r - R中的牛顿法

Question

在尝试实现用于查找平方根值的牛顿方法的代码（使用迭代）时，我遇到了一个问题。一旦达到一定的准确性，我试图让函数停止打印值，但我似乎无法让它工作。下面是我的代码。

MySqrt <- function (x, eps = 1e-6, itmax = 100, verbose = TRUE){
  i <- 1
  myvector <- integer(0)
  GUESS <- readline(prompt="Enter your guess: ")
  GUESS <- as.integer(GUESS)
  while(i <= itmax){
      GUESS <- (GUESS + (x/GUESS)) * 0.5
      myvector <- c(myvector, GUESS)
      if (abs(GUESS-x) < eps) break
      i <- i + 1
  }

  myvector

为什么 if 语句不起作用？

Answer 1

更新：

请参阅@RichieCotton 对@agstudy 回答的评论。我同意 Richie 的观点，事实上使用@agstudy 的方法更有意义。

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原答案：

你的功能很好，你的数学是关闭的。
GUESS并且x不应该（必然）接近，但GUESS * GUESS应该x接近。

MySqrt <- function (x, eps = 1e-6, itmax = 100, verbose = TRUE){
  i <- 1
  myvector <- integer(0)
  GUESS <- readline(prompt="Enter your guess: ")
  GUESS <- as.integer(GUESS)
  while(i <= itmax){
      GUESS <- (GUESS + (x/GUESS)) * 0.5
      myvector <- c(myvector, GUESS)
      browser(expr={i == 10 || abs(GUESS-x) < eps})
      if (abs((GUESS*GUESS)-x) < eps) break    ###  <~~~~  SEE HERE
      i <- i + 1
  }

  myvector
}

Answer 2

这应该有效：

MySqrt <- function (x, eps = 1e-6, itmax = 100, verbose = TRUE){
  i <- 1
  myvector <- vector(mode='numeric',itmax)  ## better to allocate memory
  GUESS <- readline(prompt="Enter your guess: ")
  GUESS <- as.numeric(GUESS)
  myvector[i] <- GUESS
  while(i <= itmax){
    GUESS <- (GUESS + (x/GUESS)) * 0.5
    if (abs(GUESS-myvector[i]) < eps) break
    i <- i + 1
    myvector[i] <-  GUESS
  }
  myvector[seq(i)]
}

MySqrt(2)
Enter your guess: 1.4
[1] 1.400000 1.414286 1.414214