4.5 缺失值，4.6 日期值

y <- c(1, 2, 3, NA)
is.na(y)

is.na用以识别是否有缺失值，返回逻辑值

# Recode 99 to missing for the variable age
leadership[age == 99, "age"] <- NA#定义列名称为age的列，此时行值为99的时候为NA
leadership

确实缺失值

# Excluding missing values from analyses
x <- c(1, 2, NA, 3)
y <- x[1] + x[2] + x[3] + x[4]
z <- sum(x)
e <- sum(x, na.rm=TRUE)#删除缺失值后运算才能有结果
y;z;e

运算前必须删除缺失值

leadership
newdata <- na.omit(leadership)
newdata

na.omit删除缺失行

mydates <- as.Date(c("2007-06-22", "2004-02-13"))
str(mydates)

![利用as.Date]函数进行日期格式的转化(https://img.haomeiwen.com/i22546862/ba79a4a4eb5f64f4.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

strDates <- c("01/05/1965", "08/16/1975")
class(strDates)
dates <- as.Date(strDates, format = "%m/%d/%Y")
class(dates)

利用format参数设定个格式

myformat <- "%m/%d/%y" 
str(leadership)
leadership$date <- as.Date(leadership$date, myformat)
str(leadership)

转换成日期格式

today <- Sys.Date()
format(today, format="%B %d %Y")
format(today, format="%A")

format指定和提取日期信息

startdate <- as.Date("2004-02-13")
enddate   <- as.Date("2009-06-22")
enddate - startdate

日期的计算

today <- Sys.Date()
dob <- as.Date("1966-10-12")
difftime(today, dob, units="days")

利用diftime函数计算日期间隔并指定时间单位

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