financial data 课本上的一道题目，参考答案给出的code有几条看不懂，也不太确定解题思路…
用R解Binomial二项式分布问题
题目：Assume that McDonald's log returns are normally distributed with mean and standard deviation equal to their estimates and that you have been made the following proposition by a friend: If at any point within the next 20 trading days, the price of McDonald's falls below 85 dollars, you will be paid $100, but if it does not, you have to pay him $1. The current price of McDonald's is at the end of the sample data, $93.07. Are you willing to make the bet? (Use 10,000 iterations in your simulation and use the command set.seed(2015) to ensure your results are the same as the answer key)
参考解答给出的code是：
> niter = 10000
> value = matrix(niter)
> set.seed(2015)
> for (i in 1:niter)
+ {
+ logR = rnorm(20,mean = mean(logReturn),sd = sd(logReturn))
+ prices = 93.07 * exp(cumsum(logR))
+ ind = (min(prices) < 85)
+ value[i] = 100*ind - (1 - ind)
+ }
> mu = mean(value)
> mu
[1] 0.0605
> mu + c(-1,1)* sqrt(mu*(1-mu)/niter)*qnorm(0.975)
[1] 0.05583 0.06517
请问其中的prices = 93.07 * exp(cumsum(logR)), 这一步的含义是什么？（为什么用93.07*exp(cumsum(logR)?）
value[i]是在求期望值吗？
题目要求里的setseed.(2015)又是什么意思/作用？