Question

我在R中创建了一个while循环，它找到了在生产率/ ha不大于最大生产率的条件下生成集合总生产（prodTarget）所需的给定区域的最小比例。

while循环本身工作正常并完全按照它应该做的，问题发生在循环之后，我从最小比例（由while循环给出）创建一个序列到1。 while循环找到的最小比例为1，seq（）命令返回错误（下面代码中的第一个版本）。但是，seq（1,1,0.1）手动正常工作并按预期返回1.

conservation_Yield<-8.4 # set minimal Yield/ha
max_current_Yield<-173.1 # the maximum observed Yield/ha
Area <- 226.02 # the area in ha

min_target <- conservation_Yield*Area
max_target <- max_current_Yield*Area
prodTargets <- seq(min_target,max_target,Area) # set a range of production targets

# Start loop over all production targets (166 in total, the last two loops are the problem)
for (i in 165: length(prodTargets)){

  currentTarget <- prodTargets[i]
  # Define the range of proportion managed land from the minimum feasable (given the target and maximum productivity) 
  #          to maximum possible

  min_proportion <- 0 # to find the minimum feasable, we set the proportion to zero
  control_Yield <- max_current_Yield*10 # set to arbitrary higher value as max possible yield

  while (control_Yield > max_current_Yield){
    min_proportion <- min_proportion+0.01 # increase the proportion of managed land
    rest_productivity <- currentTarget-conservation_Yield*Area*(1-min_proportion)
    control_Yield <- rest_productivity/(Area*min_proportion)
  }

  max_proportion <- 1 # the maximum possible proportion is always 1

  # First version: works fine for all loops except where min_proportion has to be 1 as well
  landsparing_scenarios <- seq(min_proportion,max_proportion,0.01) # returns an error

  # Second version: Check if min_proportion == 1 and create a vector of 1 manually
    if (min_proportion == 1) { # returns FALSE even though min_proportion is 1
    landsparing_scenarios <- 1
  } else {
    landsparing_scenarios <- seq(min_proportion,max_proportion,0.01)

  }

  # Third version: Check if min_proportion > 0.99 and create a vector of 1 manually
  if (min_proportion > 0.99) {
    landsparing_scenarios <- 1
  } else {
    landsparing_scenarios <- seq(min_proportion,max_proportion,0.01)

  }

}

我添加了一个if-else子句来检查最小比例是否等于1（第二个版本）。现在，即使min_proportion在控制台中键入时返回数字1，命令min_proportion == 1也会返回FALSE。变量怎么可能看起来是1而逻辑运算符不一致？

Answer 1

在while循环期间，每次迭代时将0.01的值添加到最小比例。虽然min_proportion的输出表明循环的最后一步是min_proportion = 0.99+0.01，但结果数在第16位时大于1。

尝试使用seq(min_proportion,1,0.01)创建序列会导致错误，因为min_proportion实际上是1.000000000000001，大于1，因此这个序列是不可能的。这也是测试min_proportion == 1返回FALSE，但测试min_proportion > 0.99返回TRUE的原因。

一种可能的解决方案是在while循环后将变量min_proportion舍入为2位数。另一种可能性（正如@lmo所指出的）是使用不如all.equal(min_proportion, 1)严格的函数==。

R：while循环产生数值不清楚的数字

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