R:根据先前的行创建一个新行 [英] R: Creating a new row based on previous rows

问题描述

我是R的新手，正在尝试根据先前行中的值创建新行.

I'm new to R and trying to create a new row based on values on previous rows.

样本数据:

df <- data.table(Item = c("a", "b", "c", "d"),
                 "2010FY" = c(3, 5, 2, 2),
                 "2011FY" = c(5, 6, 2, 1),
                 "2012FY" = c(-1, 2, 2, 0.5))

我想创建一个新行，将第三行除以第四行.我们将此项目称为"e"，理想情况下应这样:

I would like to create a new row that divides the 3rd row by the 4th row. Let's call this Item "e" and ideally should like this:

    Item     2010FY     2011FY     2012FY
1     a        3          5          -1
2     b        5          6           2
3     c        2          2           2
4     d        2          1          0.5
5     e        1          2           4

使用dplyr，我的第一次尝试是:

Using dplyr, my first attempt is:

 df <- bind_rows(df, e = df[Item %in% "c", ] / df[Item %in% "d", ])

这不起作用，因为字符不能被整除(即，项目"列).

This does not work because characters are not divisible (ie. The Item column).

我发现mutate_if函数可以工作，但只能按列工作.有没有办法我可以逐行执行此操作?

I found that the function mutate_if would work but only by column. Is there a way I can do this by row?

非常感谢您.

推荐答案

如上面的注释所指出的，这可行:

As pointed out in the comments above, this worked:

rbind(df, c(Item="e", df[df$Item == "c", -1] / df[df$Item == "d", -1])) 

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