Fill numeric variable while preserving group

Question

[EDITED to reflect a better example]

Say I have a dataframe like this:

df <- data.frame(x = c("A","A","B", "B"), year = c(2001,2004,2002,2005))

> df
  x year
1 A 2001
2 A 2004
3 B 2002
4 B 2005

How can I increment year by 1 while preserving x? I would like to fill in year so that the sequence is this:

  x year
1 A 2001
2 A 2002
3 A 2003
4 A 2004
5 B 2002
6 B 2003
7 B 2004
8 B 2005

Can anyone recommend a good way of doing this?

@useR recommend this approach:

> data.frame(year = min(df$year):max(df$year)) %>%
   full_join(df) %>%
   fill(x) 
Joining, by = "year"
  year x
1 2001 A
2 2002 B
3 2003 B
4 2004 A
5 2005 B

However that does not match the desired output.

Answer 1

An option using tidyr::complete and dplyr::lead can be as:

library(tidyverse)

df <- data.frame(x = LETTERS[1:3], year = c(2001,2004,2007))  

df %>% mutate(nextYear = ifelse(is.na(lead(year)),year, lead(year)-1)) %>%
  group_by(x) %>%
  complete(year = seq(year, nextYear, by=1)) %>% 
  select(-nextYear) %>%
  as.data.frame()

#   x year
# 1 A 2001
# 2 A 2002
# 3 A 2003
# 4 B 2004
# 5 B 2005
# 6 B 2006
# 7 C 2007

Edited: The solution for modified data

df <- data.frame(x = c("A","A","B", "B"), year = c(2001,2004,2002,2005))
library(tidyverse)
df %>%  group_by(x) %>%
  complete(year = seq(min(year), max(year), by=1)) %>% 
  as.data.frame()


#   x year
# 1 A 2001
# 2 A 2002
# 3 A 2003
# 4 A 2004
# 5 B 2002
# 6 B 2003
# 7 B 2004
# 8 B 2005

Answer 2

Using base R (with a little help from zoo):

full_df = data.frame(year = min(df$year):max(df$year))
df = merge(df, full_df, all = TRUE)
df = df[order(df$year), ]
df$x = zoo::na.locf(df$x)
df
#   year x
# 1 2001 A
# 2 2002 A
# 3 2003 A
# 4 2004 B
# 5 2005 B
# 6 2006 B
# 7 2007 C

Using the "tidyverse"

df <- data.frame(x = LETTERS[1:3], year = c(2001,2004,2007))
library(dplyr)
library(tidyr)
df = df %>% mutate(year = factor(year, levels = min(year):max(year))) %>%
    complete(year) %>%
    fill(x) %>%
    mutate(year = as.numeric(as.character(year)))
df
# # A tibble: 7 x 2
#    year      x
#   <dbl> <fctr>
# 1  2001      A
# 2  2002      A
# 3  2003      A
# 4  2004      B
# 5  2005      B
# 6  2006      B
# 7  2007      C