R - Number of times an item from one list appears in another list

Question

I have two lists of patient ID's. One is short, the other is very long.

I'd like to know how many times patients from the short list appear in the long list, preferably using dplyr.

library(tidyr)
library(tidyverse)

shortlist<-tribble(
  ~PatientID, 
  #--
  10,
  11,
  12,
  13,
  14,
  15
)

longlist<-tribble(
  ~PatientID,
  #--
  10,
  10,
  11,
  12,
  12,
  12,
  13,
  14,
  15,
  15,
  15,
  16,
  17
)

Desired output would be something like:

10 2

11 1

12 3

13 1

and so on

Answer 1

We filter the 'PatientID' based on the 'shortlist' and get the count

library(dplyr)
longlist %>% 
  filter(PatientID %in% shortlist$PatientID) %>% 
  count(PatientID)

-output

# A tibble: 6 × 2
  PatientID     n
      <dbl> <int>
1        10     2
2        11     1
3        12     3
4        13     1
5        14     1
6        15     3

Answer 2

shortlist %>% 
  group_by(PatientID) %>% 
  mutate(count = sum(longlist$PatientID == PatientID))

# A tibble: 6 × 2
# Groups:   PatientID [6]
  PatientID count
      <dbl> <int>
1        10     2
2        11     1
3        12     3
4        13     1
5        14     1
6        15     3

Answer 3

Another possible solution:

library(tidyverse)

table(longlist) %>% 
  stack %>% 
  mutate(PatientID = levels(ind) %>% as.numeric, ind = NULL) %>% 
  left_join(shortlist, ., by = "PatientID")

#> # A tibble: 6 × 2
#>   PatientID values
#>       <dbl>  <int>
#> 1        10      2
#> 2        11      1
#> 3        12      3
#> 4        13      1
#> 5        14      1
#> 6        15      3