Arrays R警告：“；要更换的项目数量不是更换长度的倍数”；似乎不正确

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]]))

> 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
> 

#创建示例数据
日期嗯，
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；这是秒数，对于
earlydate
，它是0

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

我真的不知道为什么R不自动将
foo
变量强制到
POSIXlt
类中；我的猜测是，强迫约会通常是很难的。当所有
foo
都是
NA
时，这里很清楚该怎么做，但是如果其中一个元素已经是整数或字符串呢？但要先自己强制执行，请使用
as.POSIXlt

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

foo