Question

我想从网站中抓取特定的html代码。所需的对象不在下面的循环的每一页上，因此，某些值为NA。但是，每当缺少一个对象时，整个循环就会停止。

我尝试使用此代码进行非循环抓取，结果证明它可以完美工作。在这里，我排除了所有无法获得或不感兴趣的值。

BJ <- data.frame( Wohnungs_ID = ID, Baujahr = ifelse (nchar(Baujahr)==0 | nchar(Baujahr)>6, NA, Baujahr), stringsAsFactors = F

在相关情况下，我想抓取两个并不总是列出的特定变量。网址是我删除了的链接的列表。

library(rvest)  
library(magrittr)
library(tidyr)  
library(ggplot2) 
library(stringr) 
library(jsonlite) 
library(purrr)  
library(dplyr)  

immo_webp <- read_html ("https://www.immobilienscout24.de/Suche/S-2/Wohnung-Miete/Rheinland-Pfalz/Koblenz")


# URLs 

base = 'https://www.immobilienscout24.de/expose/'

urls <- sapply(read_html("https://www.immobilienscout24.de/Suche/S-2/Wohnung-Miete/Rheinland-Pfalz/Koblenz")%>%
                 html_nodes('article')%>%
                 html_attr('data-obid'), function (url){paste0(base, url)})
print(urls)

# ID bilden

substring(urls, 1, 1)
ID <- substring(urls, 41)

Baujahr <- c(1 : 20)
Energie <- c(1 : 20)

for (i in urls){

  aktuellewohnung <- read_html(url(i))

  Baujahr[i] <- aktuellewohnung %>%
    html_nodes("#is24-content > div.grid-item.padding-desk-right-xl.desk-two-thirds.lap-one-whole.desk-column-left.flex-item.palm--flex__order--1.lap--flex__order--1 > div.is24-ex-details.main-criteria-headline-size.two-column-layout > div.criteriagroup.flex.flex--wrap.criteria-group--spacing.padding-top-s > div.criteriagroup.criteria-group--border.criteria-group--two-columns.criteria-group--spacing > dl:nth-child(1) > dd") %>%
    html_text()
  Energie[i] <- aktuellewohnung %>%
    html_nodes("#is24-content > div.grid-item.padding-desk-right-xl.desk-two-thirds.lap-one-whole.desk-column-left.flex-item.palm--flex__order--1.lap--flex__order--1 > div.is24-ex-details.main-criteria-headline-size.two-column-layout > div.criteriagroup.flex.flex--wrap.criteria-group--spacing.padding-top-s > div.criteriagroup.criteria-group--border.criteria-group--two-columns.criteria-group--spacing > dl:nth-child(7) > dd") %>%
    html_text()  
}

我遇到以下错误。

Error in Baujahr[i] <- aktuellewohnung %>% html_nodes("#is24-content > div.grid-item.padding-desk-right-xl.desk-two-thirds.lap-one-whole.desk-column-left.flex-item.palm--flex__order--1.lap--flex__order--1 > div.is24-ex-details.main-criteria-headline-size.two-column-layout > div.criteriagroup.flex.flex--wrap.criteria-group--spacing.padding-top-s > div.criteriagroup.criteria-group--border.criteria-group--two-columns.criteria-group--spacing > dl:nth-child(1) > dd") %>%  : 

replacement has length zero

我希望此循环有效，同时将缺少的值另存为NA。希望你能帮助我。提前谢谢。

Answer 1

使用它代替循环：

如果找不到该字段，则results_df <- lapply(urls, function(u) { message(u) aktuellewohnung <- read_html(u) Baujahr <- aktuellewohnung %>% html_nodes("#is24-content > div.grid-item.padding-desk-right-xl.desk-two-thirds.lap-one-whole.desk-column-left.flex-item.palm--flex__order--1.lap--flex__order--1 > div.is24-ex-details.main-criteria-headline-size.two-column-layout > div.criteriagroup.flex.flex--wrap.criteria-group--spacing.padding-top-s > div.criteriagroup.criteria-group--border.criteria-group--two-columns.criteria-group--spacing > dl:nth-child(1) > dd") %>% html_text() Energie <- aktuellewohnung %>% html_nodes("#is24-content > div.grid-item.padding-desk-right-xl.desk-two-thirds.lap-one-whole.desk-column-left.flex-item.palm--flex__order--1.lap--flex__order--1 > div.is24-ex-details.main-criteria-headline-size.two-column-layout > div.criteriagroup.flex.flex--wrap.criteria-group--spacing.padding-top-s > div.criteriagroup.criteria-group--border.criteria-group--two-columns.criteria-group--spacing > dl:nth-child(7) > dd") %>% html_text() tibble( Baujahr = ifelse(length(Baujahr) == 0, NA, Baujahr), Energie = ifelse(length(Energie) == 0, NA, Energie) ) }) %>% bind_rows()循环将返回lapply的列表，其中有data.frames。 NA将其转换为一个bind_rows()。

Answer 2

在分配值时尝试使用trycatch function

Baujahr[i] <- tryCatch({aktuellewohnung %>%
    html_nodes("#is24-content > div.grid-item.padding-desk-right-xl.desk-two-thirds.lap-one-whole.desk-column-left.flex-item.palm--flex__order--1.lap--flex__order--1 > div.is24-ex-details.main-criteria-headline-size.two-column-layout > div.criteriagroup.flex.flex--wrap.criteria-group--spacing.padding-top-s > div.criteriagroup.criteria-group--border.criteria-group--two-columns.criteria-group--spacing > dl:nth-child(1) > dd") %>%
    html_text()},error = function(cond){NA})

RVest：修复“替换长度为零”的错误循环

2 个答案: