xml – 循环函数中的规避错误(用于从Twitter提取数据)

我创建了一个循环函数,使用搜索API以一定的间隔提取推文(假设每隔5分钟).此功能执行以下操作：连接到twitter,提取包含特定关键字的推文,并将其保存在csv文件中.但偶尔(每天2-3次)循环因为以下两个错误之一而停止：

> htmlTreeParse中的错误(URL,useInternal = TRUE)：
为http://search.twitter.com/search.atom?q=创建解析器时出错
6.95322e-310tst&安培; RPP = 100安培;页= 10
> UseMethod中的错误(“xmlNamespaceDeFinitions”)：
没有适用于’xmlNamespaceDeFinitions’的方法应用于对象
class“NULL”

我希望你能回答我的一些问题,帮助我解决这些错误：

>是什么导致这些错误发生？
>如何调整代码以避免这些错误？
>如果遇到错误(例如使用Try函数),我如何“强制”循环继续运行？

我的功能(基于在线发现的几个脚本)如下：

library(XML)   # htmlTreeParse

    twitter.search <- "Keyword"

    QUERY <- URLencode(twitter.search)

    # Set time loop (in seconds)
    d_time = 300
    number_of_times = 3000

    for(i in 1:number_of_times){

    tweets <- NULL
    tweet.count <- 0
    page <- 1
    read.more <- TRUE

    while (read.more)
    {
    # construct Twitter search URL
    URL <- paste('http://search.twitter.com/search.atom?q=',QUERY,'&rpp=100&page=',page,sep='')
    # fetch remote URL and parse
    XML <- htmlTreeParse(URL,useInternal=TRUE,error = function(...){})

    # Extract list of "entry" nodes
    entry     <- getNodeSet(XML,"//entry")

    read.more <- (length(entry) > 0)
    if (read.more)
    {
    for (i in 1:length(entry))
    {
    subdoc     <- xmlDoc(entry[[i]])   # put entry in separate object to manipulate

    published  <- unlist(xpathApply(subdoc,"//published",xmlValue))

    published  <- gsub("Z"," ",gsub("T",published) )

    # Convert from GMT to central time
    time.gmt   <- as.POSIXct(published,"GMT")
    local.time <- format(time.gmt,tz="Europe/Amsterdam")

    title  <- unlist(xpathApply(subdoc,"//title",xmlValue))

    author <- unlist(xpathApply(subdoc,"//author/name",xmlValue))

    tweet  <-  paste(local.time," @",author,":  ",title,sep="")

    entry.frame <- data.frame(tweet,local.time,stringsAsFactors=FALSE)
    tweet.count <- tweet.count + 1
    rownames(entry.frame) <- tweet.count
    tweets <- rbind(tweets,entry.frame)
    }
    page <- page + 1
    read.more <- (page <= 15)   # Seems to be 15 page limit
    }
    }

    names(tweets)

    # top 15 tweeters
    #sort(table(tweets$author),decreasing=TRUE)[1:15]

    write.table(tweets,file=paste("Twitts - ",format(Sys.time(),"%a %b %d %H_%M_%s %Y"),".csv"),sep = ";")

    Sys.sleep(d_time)

    } # end if

解决方法

这是我的解决方案,尝试使用Twitter API的类似问题.

我在Twitter API中查询了一长串Twitter用户中每个人的关注者数量.当用户的帐户受到保护时,我会收到错误,并且在我输入try函数之前循环会中断.使用try允许循环继续工作,跳过列表中的下一个人.

这是设置

# load library
library(twitteR)
#
# Search Twitter for your term
s <- searchTwitter('#rstats',n=1500) 
# convert search results to a data frame
df <- do.call("rbind",lapply(s,as.data.frame)) 
# extract the usernames
users <- unique(df$screenName)
users <- sapply(users,as.character)
# make a data frame for the loop to work with 
users.df <- data.frame(users = users,followers = "",stringsAsFactors = FALSE)

这是循环,尝试处理错误,同时填充用户$Twitter粉丝从Twitter API获得的追随者计数

for (i in 1:nrow(users.df)) 
    {
    # tell the loop to skip a user if their account is protected 
    # or some other error occurs  
    result <- try(getUser(users.df$users[i])$followersCount,silent = TRUE);
    if(class(result) == "try-error") next;
    # get the number of followers for each user
    users.df$followers[i] <- getUser(users.df$users[i])$followersCount
    # tell the loop to pause for 60 s between iterations to 
    # avoid exceeding the Twitter API request limit
    print('Sleeping for 60 seconds...')
    Sys.sleep(60); 
    }
#
# Now inspect users.df to see the follower data