如何根据R中元素的长度对列表进行子集化 [英] How to subset a list based on the length of its elements in R
问题描述
在R中，我有一个函数(来自软件包sp的coordinates)，该函数针对您提供的每个IP地址查找11个数据字段.
In R I have a function (coordinates from the package sp ) which looks up 11 fields of data for each IP addresss you supply.
我有一个称为ip.addresses的IP列表:
I have a list of IP's called ip.addresses:
> head(ip.addresses)
[1] "128.177.90.11" "71.179.12.143" "66.31.55.111" "98.204.243.187" "67.231.207.9" "67.61.248.12"
注意:那些IP或任何其他IP都可以用来重现此问题.
Note: Those or any other IP's can be used to reproduce this problem.
因此，我使用sapply将函数应用于该对象:
So I apply the function to that object with sapply:
ips.info <- sapply(ip.addresses, ip2coordinates)
并得到一个名为ips.info的列表作为我的结果.这一切都很好，但是我不能对列表做更多的事情，因此我需要将其转换为数据框.问题在于，并非所有IP地址都在数据库中，因此某些列表元素仅具有1个字段，并且出现此错误:
and get a list called ips.info as my result. This is all good and fine, but I can't do much more with a list, so I need to convert it to a dataframe. The problem is that not all IP addresses are in the databases thus some list elements only have 1 field and I get this error:
> ips.df <- as.data.frame(ips.info)
Error in data.frame(`128.177.90.10` = list(ip.address = "128.177.90.10", :
参数暗示不同的行数:1、0
arguments imply differing number of rows: 1, 0
我的问题是-如何删除缺少/不完整数据的元素，或者将该列表转换为每个IP地址包含11列和1行的数据框?"
My question is -- "How do I remove the elements with missing/incomplete data or otherwise convert this list into a data frame with 11 columns and 1 row per IP address?"
我尝试了几件事.
-
首先，我尝试编写一个循环，以删除长度小于11的元素
First, I tried to write a loop that removes elements with less than a length of 11
for (i in 1:length(ips.info)){
if (length(ips.info[i]) < 11){
ips.info[i] <- NULL}}
这会留下一些没有数据的记录，并使其他记录说"NULL"，但是即使is.null
This leaves some records with no data and makes others say "NULL", but even those with "NULL" are not detected by is.null
-
接下来，我尝试使用双方括号进行相同的操作并得到
Next, I tried the same thing with double square brackets and get
Error in ips.info[[i]] : subscript out of bounds
我也尝试过complete.cases()看看它是否可能有用
I also tried complete.cases() to see if it could potentially be useful
Error in complete.cases(ips.info) : not all arguments have the same length
最后，我尝试了以length(ips.info[[i]] == 11为条件的for循环的变体，并将完整的记录写入了另一个对象，但是不知何故，它得到了ips.info的精确副本
Finally, I tried a variation of my for loop which was conditioned on length(ips.info[[i]] == 11 and wrote complete records to another object, but somehow it results in an exact copy of ips.info
推荐答案
这是使用内置的Filter函数
#input data
library(RDSTK)
ip.addresses<-c("128.177.90.10","71.179.13.143","66.31.55.111","98.204.243.188",
"67.231.207.8","67.61.248.15")
ips.info <- sapply(ip.addresses, ip2coordinates)
#data.frame creation
lengthIs <- function(n) function(x) length(x)==n
do.call(rbind, Filter(lengthIs(11), ips.info))
或者如果您不想使用帮助器功能
or if you prefer not to use a helper function
do.call(rbind, Filter(function(x) length(x)==11, ips.info))
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