从1~10随机有放回选7个数，求10恰好出现2次的概率．分别作理论和模拟分析
直接R模拟（大数定律） doubleten<-function(n){ m<-0 for (i in 1:n) { u<-runif(7) x<-floor(u*10)+1 if(sum(x==10)==2){m=m+1} } rt<-c('两个10的概率'=m/n);rt}doubleten(10000) 其中函数说明sum
是不是很奇怪为什么这里sum实现了统计某个值个数的操作！！！！
下面是官网文档，没有显示特别的关于统计某个值个数的操作。
Description
sum returns the sum of all the values present in its arguments.
Usage
sum(…, na.rm = FALSE)
Arguments
…
numeric or complex or logical vectors.
na.rm
logical. Should missing values (including NaN) be removed?
Examples
Pass a vector to sum, and it will add the elements together.
sum(1:5)
Pass several numbers to sum, and it also adds the elements.
sum(1, 2, 3, 4, 5)
In fact, you can pass vectors into several arguments, and everything gets added.
sum(1:2, 3:5)
If there are missing values, the sum is unknown, i.e., also missing, ….
sum(1:5, NA)
… unless we exclude missing values explicitly:
sum(1:5, NA, na.rm = TRUE)
调试分析，原来原因如下
> a<-c(1,2,2,3,3,3)> a==3[1] FALSE FALSE FALSE TRUE TRUE TRUE> sum(a==3)[1] 3赞叹此法真的巧妙！！！
运行结果 > doubleten<-function(n){+ m<-0+ for (i in 1:n) {+ u<-runif(7)+ x<-floor(u*10)+1+ if(sum(x==10)==2){m=m+1}+ }+ rt<-c('两个10的概率'=m/n);rt+ }> doubleten(10000)两个10的概率 0.122 DONE！！！