R：提取模式，不同时间

Question

我有以下问题：我有一个文本，由章节分隔并由向量存储。假设是这样的：

text <- c("Here are information about topic1.", 
"Here are some information about topic2 or topic3.", 
"Chapter number 4 is really annoying.", 
"Topic4 is discussed in this chapter.")

我想提取不同章节中提到的不同主题。所以我的输出应该是这样的：

output
      [1]       [2]
[1] "topic1"
[2] "topic2" "topic3"
[3]
[4] "topic3"

所以我有一些行有多个结果，而有些行没有匹配。

我尝试使用 str_extract_all 并取消列出列表，但遇到了导致行元素数量不同的问题。

谢谢大家！

Answer 1

您可以从plyr 使用rbind.fill.matrix。

text <- c("Here are information about topic1.", 
          "Here are some information about topic2 or topic3.", 
          "Chapter number 4 is really annoying.", 
          "Topic4 is discussed in this chapter.")

library(stringr)
library(plyr)

xy <- str_extract_all(text, pattern = "[Tt]opic\\d+")
xy <- sapply(xy, FUN = function(x) matrix(x, nrow = 1))
rbind.fill.matrix(xy) # from plyr

     1        2       
[1,] "topic1" NA      
[2,] "topic2" "topic3"
[3,] NA       NA      
[4,] "Topic4" NA