爲什麼用lapply

Question

這裏使用時，在函數中使用xpathSApply（XML封裝）拋出一個錯誤是我的函數：

# Purpose: Scrape Floraweb.de for plant species data (photograph, sociology, ecology, anatomy) 
# Author: Kay Cichini 
# Date: 2012-06-10 
# Output: PDF saved to created folder .~/FLORAWEB 
# Packages: XML, jpeg, 
# Licence: cc by-nc-sa 

floraweb_scraper <- function(input) { 

    # I didn't get around this encoding issue other than with gsub.. 
    spch_sub <- function(x) { 
     x <- gsub("ü", "ü", x) 
     x <- gsub("ä", "ä", x) 
     x <- gsub("ö", "ö", x) 
     x <- gsub("Ä", "Ä", x) 
     x <- gsub("Ãoe", "Ü", x) 
     x <- gsub("ü", "Ä", x) 
     x <- gsub("Ö", "Ö", x) 
     x <- gsub("ß", "ß", x) 
     x <- gsub("é", "é", x) 
     x <- gsub("Ã-", "í", x) 
     x <- gsub("á", "á", x) 
     x <- gsub("±", "~", x) 
     x <- gsub(" ", "", x) # pattern for backspaces 
    } 

    # automated package installation: 
    pkgs <- c("XML", "jpeg") 

    pkgs_miss <- pkgs[which(!pkgs %in% installed.packages()[, 1])] 
    if (length(pkgs_miss) > 0) { 
     install.packages(pkgs_miss) 
    } 

    # load packages: 
    require(XML) 
    require(jpeg) 


    # prepare input and get parsed script: 
    input1 <- gsub("[[:space:]]", "+", input) 
    URL <- paste("http://www.floraweb.de/pflanzenarten/taxoquery.xsql?taxname=", 
       input1, sep = "") 
    doc <- htmlParse(URL) 

    # get returned species names (dismiss last row with additional info): 
    sp <- xpathSApply(doc, "//div[@id='contentblock']//a", xmlValue) 
    sp <- sp[1:length(sp)-1] 

    # get species ids from contentblock: 
    con <- getNodeSet(doc, "//div[@id='contentblock']//a")[1:len] 
    urls <- sapply(con, xmlGetAttr, "href") 
    id_1 <- gsub("[^0-9]", "", urls) 

    # check matching and assign to resulting dataframe: 
    match <- numeric() 
    for (i in 1:len) { 
     match[i] <- sum(unlist(strsplit(tolower(sp), " ")[i]) %in% unlist(strsplit(input, 
      " ")) == 0) 
    } 
    df <- data.frame(sp, id_1, match) 

    # select the one with best match: 
    sel <- id_1[rank(df$match)][1] 

    # build urls for retrieving species data 
    url <- paste("http://www.floraweb.de/pflanzenarten/druck.xsql?suchnr=", sel, sep = "") 

    doc <- htmlParse(url) 
    img_src <- xpathSApply(doc, '//*/p[@class="centeredcontent"]/img/@src') 
    img_url <- gsub("../", "http://www.floraweb.de/", img_src, fixed = T) 

    # get infos: 
    infos <- xpathSApply(doc, "//div[@id='content']//p", xmlValue)[c(2, 7, 22, 33, 35, 14)] 

    # replace special characters: 
    infos <- spch_sub(infos) 

    # make dir to save data: 
    dir.create(path.expand("~/FLORAWEB/"), showWarnings = F) 
    setwd(path.expand("~/FLORAWEB/")) 

    # download image: 
    download.file(img_url, "floraweb.jpg", mode = "wb") 

    # open device: 
    pdf(paste(spch_sub(df$sp[df$id_1 == sel]), ".FloraWeb.pdf", sep = ""), paper = "a4") 

    # read image: 
    img <- readJPEG("floraweb.jpg") 
    w <- dim(img)[2] 
    h <- dim(img)[1] 

    # print img to plot region: 
    par(mar = rep(0, 4), oma = rep(0, 4), mfrow = c(2, 1)) 
    plot(NA, xlim = c(0, w), ylim = c(0, h), xlab = "", ylab = "", axes = F, 
    type = "n", yaxs = "i", xaxs = "i", asp = 1) 
    rasterImage(img, 0, 0, w, h) 

    # print text: 
    plot(NA, xlim = c(0, 1), ylim = c(0, 1), xlab = "", ylab = "", axes = F, type = "n", 
     yaxs = "i", xaxs = "i") 
    # text left intendent and center adjustment: 
    l <- 0.5 
    c_adj <- c(0.5, 0.5) 

    # plot text: 
    text(l, 0.95, paste("Eingabe = ", input, "/Gefunden = ", infos[1], sep = ""), font = 2, adj = c_adj, cex = 0.7) 
    text(l, 0.5, paste(strwrap(infos[-1], width = 112), collapse = "\n"), adj = c_adj, cex = 0.7) 

    # Credit: 
    text(l, 0.05, "Die hier verwendeten Daten sind der Internet-Seite FloraWeb.de entnommen.", 
     adj = c_adj, cex = 0.4, font = 3) 

    graphics.off() 
    message(paste(sp_name, "PDF wurde erzeugt\n\n", sep = "\n -- ")) 

    # remove jpegs: 
    unlink(dir(pattern = ".jpg")) 
} 

# Examples: 
floraweb_scraper("Poa alp") 

pfl_liste <- c("leuc alp", "Poa badensis", "Poa alp") 
lapply(pfl_liste, FUN = floraweb_scraper) 

它運作良好，在第一個例子，但與lapply使用時拋出錯誤 - 有人有線索嗎？

Answer 1

對象len未定義

for (i in 1:len) { 
     match[i] <- sum(unlist(strsplit(tolower(sp), " ")[i]) %in% unlist(strsplit(input, 
      " ")) == 0) 
    } 

你可能在你的工作空間中定義。當你調用第一個函數時，一切都很好。在使用lapply時，您可能遇到範圍問題。在你的函數中定義len可能會解決問題