提取数据框中变量第一次出现的行 [英] Extract rows for the first occurrence of a variable in a data frame

问题描述

我有一个包含两个变量 Date 和 Taxa 的数据框，我想获取每个分类群第一次出现的日期.在由 172 行组成的数据框中有 9 个不同的日期和 40 个不同的分类群，但我的答案应该只有 40 行.

I have a data frame with two variables, Date and Taxa and want to get the date for the first time each taxa occurs. There are 9 different dates and 40 different taxa in the data frame consisting of 172 rows, but my answer should only have 40 rows.

Taxa 是一个因子，Date 是一个日期.

Taxa is a factor and Date is a date.

例如，我的数据框(称为物种")设置如下:

For example, my data frame (called 'species') is set up like this:

Date          Taxa
2013-07-12    A
2011-08-31    B
2012-09-06    C
2012-05-17    A
2013-07-12    C
2012-09-07    B

我会寻找这样的答案:

Date          Taxa
2012-05-17    A
2011-08-31    B
2012-09-06    C

我尝试使用:

t.first <-  species[unique(species$Taxa),]

它给了我正确的行数，但重复了分类群.如果我只使用 unique(species$Taxa) 它似乎给了我正确的答案，但是我不知道它第一次发生的日期.

and it gave me the correct number of rows but there were Taxa repeated. If I just use unique(species$Taxa) it appears to give me the right answer, but then I don't know the date when it first occurred.

感谢您的帮助.

推荐答案

t.first <- species[match(unique(species$Taxa), species$Taxa),]

应该给你你正在寻找的东西.match 返回比较向量中第一个匹配项的索引，从而为您提供所需的行.

should give you what you're looking for. match returns indices of the first match in the compared vectors, which give you the rows you need.

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