R 中的嵌套 For 循环错误 "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。该代码确实打印了这些组合，但是我收到一个错误，关于只对其中一个元素而不是每一行执行此操作数据框。

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"

这里有 2 件事情不起作用。一个是启动的 var output 使用第一个模式的长度（长度 = 6）和基于该长度的打印组合，但是我正在寻找一个输出，它是字符串的长度（长度 = 9 ).预期输出如下，未使用嵌套 for 循环。

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

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

我 trim 进一步向下，以便每个字符串只有 3 个连续字符的组合列表。

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仅打印列表中第二个字符串的组合。我已尝试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。预期输出如下。

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

使用列表的解决方案 - 正如 Gregor Thomas 所建议的。

R 中的嵌套 For 循环错误 "numerical expression has 2 elements: only the first used"

Nested For loop in R with error "numerical expression has 2 elements: only the first used"

r

for-loop

nested-for-loop