I’ve been exploring the feasibility of aggregating data on prices of replacement parts for heavy machinery. There are a number of websites which list this sort of data. I’m focusing on the static sites for the moment.

I’m using are R with {rvest} (and a few other Tidyverse packages thrown in for good measure).

library(glue)
library(dplyr)
library(purrr)
library(stringr)
library(rvest)

The data are paginated. Fortunately the URL includes the page number as a GET parameter, so stepping through the pages is simple. I defined a global variable, URL, with a {glue} placeholder for the page number.

This is how I’m looping over the pages. The page number, page, is set to one initially and incremented after each page of results is scraped. When it gets to a page without at results, the loop is stopped.

page = 1

while (TRUE) {
  items <- read_html(glue(URL)) %>% html_nodes(SELECTOR)
  #
  if (length(items) == 0) break                  # Check if gone past last page.

  # Extract data for each item here...

  page <- page + 1                               # Advance to next page.
}

That’s the mechanics. Within the loop I then used map_dfr() from {purrr} to iterate over items, delving into each item to extract its name and price.

map_dfr(items, function(item) {
    tibble(
      part = item %>% html_node("p") %>% html_text(),
      price = item %>% html_node(".product-item--price small") %>% html_text()
    )
  })

The results from each page are appended to a list and finally concatenated using bind_rows().

> dim(parts)
[1] 987   2

Scraping a single category yields 987 parts. Let’s take a look at the first few.

> head(parts)
  part                                                                                   price    
  <chr>                                                                                  <chr>    
1 R986110000 Bosch Rexroth New Replacement Hydraulic Axial Piston Motor For CAT 240-7732 $1,534.00
2 R986110001 Bosch Rexroth New Replacement Hydraulic Axial Piston Motor For CAT 213-5268 $1,854.00
3 R986110002 Bosch Rexroth New Replacement Hydraulic Axial Piston Motor For CAT 266-8034 $1,374.00
4 R986110003 Bosch Rexroth New Replacement Hydraulic Axial Piston Motor For CAT 296-6728 $1,754.00
5 R986110004 Bosch Rexroth New Replacement Hydraulic Axial Piston Motor For CAT 136-8869 $1,494.00
6 R986110005 Bosch Rexroth New Replacement Hydraulic Axial Piston Motor For CAT 255-6805 $1,534.00

That’s looking pretty good already. There’s one final niggle: the data in the price column are strings. Ideally we’d want those to be numeric. But to do that we have to strip off some punctuation. Not a problem thanks to functions from {stringr}.

parts <- parts %>%
  mutate(
    price = str_replace(price, "^\\$", ""),      # Strip off leading "$"
    price = str_replace_all(price, ",", ""),     # Strip out comma separators
    price = as.numeric(price)
  )

Success!

> head(parts)
  part                                                                                   price
  <chr>                                                                                  <dbl>
1 R986110000 Bosch Rexroth New Replacement Hydraulic Axial Piston Motor For CAT 240-7732  1534
2 R986110001 Bosch Rexroth New Replacement Hydraulic Axial Piston Motor For CAT 213-5268  1854
3 R986110002 Bosch Rexroth New Replacement Hydraulic Axial Piston Motor For CAT 266-8034  1374
4 R986110003 Bosch Rexroth New Replacement Hydraulic Axial Piston Motor For CAT 296-6728  1754
5 R986110004 Bosch Rexroth New Replacement Hydraulic Axial Piston Motor For CAT 136-8869  1494
6 R986110005 Bosch Rexroth New Replacement Hydraulic Axial Piston Motor For CAT 255-6805  1534

The great thing about scripting a scraper is that, provided that the website has not been dramatically restructured, the scraper can be run at any time to gather an updated set of results.