英文:
Error when iterating over a character vector but omitting some elements
问题
我对R语言非常陌生，遇到了一些在遍历字符向量时的问题。
当遍历数值向量时，使用否定运算符'-'来省略其中一个元素相对容易。例如：
for (i in (1:5)[-4]){
print(i)
}
会输出：
[1] 1
[1] 2
[1] 3
[1] 5
然而，将相同的方法应用于字符向量会引发错误：
X = c("a", "b", "c", "d", "e")
for (letter in (X)[-"d"]){
print(letter)
}
Error in -"d" : invalid argument to unary operator
我尝试了多种变体，但似乎都返回相同的错误。我想知道为什么这两种情况会被不同对待，以及正确的语法是什么？
英文:
I'm very new to R and am having some trouble when iterating through a character vector.
When iterating through a numeric vector, it is rather easy to omit one of the elements using the negation operator '-'. For example
for (i in (1:5)[-4]){
print(i)
}
yields
[1] 1
[1] 2
[1] 3
[1] 5
However, applying the same method to a character vector raises an error
X = c("a", "b", "c", "d", "e")
for (letter in (X)[-"d"]){
print(letter)
}
Error in -"d" : invalid argument to unary operator
I have tried multiple variations on the above but all seem to return the same error. I was wondering if someone could tell me why these two cases are treated differently and what the correct syntax is?
答案1
得分: 2
使用此代码:
X <- c("a", "b", "c", "d", "e")
for (letter in X[X != "d"]) {
print(letter)
}
输出将是:
<details>
<summary>英文:</summary>
use this:
X <- c("a", "b", "c", "d", "e")
for (letter in X[X != "d"]) {
print(letter)
}
output will be:
</details>
# 答案2
**得分**: 2
你可以使用以下代码来实现：
```R
setdiff(X, 'd')
或者
X[-grep('d', X)]
此外，R 中许多函数（如 print）都是向量化的，因此它们会处理向量的所有元素，而不需要显式循环。
另外，对于许多情况，R 有更简洁、可读性更好、速度更快的替代循环的方式：整个 ...apply 系列，Map，Reduce（以及它们来自 {purrr} 的增强变体）等等。
你可以尝试 letters[1:5] |> setdiff('d') |> paste('is not d') 来看到管道操作符的魔力，也可以了解更多信息请查看 {purrr}。
英文:
You could use
setdiff(X, 'd')
or
X[-grep('d', X)]
Aside 1: many functions in R (like print) are vectorised, so they will digest all elements of a vector without explicitly being told to loop anyway.
Aside 2: R has more concise/readable/faster equivalents to loops in many cases: the whole ...apply family, Map, Reduce (and their steroid variants from {purrr}) and such.
Try letters[1:5] |> setdiff('d') |> paste('is not d') to watch the pipe operator doing its magic, too 
答案3
得分: 1
为了在[中使用-，我们需要使用数字索引。因此，我们需要使用which来找出X中“d”的位置。
for (letter in X[-which(X == "d")]){
print(letter)
}
[1] "a"
[1] "b"
[1] "c"
[1] "e"
英文:
To use - in [, we need numeric indices. So we need to find out the position of "d" in X by using which.
for (letter in X[-which(X == "d")]){
print(letter)
}
[1] "a"
[1] "b"
[1] "c"
[1] "e"
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