subtract column1 (dataframe1) from column2 (dataframe2) based on matching column in both R

Question

Dataframe1 has two columns: num_movies and userId. Dataframe2 has two columns: No_movies and userId. But Dataframe2 has 2106 rows and Dataframe1 has 1679 rows. I want to subtract the number of movies in Dataframe2 from Dataframe1 based on matching userId values. I have written the following line:

df1$num_movies = df1$num_movies - df2$No_movies[df1$userId %in% df2$userId]

and I get the following error:

Error in `$<-.data.frame`(`*tmp*`, "num_movies", value = c(2, 9, 743,  : 
  replacement has 2106 rows, data has 1679
In addition: Warning message:
In df1$num_movies - df2$No_movies[df1$userId %in%  :
  longer object length is not a multiple of shorter object length

Elsewhere it has been proposed that I upgrade from 3.0.2 to 3.1.2 to solve this problem. But I still have the same error after the upgrade. What I have written seems logical for me. I intend to pick only 1679 userIds out of 2106. Why is it selecting all of them? How do I circumvent this error?

Answer 1

You can use the match function to find the corresponding row from Dataframe2 for each row in Dataframe1.

matched.movies <- Dataframe2$No_movies[match(Dataframe1$userId, Dataframe2$userId)]
matched.movies[is.na(matched.movies)] <- 0
Dataframe1$num_movies <- Dataframe1$num_movies - matched.movies
Dataframe1
#   num_movies userId
# 1         10      1
# 2          7      2
# 3          6      3

Data:

(Dataframe1 <- data.frame(num_movies=rep(10, 3), userId=1:3))
#   num_movies userId
# 1         10      1
# 2         10      2
# 3         10      3
(Dataframe2 <- data.frame(No_movies=2:6, userId=c(0, 2, 3, 9, 10)))
#   No_movies userId
# 1         2      0
# 2         3      2
# 3         4      3
# 4         5      9
# 5         6     10