R中嵌套的For循环出现错误"numerical expression has 2 elements: only the first used“

Question

我正在尝试使用R中的嵌套for循环打印字符串中3个连续字符的组合。代码确实打印了这些组合，但我得到了一个错误，即只对其中一个元素而不是数据帧中的每一行执行此操作。

x <- data.frame(Pattern = c("abcdef", "hijklmnop"), id = 1:2)

output <- vector("character", length(x$Pattern))

for (i in 1:nrow(x)) {  

  file <- x$Pattern[i]

  for (j in 1:(str_length(x$Pattern))) {
    output[j] <- substr(file, j, j+2)
  }

}

numerical expression has 2 elements: only the first usednumerical expression has 2 elements: only the first used
> 
> output
[1] "hij" "ijk" "jkl" "klm" "lmn" "mno"

这里发生了两件不起作用的事情。其一是启动的var output使用第一个模式的长度(length = 6)和基于该长度的打印组合，但是我寻找的输出是字符串的长度( length = 9)。不使用嵌套for循环的预期输出如下所示。

  for (j in 1:9) {
    
    output[j] <- substr(file, j, j+2)
    
  }

output
[1] "hij" "ijk" "jkl" "klm" "lmn" "mno" "nop" "op"  "p"  

我进一步削减了它，这样我就只有一个组合列表，每个字符串有3个连续的字符。

list(output[1:(length(output)-3)])
[[1]]
[1] "hij" "ijk" "jkl" "klm" "lmn" "mno"

我遇到的第二个问题是，输出只打印列表中第二个字符串的组合。我试着按照其他帖子中的建议将1:nrow(a)更改为seq_along和length(a)，但这不起作用。预期的输出如下所示。

a$combo <- output

a$combo
[1] c("abc","bcd","cde","def") c("hij","ijk","jkl","klm","lmn","mno")

Answer 1

x <- data.frame(Pattern = c("abcdef", "hijklmnop"), id = 1:2)

# number of additional letters in individual character string
add_letters = 2

library(stringr)

output = list()


for (i in 1:nrow(x)) {  
    
    file <- x$Pattern[i]
    
    l = list()
    
    for (j in 1:(str_length(x$Pattern[i])-add_letters)) {
        
        l[j] <- c(substr(file, j, j+add_letters))
    
    }
    
    output[[i]] = l 
    
}

x$combo = output

一个使用列表的解决方案--正如Gregor Thomas建议的那样。