Summarize with conditions in dplyr

问题

I'll illustrate my question with an example.

Sample data:

 df <- data.frame(ID = c(1, 1, 2, 2, 3, 5), A = c("foo", "bar", "foo", "foo", "bar", "bar"), B =     c(1, 5, 7, 23, 54, 202))

df
  ID   A   B
1  1 foo   1
2  1 bar   5
3  2 foo   7
4  2 foo  23
5  3 bar  54
6  5 bar 202

What I want to do is to summarize, by ID, the sum of B and the sum of B when A is "foo". I can do this in a couple steps like:

require(magrittr)
require(dplyr)

df1 <- df %>%
  group_by(ID) %>%
  summarize(sumB = sum(B))

df2 <- df %>%
  filter(A == "foo") %>%
  group_by(ID) %>%
  summarize(sumBfoo = sum(B))

left_join(df1, df2)

  ID sumB sumBfoo
1  1    6       1
2  2   30      30
3  3   54      NA
4  5  202      NA

However, I'm looking for a more elegant/faster way, as I'm dealing with 10gb+ of out-of-memory data in sqlite.

require(sqldf)
my_db <- src_sqlite("my_db.sqlite3", create = T)
df_sqlite <- copy_to(my_db, df)

I thought of using mutate to define a new Bfoo column:

df_sqlite %>%
  mutate(Bfoo = ifelse(A=="foo", B, 0))

Unfortunately, this doesn't work on the database end of things.

Error in sqliteExecStatement(conn, statement, ...) : 
  RS-DBI driver: (error in statement: no such function: IFELSE)

回答1:

You can do both sums in a single dplyr statement:

df1 <- df %>%
  group_by(ID) %>%
  summarize(sumB = sum(B),
            sumBfoo = sum(B[A=="foo"]))

回答2:

Writing up @hadley's comment as an answer

df_sqlite %>%
  group_by(ID) %>%
  mutate(Bfoo = if(A=="foo") B else 0) %>%
  summarize(sumB = sum(B),
            sumBfoo = sum(Bfoo)) %>%
  collect

回答3:

If you want to do counting instead of summarizing, then the answer is somewhat different. The change in code is small, especially in the conditional counting part.

df1 <- df %>%
    group_by(ID) %>%
    summarize(countB = n(),
              countBfoo = sum(A=="foo"))

df1
Source: local data frame [4 x 3]

  ID countB countBfoo
1  1      2         1
2  2      2         2
3  3      1         0
4  5      1         0

回答4:

If you wanted to count the rows, instead of summing them, can you pass a variable to the function:

    df1 <- df %>%
group_by(ID) %>%
summarize(RowCountB = n(),
          RowCountBfoo = n(A=="foo"))

I get an error both with n() and nrow().

来源：https://stackoverflow.com/questions/23528862/summarize-with-conditions-in-dplyr

标签

dplyr

sqldf