When and how to use "." , ".$"and ".x", ".y" (pipe)

[Rsky]

Map + write_csv

Let's say I'm using the gapminder dataset. I'm not, but my problem is very similar. What I want to do is to split file and write multiple csv files (one for each country). But I don't know how to handle the different names. I would like to name each csv file with the name of the country: Afghanistan.csv Albania.csv Algeria.csv .... I tried the following, but I got multiple errors: gapminder %>% nest(-country) %>% map(data,write_csv)

gapminder %>%
    nest(-country) %>%
    pwalk(~write_csv(x = .y, path = paste0(.x, ".csv") ) )

I've been using the tidyverse and using the dot symbol sensibly.
I had been passing lists, columns, etc. I'm not sure why it was working, but I was getting over it by trial and error anyway.

However, when I looked at this code, it specifies the columns by x,y.
This code is . . $data or . $country will not work.

How can I figure out how to use the dot symbol differently?

When "." and When should I use ".$col" and when should I use ".x"?

I'd like to know your advice and references.

thank you

[nirgrahamuk]

It's for passing more than one param into a function

Map over two inputs — map2

[Rsky]

@Rsky

gapminder %>%
    nest(-country) %>%
    pwalk(~write_csv(x = .y, path = paste0(.x, ".csv") ) )

gapminder %>%
    nest(-country) %>%
    pwalk(~write_csv(x = .$data, path = paste0(.$country, ".csv") ) )

gapminder %>%
  group_by(country) %>% 
  nest()%>%
  pwalk(list(.x = .$country,.y=.$data),~write_csv(x = .y,file= paste0(.x, ".csv") ) )

Thanks for the link.
I've just read it.
I couldn't figure out when and which symbols to use to be successful.
why can't Success above code?

The code below will succeed

data.frame(mean=0,sd=1) %>% 
map2(.x=.$mean,.y=.$sd,rnorm(.x,.y),n=10)

[sturu]

I think your confusion is because of how pwalk (& pmap etc) is passing the data into the function, especially after the nest call. They all pass a single list into the .f argument, unlike map2 etc, so when you use a ~ function the .f arguments for the two are:

function(..., .x = ..1, .y = ..2, . = ..1) {
    # do something here
}

You're also currently trying to use the '.' to reference the data frame within the called function (after the ~) which won't work as you now have a different context - in a ~ function the '.' now refers to the first argument, which walks across the unique values of 'country' in your pwalk example. You could learn more about the new context by running:

data.frame(mean=c(0),sd=c(1)) %>% 
map2(.x=.$mean,.y=.$sd,~{ 
                   browser()
            }, n=10)

and

gapminder %>%
            group_by(country) %>%
            nest() %>%
            pwalk(~{ 
                   browser()
            })

and these will probably work:

gapminder %>%
            group_by(country) %>%
            nest() %>%
            pwalk(~write_csv(list(...)$data, paste0(list(...)$country, ".csv")))

gapminder %>%
            group_by(country) %>%
            nest() %>%
            pwalk(~write_csv(tibble(...)$data, paste0(tibble(...)$country[1], ".csv")))

gapminder %>%
            group_by(country) %>%
            nest() %>%
            pwalk(~write_csv(.y$data, paste0(.x, ".csv")))

gapminder %>%
            group_by(country) %>%
            nest() %>%
            pwalk(~write_csv(..2$data, paste0(..1, ".csv")))

gapminder %>%
    group_by(country) %>%
    nest() %>%
    select(data, country) %>%
    pwalk(~write_csv(x = . , path = paste0(.y, ".csv") ) 

gapminder %>%
            group_by(country) %>%
            nest() %>%
            pwalk(function(country, data, ...) {
                             write_csv(data, paste0(country, ".csv"))
                         })

I have not tested these, & they may not all work, but it'll give you some idea how to troubleshoot these things in future. Hope it helps.

Edit: now tested & errors fixed!