原程序运行都正常，原程序：     
    b <- .2          #actual value of beta
    w <- .25         #width of the uniform support set
   m <- 5000        #length of the chain
    burn <- 1000     #burn-in time
   days <- 250
    x <- numeric(m)  #the chain
    # generate the observed frequencies of winners
    i <- sample(1:5, size=days, replace=TRUE,
            prob=c(1, 1-b, 1-2*b, 2*b, b))
    win <- tabulate(i)
    print(win)
    prob <- function(y, win) {
        # computes (without the constant) the target density
        if (y < 0 || y >= 0.5)
            return (0)
        return((1/3)^win[1] *
            ((1-y)/3)^win[2] * ((1-2*y)/3)^win[3] *
                ((2*y)/3)^win[4] * (y/3)^win[5])
    }
    u <- runif(m)         #for accept/reject step
    v <- runif(m, -w, w)  #proposal distribution
    x[1] <- .25
    for (i in 2:m) {
        y <- x[i-1] + v
        if (u <= prob(y, win) / prob(x[i-1], win))
            x <- y  else
                x <- x[i-1]
    }
    print(win)
    print(round(win/days, 3))
    print(round(c(1, 1-b, 1-2*b, 2*b, b)/3, 3))
    xb <- x[(burn+1):m]
    print(mean(xb))
   
    par(mfrow=c(1,2))
    plot(x, type="l")
    abline(h=b, v=501, lty=3)
    xb <- x[- (1:501)]
    hist(xb, freq=F, xlab=bquote(beta), ylab="X", main="")
    z <- seq(min(xb), max(xb), length=100)
    lines(z, dnorm(z, mean(xb), sd(xb)))
  
改动后的程序出现了错误，我只把样本改成了4个，和prob改动了一点，改动后程序：
> b <- .2
> w <- .25
> m <- 10000
> burn <- 1000
> days <- 1604
> x <- numeric(m)
> i <- sample(1:4,size=days,replace=TRUE,prob=c(2*b,1-3*b,b,1))
> win <- tabulate(i)
> print(win)
[1] 294 327 146 837
> prob <- function(y,win) {
    if (y < 0 || y >= 0.5)
       return(0)
   return(((2*y)/2)^win[1]*((1-3*y)/2)^win[2]*((y)/2)^win[3]*(1/2)^win[4])
}
> u <- runif(m)
> v <- runif(m,-w,w)
> x[1] <- .25
> for (i in 2:m) {
  y <- x[i-1]+v
   if (u <= prob(y,win)/prob(x[i-1],win))
     x <- y
      else
      x <- x[i-1]
}
错误于if (u <= prob(y, win)/prob(x[i - 1], win)) x <- y else x <- x[i -  :
  需要TRUE/FALSE值的地方不可以用缺少值

原程序和改动后的程序红色字体是我对应改过的地方，可运行后就出现错误，原程序能够正常运行，请教指导！！谢谢！！