- 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：
代码语言：javascript复制j <- LETTERS[1:4]
for ( i in j) {
print(i)
}它产生如下结果：
代码语言：javascript复制[1] "A"
[1] "B"
[1] "C"
[1] "D"下标访问
代码语言：javascript复制> 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
代码语言：javascript复制> 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
代码语言：javascript复制> 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
......
......
