How to feed a list of unquoted column names into `lapply` (so that I can use it with a `dplyr` function)

Question

I am trying to write a function in tidyverse/dplyr that I want to eventually use with lapply (or map). (I had been working on it to answer

Answer 1

as.name will convert a string to a name and that can be passed to report:

lapply(cat.list, function(x) do.call("report", list(as.name(x))))

character argument An alternative is to rewrite report so that it accepts a character string argument:

report_ch <- function(colname) {  
    report_cat <- rlang::sym(colname)   # as.name(colname) would also work here
    sample_data %>%
                group_by(!!report_cat, YEAR) %>%
                summarize(num = n(), total = sum(AMOUNT)) %>% 
                rename(REPORT_VALUE = !!report_cat) %>% 
                mutate(REPORT_CATEGORY = colname)
}

lapply(cat.list, report_ch)

wrapr An alternate approach is to rewrite report using the wrapr package which is an alternative to rlang/tidyeval:

library(dplyr)
library(wrapr)

report_wrapr <- function(colname) 
  let(c(COLNAME = colname),
      sample_data %>%
                  group_by(COLNAME, YEAR) %>%
                  summarize(num = n(), total = sum(AMOUNT)) %>%
                  rename(REPORT_VALUE = COLNAME) %>%
                  mutate(REPORT_CATEGORY = colname)
   )

lapply(cat.list, report_wrapr)

Of course, this whole problem would go away if you used a different framework, e.g.

plyr

library(plyr)

report_plyr <- function(colname)
  ddply(sample_data, c(REPORT_VALUE = colname, "YEAR"), function(x)
     data.frame(num = nrow(x), total = sum(x$AMOUNT), REPORT_CATEOGRY = colname))

lapply(cat.list, report_plyr)

sqldf

library(sqldf)

report_sql <- function(colname, envir = parent.frame(), ...)
  fn$sqldf("select [$colname] REPORT_VALUE,
                   YEAR,
                   count(*) num,
                   sum(AMOUNT) total,
                   '$colname' REPORT_CATEGORY
            from sample_data
            group by [$colname], YEAR", envir = envir, ...)

lapply(cat.list, report_sql)              

base - by

report_base_by <- function(colname)
      do.call("rbind", 
        by(sample_data, sample_data[c(colname, "YEAR")], function(x)
            data.frame(REPORT_VALUE = x[1, colname], 
                       YEAR = x$YEAR[1], 
                       num = nrow(x), 
                       total = sum(x$AMOUNT), 
                       REPORT_CATEGORY = colname)
         )
      )

lapply(cat.list, report_base_by)

data.table The data.table package provides another alternative but that has already been covered by another answer.

Update: Added additional alternatives.

Answer 2

Let me first point out that in your initial report function, you can use quo_name to convert the quosure into a string, which you can then use in mutate like the following:

library(dplyr)
library(rlang)

report <- function(report_cat){
  report_cat <- enquo(report_cat)

  sample_data %>%
    group_by(!!report_cat, YEAR) %>%
    summarize(num=n(),total=sum(AMOUNT)) %>%
    rename(REPORT_VALUE = !!report_cat) %>%
    mutate(REPORT_CATEGORY = quo_name(report_cat))
}

report(REPORT_CODE)

Now, to address your question of "how to feed a list of unquoted strings through lapply or map to make it work inside dplyr functions", I propose two ways of doing it.

1. Use rlang::sym to parse your strings and unquote it when feeding into lapply or map

library(purrr)

cat.list <- c("REPORT_CODE","PAYMENT_METHOD","INBOUND_CHANNEL","AMOUNT_CAT")

map_df(cat.list, ~report(!!sym(.)))    

or with syms you can parse all elements of a vector at once:

map_df(syms(cat.list), ~report(!!.))

Result:

# A tibble: 27 x 5
# Groups:   REPORT_VALUE [16]
   REPORT_VALUE  YEAR   num total REPORT_CATEGORY
          <chr> <chr> <int> <int>           <chr>
 1            J  FY14     1    25     REPORT_CODE
 2            Q  FY16     1     1     REPORT_CODE
 3            Q  FY17     1   100     REPORT_CODE
 4            R  FY17     1    50     REPORT_CODE
 5            R  FY18     2    75     REPORT_CODE
 6            S  FY17     2   400     REPORT_CODE
 7            S  FY18     2   530     REPORT_CODE
 8        Check  FY14     1    25  PAYMENT_METHOD
 9        Check  FY17     1    50  PAYMENT_METHOD
10        Check  FY18     2    55  PAYMENT_METHOD
# ... with 17 more rows 

2. Rewrite your report function by placing lapply or map inside so that report can do NSE

report <- function(...){
  report_cat <- quos(...)

  map_df(report_cat, function(x) sample_data %>%
             group_by(!!x, YEAR) %>%
             summarize(num=n(),total=sum(AMOUNT)) %>%
             rename(REPORT_VALUE = !!x) %>%
             mutate(REPORT_CATEGORY = quo_name(x)))
}

By placing map_df inside report, you can take advantage of quos, which converts ... to list of quosures. They are then fed into map_df and unquoted one by one using !!.

report(REPORT_CODE, PAYMENT_METHOD, INBOUND_CHANNEL, AMOUNT_CAT)

Another advantage of writing it like this is that you can also supply a vector of string symbols and splice them using !!! like the following:

report(!!!syms(cat.list))

Result:

# A tibble: 27 x 5
# Groups:   REPORT_VALUE [16]
   REPORT_VALUE  YEAR   num total REPORT_CATEGORY
          <chr> <chr> <int> <int>           <chr>
 1            J  FY14     1    25     REPORT_CODE
 2            Q  FY16     1     1     REPORT_CODE
 3            Q  FY17     1   100     REPORT_CODE
 4            R  FY17     1    50     REPORT_CODE
 5            R  FY18     2    75     REPORT_CODE
 6            S  FY17     2   400     REPORT_CODE
 7            S  FY18     2   530     REPORT_CODE
 8        Check  FY14     1    25  PAYMENT_METHOD
 9        Check  FY17     1    50  PAYMENT_METHOD
10        Check  FY18     2    55  PAYMENT_METHOD
# ... with 17 more rows

Answer 3

I'm not really a dplyr afficionado, but for what its worth here is how you could achieve this using library(data.table) instead:

setDT(sample_data)

gen_report <- function(report_cat){
  sample_data[ , .(num = .N, total = sum(AMOUNT), REPORT_CATEGORY = report_cat), 
               by = .(REPORT_VALUE = get(report_cat), YEAR)] 
}

gen_report('REPORT_CODE')
lapply(cat.list, gen_report)