字符串中NA的传染性和特殊性

NA ^ 0
#> [1] 1
NA | TRUE
#> [1] TRUE
NA & FALSE
#> [1] FALSE

3. Attributes

1. matrices, arrays, factors, or date-times.是在atomic vectors的基础上添加属性完成的
2. Individual attributes can be retrieved and modified with attr(), or retrieved en masse with attributes(), and set en masse with structure(). ```r a <- 1:3 attr(a, “x”) <- “abcdef” attr(a, “x”)

   > [1] “abcdef”

attr(a, “y”) <- 4:6 str(attributes(a))

> List of 2

> $ x: chr “abcdef”

> $ y: int [1:3] 4 5 6

Or equivalently

a <- structure( 1:3, x = “abcdef”, y = 4:6 ) str(attributes(a))

> List of 2

> $ x: chr “abcdef”

> $ y: int [1:3] 4 5 6

属性通常应该被认为是短暂的。例如，大多数操作都会丢失大多数属性：
```r
attributes(a[1])
#> NULL
attributes(sum(a))
#> NULL

如果你想要保留其他属性，您需要创建自己的 S3 类

3.1 summary

4. S3 atomic vectors

最重要的向量属性之一是class，拥有一个类属性（class）会将一个对象变成一个S3 对象，
S3 对象被传递给一个通用函数（generic function）时，它的行为将与一个常规向量不同。

4.1

four important S3 vectors used in base R:

• Categorical data, where values come from a fixed set of levels recorded in factor vectors.
• Dates (with day resolution), which are recorded in Date vectors.
• Date-times (with second or sub-second resolution), which are stored in POSIXct vectors.
• Durations, which are stored in difftime vectors.
•