如何在不强制转换为数字的情况下迭代日期列表? [英] How to iterate over list of Dates without coercion to numeric?

问题描述

这与循环日期或POSIXct 对象产生一个数字迭代器

> dates <- as.Date(c("2013-01-01", "2013-01-02"))
> class(dates)
[1] "Date"
> for(d in dates) print(class(d))
[1] "numeric"
[1] "numeric"

我有两个问题:

1. 迭代 Date 对象列表的首选方法是什么?
2. 我不明白约书亚的回答(从上面链接的问题中接受的答案)，我会在这里引用它:所以你的 Date 向量被强制为 numeric 因为 Date 对象不是严格的向量".那么如何确定应该将Date 强制转换为numeric?

1. What is the preferred way to iterate over a list of Date objects?
2. I don't understand Joshua's answer (accepted answer from the question linked above), I'll quote it here: "So your Date vector is being coerced to numeric because Date objects aren't strictly vectors". So how is it determined that Date should be coerced to numeric?

推荐答案

这里有两个问题.一是输入是否从 Date 强制转换为 numeric.另一个是输出是否被强制为 numeric.

There are two issues here. One is whether the input gets coerced from Date to numeric. The other is whether the output gets coerced to numeric.

输入

For 循环将 Date 输入强制转换为 numeric，因为正如@DWin 和 @JoshuaUlrich 指出的那样，for 循环采用 vectors 和 Date 在技术上不是向量.

For loops coerce Date inputs to numeric, because as @DWin and @JoshuaUlrich point out, for loops take vectors, and Dates are technically not vectors.

> for(d in dates) print(class(d))
[1] "numeric"
[1] "numeric"

另一方面，lapply 和它的简化后代 sapply 没有这样的限制.

On the other hand, lapply and its simplifier offspring sapply have no such restrictions.

> sapply( dates, function(day) class(day) )
[1] "Date" "Date"

输出

但是！上面 class() 的输出是一个字符.如果您尝试实际返回日期对象，则 sapply 不是您想要的.

However! The output of class() above is a character. If you try actually returning a date object, sapply is not what you want.

lapply 不会强制转换为向量，但 sapply 会:

lapply does not coerce to a vector, but sapply does:

> lapply( dates, identity )
[[1]]
[1] "2013-01-01"

[[2]]
[1] "2013-01-02"

> sapply( dates, identity )
[1] 15706 15707

那是因为 sapply 的简化函数将输出强制转换为向量.

That's because sapply's simplification function coerces output to a vector.

总结

所以:如果你有一个 Date 对象并且想要返回一个非 Date 对象，你可以使用 lapply 或 应用.如果您有一个非Date 对象，并且想要返回一个Date 对象，您可以使用for 循环或lapply.如果您有一个 Date 对象并且想要返回一个 Date 对象，请使用 lapply.

So: If you have a Date object and want to return a non-Date object, you can use lapply or sapply. If you have a non-Date object, and want to return a Date object, you can use a for loop or lapply. If you have a Date object and want to return a Date object, use lapply.

了解更多信息的资源

如果您想更深入地研究向量，可以从 John Cook 的笔记开始，继续R Inferno，继续SDA.

If you want to dig deeper into vectors, you can start with John Cook's notes, continue with the R Inferno, and continue with SDA.

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