R transform dates table by subsetting for top n most recent dates

Question

I've got a dataset that looks like this

#Determine the M, M-1 and M-2 values for every site

df = data.frame(Site=c(rep('x',5),rep('y',5),rep('z',5)),
                Date=c(as.Date('2020/09/11'),as.Date('2020/09/11'),
                       as.Date('2020/08/02'),as.Date('2020/07/09'),
                       as.Date('2020/05/15'),as.Date('2020/08/02'),
                       as.Date('2020/07/10'),as.Date('2020/07/19'),
                       as.Date('2020/06/27'),as.Date('2020/03/01'),
                       as.Date('2020/04/07'),as.Date('2020/01/01'),
                       as.Date('2019/11/10'),as.Date('2019/11/11'),
                       as.Date('2019/11/07')))

I'm trying to transform the dataset such that

1. I pick the top 3 most recent months from the Date column for each Site

2. Spread the Date column such that it is in 'M' 'M-1' and 'M-2' format

3. 'M' being most recent month, 'M-1' being second most recent month, 'M-2' being third most recent month

4. Convert Date from YYYY/MM/DD format to Month-Year format eg 2020/01/01 = JAN 2020

My attempt

For step 1.

transformed = df %>%
  group_by(Site) %>%
  arrange(desc(Date)) %>%
  slice(1:3)

This works fine up till here. I've been trying various ways of spreading the dates to the desired format, but I haven't succeeded. I'm a bit new to this so any inputs would help a lot.

Desired output

I'd like my desired output to look like this

#Output table

output = data.frame(Site=c('x','y','z'),
                     M=c('SEP 2020','AUG 2020','APR 2020'),
                    `M-1`=c('AUG 2020','JUL 2020','JAN 2020'),
                    `M-2`=c('JUL 2020','JUN 2020','NOV 2019'))

colnames(output)=c('Site','M','M-1','M-2')

Modification: September 21, 2020

I've slightly modified the original dataframe df to factor in cases where we have more than one date in a given month. In such a situation, how can the solutions you've proposed be modified to get the same output ?

Answer 1

Try:

df %>%
  group_by(Site) %>%
  arrange(desc(Date)) %>%
  slice(1:3) %>%
  mutate(
    id_within = str_c("M_", 1:3),
    Date = format(Date, "%b %Y")
  ) %>%
  pivot_wider(
    id_cols = Site,
    names_from = id_within,
    values_from = Date
  )

Answer 2

You can also try this solution which is close to what you want:

library(tidyverse)
#Code
df %>% 
  arrange(Site,desc(Date)) %>%
  group_by(Site) %>%
  mutate(Month=format(Date,'%b %Y')) %>%
  #Index for top month
  mutate(index=1:n()) %>%
  #Filter
  filter(index<=3) %>% 
  #Labels
  mutate(Label=paste0('M-',(1:n())-1)) %>%
  ungroup() %>%
  select(-c(Date,index)) %>%
  pivot_wider(names_from=Label,values_from=Month)

Output:

# A tibble: 3 x 4
  Site  `M-0`    `M-1`    `M-2`   
  <fct> <chr>    <chr>    <chr>   
1 x     Sep 2020 Aug 2020 Jul 2020
2 y     Aug 2020 Jul 2020 Jun 2020
3 z     Apr 2020 Jan 2020 Nov 2019

Update: You can try this alternative for you new data:

#Code 2
df %>% 
  arrange(Site,desc(Date)) %>%
  group_by(Site) %>%
  mutate(Month=format(Date,'%b %Y')) %>%
  #Remove duplicated dates
  filter(!duplicated(Month)) %>%
  #Index for top month
  mutate(index=1:n()) %>%
  #Filter
  filter(index<=3) %>% 
  #Labels
  mutate(Label=paste0('M-',(1:n())-1)) %>%
  ungroup() %>%
  select(-c(Date,index)) %>%
  pivot_wider(names_from=Label,values_from=Month)

Output:

# A tibble: 3 x 4
  Site  `M-0`    `M-1`    `M-2`   
  <fct> <chr>    <chr>    <chr>   
1 x     Sep 2020 Aug 2020 Jul 2020
2 y     Aug 2020 Jul 2020 Jun 2020
3 z     Apr 2020 Jan 2020 Nov 2019

Answer 3

Here is one way with dplyr :

library(dplyr)

df %>%
  #Get month and year information from date
  mutate(month = toupper(format(Date, '%b %Y'))) %>%
  #arrange data in descending order
  arrange(Site, desc(Date)) %>%
  #for each site
  group_by(Site) %>%
  #Create a unique id number
  mutate(id = match(month, unique(month))) %>%
  #select only 3 unique id's for each site
  filter(id <= 3) %>%
  #Create column name
  mutate(col = c('M', 'M-1', 'M-2')[id]) %>%
  #remove columns which are not required
  select(-Date, -id) %>%
  #get data in wide format
  tidyr::pivot_wider(names_from = col, values_from = month, values_fn = unique)

#   Site  M        `M-1`    `M-2`   
#  <chr> <chr>    <chr>    <chr>   
#1 x     SEP 2020 AUG 2020 JUL 2020
#2 y     AUG 2020 JUL 2020 JUN 2020
#3 z     APR 2020 JAN 2020 NOV 2019

Answer 4

Use slice_max :

library(tidyverse)

tibble(df) %>%
  group_by(Site) %>%
  slice_max(order_by = Date, n = 3) %>%
  mutate(Date = format(Date, '%b %Y') %>% toupper(),
         m = paste0("M-", 0:(n()-1)) %>% str_replace(., "-0", "")) %>%
  pivot_wider(names_from = m, values_from = Date) %>% ungroup()

Output:

# A tibble: 3 x 4
  Site  M        `M-1`    `M-2`   
  <fct> <chr>    <chr>    <chr>   
1 x     SEP 2020 AUG 2020 JUL 2020
2 y     AUG 2020 JUL 2020 JUN 2020
3 z     APR 2020 JAN 2020 NOV 2019