#' Function to retrive data from a service
#'
#' Retrive the pmid (PubMed identifier).
#'
#' @param expr is expression to look for (e.g., H2AX)
#' @param retmax is the total number of UIDs from the retrieved set to be shown in the XML output
#' @param service is the service name (e.g., Entrez)
#' @details The function uses internally the seRvice function to set an service,
#' from which the data are retrived. The Entrez service is used by default. The
#' result of the scRapper function is a XML structured output.
#' @author Stefan Roediger, Michal Burdukiewcz
#' @references Eric Sayers, The E-utilities In-Depth: Parameters, Syntax and More, 2015, http://www.ncbi.nlm.nih.gov/books/NBK25499/
#' @examples scrpR("apoptosis", retmax = 3)
#' @keywords scraper
#' @export scrpR
scrpR <- function(expr = NULL, retmax = 20, service = "Entrez") {
# check for number of abstracts to retrieve information from
ifelse(retmax < 1, retmax <- 1, retmax)
ifelse(retmax > 100000, warning("use different restart value"), retmax)
# use the service function to select a data base / source
seRvice(sl = service)
if(service == "Entrez") {
service.url <- getOption("Entrez.service.url")
expression.url <- paste0(service.url, "esearch.fcgi?db=pubmed&term=", expr, "&retmax=", retmax)
} else {
stop("Unknown service! Check spelling!")
}
# parse the XML structure
try(xmlTreeParse(file = expression.url, isURL = TRUE))
}
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