not figure out how to adapt the solutions there to multiple count columns (and also appending another class column).
Also, my actual dataset has many more rows and classes (say 1000 rows and 20 classes), so ideally I want a solution that is as efficient as possible.
I wonder if anyone can help me on this. Thanks in advance.

Answer 1

Here is a tidyverse option. We can use uncount from tidyr to duplicate the rows according to the count in value (i.e., from the var columns) after pivoting to long format.
library(tidyverse)
df %>%
pivot_longer(starts_with("var"), names_to = "class") %>%
filter(value != 0) %>%
uncount(value) %>%
mutate(class = str_extract(class, "\\d+"))
Output
f1 f2 class
<chr> <chr> <chr>
1 a c 1
2 a c 3
3 a c 3
4 a c 3
5 b d 1
6 b d 2
7 b d 2
Another slight variation is to use expandrows from splitstackshape in conjunction with tidyverse.
library(splitstackshape)
df %>%
pivot_longer(starts_with("var"), names_to = "class") %>%
filter(value != 0) %>%
expandRows("value") %>%
mutate(class = str_extract(class, "\\d+"))

Answer 2

base R
Row order (and row names) notwithstanding:
tmp <- subset(reshape2::melt(df, id.vars = c("f1","f2"), value.name = "class"), class > 0, select = -variable)
tmp[rep(seq_along(tmp$class), times = tmp$class),]
# f1 f2 class
# 1 a c 1
# 2 b d 1
# 4 b d 2
# 4.1 b d 2
# 5 a c 3
# 5.1 a c 3
# 5.2 a c 3
dplyr
library(dplyr)
# library(tidyr) # pivot_longer
df %>%
pivot_longer(-c(f1, f2), values_to = "class") %>%
dplyr::filter(class > 0) %>%
select(-name) %>%
slice(rep(row_number(), times = class))
# # A tibble: 7 x 3
# f1 f2 class
# <chr> <chr> <dbl>
# 1 a c 1
# 2 a c 3
# 3 a c 3
# 4 a c 3
# 5 b d 1
# 6 b d 2
# 7 b d 2