Add rows by group and fill them with zero in R with dplyr

Question

I have a huge data frame that looks like this

gene=c("A","A","A","A","B","B")
frequency=c(abs(rnorm(6,0.5,1)))
time=c(1,2,3,4,1,2)
df <- data.frame(gene,frequency,time)

  gene  frequency time
1    A 0.08463914    1
2    A 1.55639512    2
3    A 1.24172246    3
4    A 0.75038980    4
5    B 1.13189855    1
6    B 0.56896895    2

For the gene B I have data only for the time points 1 and 2. I want to fill the data of time point 3 and 4 with zeros so as my data look like this

  gene  frequency time
1    A 0.08463914    1
2    A 1.55639512    2
3    A 1.24172246    3
4    A 0.75038980    4
5    B 1.13189855    1
6    B 0.56896895    2
7    B      0        3
8    B      0        4

Overall I have multiple groups (aka genes) that I want to do this for. Any help or hint are highly appreciated.

Answer 1

We can use complete

library(dplyr)
library(tidyr)
df %>% 
    complete(gene, time = 1:4, fill = list(frequency = 0)) %>%
    select(names(df))

-output

# A tibble: 8 x 3
  gene  frequency  time
  <chr>     <dbl> <dbl>
1 A         0.590     1
2 A         0.762     2
3 A         0.336     3
4 A         0.437     4
5 B         0.904     1
6 B         1.97      2
7 B         0         3
8 B         0         4

Answer 2

akrun's answer is best! Here is a way with pivoting: In essence: during the pivoting procedure NA were produced, these could be than replaced by 0:

library(tidyr)
library(dplyr)

df %>% 
    pivot_wider(
        names_from = gene,
        values_from = frequency
    ) %>% 
    pivot_longer(2:3, 
                 names_to = "gene",
                 values_to = "frequency") %>% 
    mutate(frequency = replace_na(frequency, 0)) %>% 
    arrange(gene, time) %>% 
    select(-time, time)

  gene  frequency  time
  <chr>     <dbl> <dbl>
1 A         1.00      1
2 A         0.413     2
3 A         0.539     3
4 A         1.08      4
5 B         0.473     1
6 B         1.79      2
7 B         0         3
8 B         0         4