I am trying to extract values from a vector using numeric vectors expressed in two seemingly equivalent ways:
我试图使用以两种看似相同的方式表示的数字向量从向量中提取值:
x <- c(1,2,3)
x[2:3]
# [1] 2 3
x[1+1:3]
# [1] 2 3 NA
I am confused why the expression x[2:3] produces a result different from x[1+1:3] -- the second includes an NA value at the end. What am I missing?
我很困惑,为什么表达式x [2:3]产生的结果与x [1 + 1:3]不同 - 第二个包括结尾的NA值。我错过了什么?
1 个解决方案
#1
10
Because the operator : has precedence over + so 1+1:3 is really 1+(1:3) (i. e. 2:4) and not 2:3. Thus, to change the order of execution as defined operator precedence, use parentheses ()
因为运算符:优先于+ 1所以1 + 1:3实际上是1+(1:3)(即2:4)而不是2:3。因此,要根据定义的运算符优先级更改执行顺序,请使用括号()
You can see the order of precedence of operators in the help file ?Syntax. Here is the relevant part:
您可以在帮助文件中查看运算符的优先顺序?语法。以下是相关部分:
The following unary and binary operators are defined. They are listed in precedence groups, from highest to lowest.
:::::access variables in a namespace$@component / slot extraction[[[indexing^exponentiation (right to left)-+unary minus and plus:sequence operator%any%special operators (including%%and%/%)*/multiply, divide+-(binary) add, subtract定义了以下一元和二元运算符。它们列在优先级组中,从最高到最低。 ::::访问命名空间中的变量$ @ component / slot extract [[[indexing ^ exponentiation(从右到左) - +一元减号和加号:序列运算符%任何%特殊运算符(包括%%和%/%) * /乘,除+ - (二进制)加,减
#1
10
Because the operator : has precedence over + so 1+1:3 is really 1+(1:3) (i. e. 2:4) and not 2:3. Thus, to change the order of execution as defined operator precedence, use parentheses ()
因为运算符:优先于+ 1所以1 + 1:3实际上是1+(1:3)(即2:4)而不是2:3。因此,要根据定义的运算符优先级更改执行顺序,请使用括号()
You can see the order of precedence of operators in the help file ?Syntax. Here is the relevant part:
您可以在帮助文件中查看运算符的优先顺序?语法。以下是相关部分:
The following unary and binary operators are defined. They are listed in precedence groups, from highest to lowest.
:::::access variables in a namespace$@component / slot extraction[[[indexing^exponentiation (right to left)-+unary minus and plus:sequence operator%any%special operators (including%%and%/%)*/multiply, divide+-(binary) add, subtract定义了以下一元和二元运算符。它们列在优先级组中,从最高到最低。 ::::访问命名空间中的变量$ @ component / slot extract [[[indexing ^ exponentiation(从右到左) - +一元减号和加号:序列运算符%任何%特殊运算符(包括%%和%/%) * /乘,除+ - (二进制)加,减