• for循环是一种重复控制结构，可以让您有效地编写一个需要执行特定次数的循环。

for (value in vector) {
   statements
}

示例：

j <- LETTERS[1:4]
for ( i in j) {
   print(i)
}

它产生如下结果：

[1] "A"
[1] "B"
[1] "C"
[1] "D"

下标访问

> a <- c("a","b","c","d")
> for (i in c(1:length(a)))
+ {print(i)}

[1] 1
[1] 2
[1] 3
[1] 4

元素访问

> a <- c("a","b","c","d")
> for (i in c(1:length(a)))
+ {print(a[i])}

[1] "a"
[1] "b"
[1] "c"
[1] "d"

循环嵌套

> a <- matrix(c(1:50),nrow=5)
> a
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,]    1    6   11   16   21   26   31   36   41    46
[2,]    2    7   12   17   22   27   32   37   42    47
[3,]    3    8   13   18   23   28   33   38   43    48
[4,]    4    9   14   19   24   29   34   39   44    49
[5,]    5   10   15   20   25   30   35   40   45    50
> for (i in c(1:nrow(a)))
+   for (j in c(1:ncol(a)))
+   {print(a[i,j])}
[1] 1
[1] 6
[1] 11
[1] 16
[1] 21
[1] 26
[1] 31
[1] 36
[1] 41
[1] 46
[1] 2
[1] 7
[1] 12
[1] 17
[1] 22
[1] 27
[1] 32
......
......