从另一个向量中删除向量中第一次出现的元素 [英] Remove first occurrence of elements in a vector from another vector

问题描述

我有一个字符向量，其中包括一些重复的元素，例如

v <- c("d09", "d11", "d13", "d01", "d02", "d10", "d13")

另一个包含单个字符计数的向量，例如

x <- c("d10", "d11", "d13")

我只想从第二个矢量v中删除x中每个元素的 first 出现.在此示例中，d13出现在x中，两次出现在v中，但是仅从v中删除了第一个匹配项，并保留了重复项.因此，我想结束:

"d09", "d01", "d02", "d13"

我一直在尝试各种事情，例如z <- v[!(v %in% x)]，但它会不断删除x中字符的所有个实例，而不仅仅是第一个，因此我最终得到了这个:

"d09", "d01", "d02"

我该怎么做才能仅删除一个重复元素的实例?

解决方案

您可以使用match和负索引.

v[-match(x, v)]

产生

[1] "d09" "d01" "d02" "d13"

match仅返回值的第一个匹配项的位置，在这里我们利用它来发挥优势.

请注意，%in%和is.element是match的简并版本.比较:

match(x, v)            # [1] 6 2 3
match(x, v) > 0        # [1] TRUE TRUE TRUE
x %in% v               # [1] TRUE TRUE TRUE
is.element(x, v)       # [1] TRUE TRUE TRUE

最后三个都是相同的，并且基本上是第一个的逻辑版本(实际上，请参见%in%和is.element的代码).这样做会丢失键信息，键信息是v中x的第一个匹配项的位置，并且仅知道v中存在x值才被留下.

相反，v %in% x表示与您想要的东西不同的东西，即"v中的值位于x中"，由于所有重复的值都满足该条件，因此不符合您的要求./p>

I have a character vector, including some elements that are duplicates e.g.

v <- c("d09", "d11", "d13", "d01", "d02", "d10", "d13")

And another vector that includes single counts of those characters e.g.

x <- c("d10", "d11", "d13")

I want to remove only the first occurrence of each element in x from the 2nd vector v. In this example, d13 occurs in x and twice in v, but only the first match is removed from v and the duplicate is kept. Thus, I want to end up with:

"d09", "d01", "d02", "d13"

I've been trying various things e.g. z <- v[!(v %in% x)] but it keeps removing all instances of the characters in x, not just the first, so I end up with this instead:

"d09", "d01", "d02"

What can I do to only remove one instance of a duplicated element?

解决方案

You can use match and negative indexing.

v[-match(x, v)]

produces

[1] "d09" "d01" "d02" "d13"

match only returns the location of the first match of a value, which we use to our advantage here.

Note that %in% and is.element are degenerate versions of match. Compare:

match(x, v)            # [1] 6 2 3
match(x, v) > 0        # [1] TRUE TRUE TRUE
x %in% v               # [1] TRUE TRUE TRUE
is.element(x, v)       # [1] TRUE TRUE TRUE

The last three are all the same, and are basically the coerced to logical version of the first (in fact, see code for %in% and is.element). In doing so you lose key information, which is the location of the first match of x in v and are left only knowing that x values exist in v.

The converse, v %in% x means something different from what you want, which is "which values in v are in x", which won't meet your requirement since all duplicate values will satisfy that condition.

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