Make a function to filter and summarize using R

Question

I have these two tables;

   <A>                       <B>
a1    a2                     b1   
ABC   CAFE                   AB
ABD   DRINK                  BF
ABF   CAFE                   ..
ABFF  DRINK
..     ..

I would like to know the summarize table containing B to a1 in table A like this;

library(dplyr)
library(stringr)

A1 <- A %>%
filter(str_detect(a1, "AB")) %>%
group_by(a2) %>%
summarize(n())

A2 <- A %>%
filter(str_detect(a1, "BF")) %>%
group_by(a2) %>%
summarize(n())

However, I should make the code several times so that I would like to a function to input the B table in the str_detect function... How do I make the function?

Answer 1

Here I designed a function called count_fun, which has four arguments. dat is a data frame like A, Scol is a column with strings, Gcol is the grouping column, and String is the test string. See https://cran.r-project.org/web/packages/dplyr/vignettes/programming.html to learn how to design a function using dplyr.

library(dplyr)
library(stringr)

count_fun <- function(dat, Scol, Gcol, String){

  Scol <- enquo(Scol)
  Gcol <- enquo(Gcol)

  dat2 <- dat %>%
    filter(str_detect(!!Scol, String)) %>%
    group_by(!!Gcol) %>%
    summarize(n())
  return(dat2)
}

count_fun(A, a1, a2, "AB")
# # A tibble: 2 x 2
#   a2    `n()`
#   <chr> <int>
# 1 CAFE      2
# 2 DRINK     2

count_fun(A, a1, a2, "BF")
# # A tibble: 2 x 2
#   a2    `n()`
#   <chr> <int>
# 1 CAFE      1
# 2 DRINK     1

We can then apply count_fun using lapply to loop through every elements in B.

lapply(B$b1, function(x){
  count_fun(A, a1, a2, x)
})

# [[1]]
# # A tibble: 2 x 2
#   a2    `n()`
#   <chr> <int>
# 1 CAFE      2
# 2 DRINK     2
# 
# [[2]]
# # A tibble: 2 x 2
#   a2    `n()`
#   <chr> <int>
# 1 CAFE      1
# 2 DRINK     1

DATA

A <- read.table(text = "a1    a2
ABC   CAFE
                ABD   DRINK 
                ABF   CAFE
                ABFF  DRINK
                ",
                header = TRUE, stringsAsFactors = FALSE)

B <- data.frame(b1 = c("AB", "BF"), stringsAsFactors = FALSE)