Using R to get download URL by link name

Question

I\'m trying to use rvest to download a list of files from this site. The file names are regular, but the download URLs don\'t match a pattern (just dozens of digits

Answer 1

Slightly different approach.

Grab all downloadable filenames and URLs:

library(httr)
library(rvest)
library(tidyverse)

pg <- read_html("http://www-air.larc.nasa.gov/cgi-bin/ArcView/actamerica.2016?C130=1")

fils <- html_nodes(pg, xpath=".//a[contains(@href, 'cgi-bin/enzFile')]")

data_frame(
  filename = html_text(fils),
  link = sprintf("http://www-air.larc.nasa.gov%s", html_attr(fils, "href"))
) -> xdf

glimpse(xdf)
## Observations: 719
## Variables: 2
## $ filename  "ACTAMERICA-Elevation_C130_20160711_R0.ict", "ACTAMERICA-Elevation_C130_20160715_R0.ict", "ACTAMERI...
## $ link      "http://www-air.larc.nasa.gov/cgi-bin/enzFile?f49DA0512C4E81E3C01FDB44A33CD88AAFE2f7075622d6169722f...

xdf
## # A tibble: 719 x 2
## filename                                                                                                                                                                                                                                                                           link
##                                                                                                                                                                                                                                                                           
## 1 ACTAMERICA-Elevation_C130_20160711_R0.ict http://www-air.larc.nasa.gov/cgi-bin/enzFile?f49DA0512C4E81E3C01FDB44A33CD88AAFE2f7075622d6169722f414354414d45524943412f323031362f433133305f41495243524146542f444947414e47492e4a4f534855412f414354414d45524943412d456c65766174696f6e5f433133305f32303136303731315f52302e696374
## 2 ACTAMERICA-Elevation_C130_20160715_R0.ict http://www-air.larc.nasa.gov/cgi-bin/enzFile?f49DA0512C4E81E3C01FDB44A33CD88AAFE2f7075622d6169722f414354414d45524943412f323031362f433133305f41495243524146542f444947414e47492e4a4f534855412f414354414d45524943412d456c65766174696f6e5f433133305f32303136303731355f52302e696374
## 3 ACTAMERICA-Elevation_C130_20160718_R0.ict http://www-air.larc.nasa.gov/cgi-bin/enzFile?f49DA0512C4E81E3C01FDB44A33CD88AAFE2f7075622d6169722f414354414d45524943412f323031362f433133305f41495243524146542f444947414e47492e4a4f534855412f414354414d45524943412d456c65766174696f6e5f433133305f32303136303731385f52302e696374
## 4 ACTAMERICA-Elevation_C130_20160719_R0.ict http://www-air.larc.nasa.gov/cgi-bin/enzFile?f49DA0512C4E81E3C01FDB44A33CD88AAFE2f7075622d6169722f414354414d45524943412f323031362f433133305f41495243524146542f444947414e47492e4a4f534855412f414354414d45524943412d456c65766174696f6e5f433133305f32303136303731395f52302e696374
## 5 ACTAMERICA-Elevation_C130_20160721_R0.ict http://www-air.larc.nasa.gov/cgi-bin/enzFile?f49DA0512C4E81E3C01FDB44A33CD88AAFE2f7075622d6169722f414354414d45524943412f323031362f433133305f41495243524146542f444947414e47492e4a4f534855412f414354414d45524943412d456c65766174696f6e5f433133305f32303136303732315f52302e696374
## 6 ACTAMERICA-Elevation_C130_20160722_R0.ict http://www-air.larc.nasa.gov/cgi-bin/enzFile?f49DA0512C4E81E3C01FDB44A33CD88AAFE2f7075622d6169722f414354414d45524943412f323031362f433133305f41495243524146542f444947414e47492e4a4f534855412f414354414d45524943412d456c65766174696f6e5f433133305f32303136303732325f52302e696374
## 7 ACTAMERICA-Elevation_C130_20160725_R0.ict http://www-air.larc.nasa.gov/cgi-bin/enzFile?f49DA0512C4E81E3C01FDB44A33CD88AAFE2f7075622d6169722f414354414d45524943412f323031362f433133305f41495243524146542f444947414e47492e4a4f534855412f414354414d45524943412d456c65766174696f6e5f433133305f32303136303732355f52302e696374
## 8 ACTAMERICA-Elevation_C130_20160726_R0.ict http://www-air.larc.nasa.gov/cgi-bin/enzFile?f49DA0512C4E81E3C01FDB44A33CD88AAFE2f7075622d6169722f414354414d45524943412f323031362f433133305f41495243524146542f444947414e47492e4a4f534855412f414354414d45524943412d456c65766174696f6e5f433133305f32303136303732365f52302e696374
## 9 ACTAMERICA-Elevation_C130_20160727_R0.ict http://www-air.larc.nasa.gov/cgi-bin/enzFile?f49DA0512C4E81E3C01FDB44A33CD88AAFE2f7075622d6169722f414354414d45524943412f323031362f433133305f41495243524146542f444947414e47492e4a4f534855412f414354414d45524943412d456c65766174696f6e5f433133305f32303136303732375f52302e696374
## 10 ACTAMERICA-Elevation_C130_20160801_R0.ict http://www-air.larc.nasa.gov/cgi-bin/enzFile?f49DA0512C4E81E3C01FDB44A33CD88AAFE2f7075622d6169722f414354414d45524943412f323031362f433133305f41495243524146542f444947414e47492e4a4f534855412f414354414d45524943412d456c65766174696f6e5f433133305f32303136303830315f52302e696374
## # ... with 709 more rows

Get the ones you care about:

picarro <- filter(xdf, grepl("PICARRO", filename))

Download them:

walk2(picarro$link, picarro$filename, download.file)
## trying URL 'http://www-air.larc.nasa.gov/cgi-bin/enzFile?f49DA0512C4E81E3C01FDB44A33CD88AAFE2f7075622d6169722f414354414d45524943412f323031362f433133305f41495243524146542f444947414e47492e4a4f534855412f414354414d45524943412d5049434152524f5f433133305f32303136303532375f52422e696374'
## Content type 'text/plain' length 1023662 bytes (999 KB)
## ==================================================
##   downloaded 999 KB
## 
## trying URL 'http://www-air.larc.nasa.gov/cgi-bin/enzFile?f49DA0512C4E81E3C01FDB44A33CD88AAFE2f7075622d6169722f414354414d45524943412f323031362f433133305f41495243524146542f444947414e47492e4a4f534855412f414354414d45524943412d5049434152524f5f433133305f32303136303731315f52302e696374'
## Content type 'text/plain' length 886392 bytes (865 KB)
## ==================================================
##   downloaded 865 KB
## 
## trying URL 'http://www-air.larc.nasa.gov/cgi-bin/enzFile?f49DA0512C4E81E3C01FDB44A33CD88AAFE2f7075622d6169722f414354414d45524943412f323031362f433133305f41495243524146542f444947414e47492e4a4f534855412f414354414d45524943412d5049434152524f5f433133305f32303136303731355f52302e696374'
## Content type 'text/plain' length 530339 bytes (517 KB)
## ==================================================
##   downloaded 517 KB

etc.