The form of the assignment expression is generally left-hand-side <- right-hand-side. Where the left-hand-side, when evaluated, yields a symbol while the right-hand-side yields a value.
The assignment operator, <-, is overloaded. An operator is said to be overloaded if it performs more than one task. In R and S the assignment operator performs two tasks. If the left-hand side is a symbol that already exists in the current environment, then the value of that symbol is changed to the right-hand side. If the symbol does not exist in the current environment, then it is created and the value of the right-hand side is assigned to it.
Most programming languages separate these two tasks. Overloading them can make programming easier but can also create serious problems in writing compilers and in understanding the code. Consider the following unusual, but legal, code segment:
foo <-
function(x) {
if (x < 10)
y <- 12
x + y
}
If
>foo(11)
错误于foo(11) : 找不到对象'y'
> foo(5)
[1] 17
Now, one cannot determine whether y is a local variable or a global variable until the function is evaluated. If x<10, then y in x+y is a local variable and otherwise it is a global variable.
However, the second type of assignment, <<- can deal with this problem.
In R, the semantics of the operator are such that starting with the parent of the current environment and searching up through the enclosing environments the symbol on the left-hand side is sought. If it is found then the value of the right hand side is associated with it. If it is not found once the top-level environment has been searched, then the symbol on the left-hand side is assigned to it.
foo <-
function(x) {
if (x < 10)
y <<- 12
x + y
}
> foo(5)
[1] 17
> foo(11)
[1] 23
PS
An environment is a collection of symbols and associated values.
A function closure is a function together with an environment.
The symbols that occur in the body of a function can be divided into two groups; bound variables and free variables. The formal parameters of a function are those occurring in the argument list of the function. Any variable in the body of the function that matches one of the formal parameters is bound to that formal parameter. Local variables, those whose values are determined by the evaluation of expressions in the body of the functions, are bound to the values of the expressions. All other variables are called free variables. At the time that the function is evaluated, a binding must be obtained for all variables
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