Question

我有一个示例R脚本，如下所示：

# Create example data
date <- c("11/09/2016", "11/02/2016", "11/16/2016", "11/23/2016")
column_two <- c(4, 2, 3, 4)
# Populate a data frame and make sure the dates have the correct class
mydata <- data.frame(date, column_two)
mydata$date <- strptime(mydata$date, format="%m/%d/%Y")

print("The contents of mydata are:")
print(mydata)

# Create a dummy list (or vector, or array, or what is it?)
foo <- rep(NA, 5)
print("foo is initialized to:")
print(foo)
print("The class of foo is:")
print(class(foo))

earlydate <- min(mydata$date)
print(sprintf("Earliest date is: %s", earlydate))
print("The class of earlydate is:")
print(class(earlydate))
print(sprintf("Length of earliest date is: %d", length(earlydate)))
print(sprintf("Length of foo[2] is: %d", length(foo[2])))

# Attempt to set one variable equal to another
foo[2] <- earlydate

print("After assignment, foo looks like this:")
print(foo)
print("Now the classes of foo, foo[2], and foo[[2]] are:")
print(class(foo))
print(class(foo[2]))
print(class(foo[[2]]))

脚本的打印输出如下所示：

> source("test_warning.R")
[1] "The contents of mydata are:"
        date column_two
1 2016-11-09          4
2 2016-11-02          2
3 2016-11-16          3
4 2016-11-23          4
[1] "foo is initialized to:"
[1] NA NA NA NA NA
[1] "The class of foo is:"
[1] "logical"
[1] "Earliest date is: 2016-11-02"
[1] "The class of earlydate is:"
[1] "POSIXlt" "POSIXt" 
[1] "Length of earliest date is: 1"
[1] "Length of foo[2] is: 1"
[1] "After assignment, foo looks like this:"
[[1]]
[1] NA

[[2]]
[1] 0

[[3]]
[1] NA

[[4]]
[1] NA

[[5]]
[1] NA

[1] "Now the classes of foo, foo[2], and foo[[2]] are:"
[1] "list"
[1] "list"
[1] "numeric"
Warning message:
In foo[2] <- earlydate :
  number of items to replace is not a multiple of replacement length
>

我有很多问题：

为什么foo[2]和earlydate明显长度相同时会收到警告？
为什么foo [2]的值设置为0而不是earlydate的值？
很明显，R会自动将foo变量及其元素强制转换为新类;为什么这些课程（foo，foo[2]或foo[[2]]）似乎与"POSIXlt" "POSIXt"的班级（earlydate）相匹配变量？
如何正确初始化foo，使其成为包含单个元素的向量，列表或类数组的东西，这些元素能够支持此类值赋值而不会引发警告并导致此类型反直觉行为？

Answer 1

嗯，POSIXlt的幕后实际上是一个列表。

> class(unclass(earlydate))
[1] "list"
> length(unclass(earlydate))
[1] 11

赋值为0，因为这是列表的第一个元素;它是秒数，earlydate为0。

> unclass(earlydate)[1]
$sec
[1] 0

我真的不知道为什么R不会自动将foo变量强制转换为POSIXlt类;我的猜测是，对一般日期的强制是很难的。很清楚在foo所有NA都在这里做什么，但如果其中一个元素已经是整数或字符串呢？但要先自己强迫它，请使用as.POSIXlt。

foo <- rep(as.POSIXlt(NA), 5)

根据您对其进行的其他操作，您可能还会发现使用POSIXct作为更好的解决方案，正如Integer to POSIXlt的答案所述。它将日期存储为自原点以来的秒数，因此不是下面的列表，因此更适合某些类型的计算和存储。

R警告：＆＃34;要更换的项目数量不是更换长度的倍数＆＃34;似乎不正确

1 个答案: