Iterate over x rates to scrap currency. #purrr

xrates.R

# Reference
# http://www.x-rates.com/average/?from=CNY&to=EUR&amount=100&year=2017

library(rvest)

# 
base <- "http://www.x-rates.com/average/?amount=100"

# Currency
cur <- c("USD", "CNY")
year_range <- 2017:2018

# Permutation of above currency
cur_df <- expand.grid(cur, cur) %>% 
    # remove same-currency pair
    filter(Var1 != Var2) %>% 
    # make into data frame 
    mutate(url = paste0(base, "&from=", Var1, "&to=", Var2, "&year="))
head(cur_df)

# Iterate over multiple year to produce list
urls <- purrr::map(year_range, ~paste0(cur_df$url, .))
urls

# single case
url = "https://www.x-rates.com/average/?amount=100&from=JPY&to=CNY&year=2015"

xml %>% html_nodes(".avgRate") %>% html_text() %>% list() 

# obtain 12-month average
get_avg_rate <- function(url){
    url %>% read_html() %>% html_nodes(".avgRate") %>% html_text() %>% list() 
}
# a safer approach
get_safely <- safely(get_avg_rate)


# urls %>% map( ~ map(., get_safely))
get_from_list <- function(url_list){
    map(url_list, get_safely)
}

map(urls, get_from_list)