一些函数

table()函数

table()的输出可以看成是一个带名字的数字向量。可以用names()和as.numeric()分别得到名称和频数：

> x <- sample(c("a", "b", "c"), 100, replace=TRUE)
> names(table(x))
[1] "a" "b" "c"
> as.numeric(table(x))
[1] 42 25 33

或者，可以直接把输出结果转化为数据框，as.data.frame()：

> as.data.frame(table(x))
  x Freq
1 a   42
2 b   25
3 c   33

为一个变量名称赋值。 Assign a Value to a Name Assign a value to a name in an environment.

用法：

assign(x, value, pos = -1, envir = as.environment(pos),
inherits = FALSE, immediate = TRUE)

参数：

x：变量名称，字符串类型，x只能是一个字符串，如果为字符串向量，则会使用向量的第一个元素，并给出warning；
- a variable name, given as a character string. No coercion is done, and the first element of a character vector of length greater than one will be used, with a warning.
value：赋给x的变量值；
pos：where to do the assignment. By default, assigns into the current environment.
envir：the environment to use.
inherits：should the enclosing frames of the environment be inspected?
immediate：an ignored compatibility feature.

根据变量名称获取该变量的值。 Return the Value of a Named Object； Search by name for an object (get) or zero or more objects (mget)

用法：

get(x, pos = -1, envir = as.environment(pos), mode = "any",
inherits = TRUE)

mget(x, envir = as.environment(-1), mode = "any", 
ifnotfound, inherits = FALSE)

参数：

x：
- For get, an object name (given as a character string).
- For mget, a character vector of object names.
pos, envir：where to look for the object ; if omitted search as if the name of the object appeared unquoted in an expression.
mode： the mode or type of object sought;
inherits： should the enclosing frames of the environment be searched?
ifnotfound： For mget, a list of values to be used if the item is not found: it will be coerced to a list if necessary.

> for(i in 1 : 6){
+   assign(paste('var', i, sep = ''), i)
+   print(get(paste('var', i, sep = '')))  
+ }
[1] 1
[1] 2
[1] 3
[1] 4
[1] 5
[1] 6

函数attach()可将数据框添加到R的搜索路径中。R在遇到一个变量名以后，将检查搜索路径中的数据框，以定位到这个变量; detach()去除一个数据框的绑定。

函数 search 显示当前的搜索路径。它可以用来跟踪已经被绑定或者绑定去除的列表和数据框(以及包)。