使用“:”（冒号）时的运算符优先顺序

Question

I am trying to extract values from a vector using numeric vectors expressed in two seemingly equivalent ways:

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?

Answer 1

Because the operator : has precedence over + so 1+1:3 is really 1+(1:3) (ie 2:4 ) and not 2:3 . Thus, to change the order of execution as defined operator precedence, use parentheses ()

You can see the order of precedence of operators in the help file ?Syntax . Here is the relevant part:

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