Eval替代品 [英] Eval alternative

问题描述

此代码用作计算器，但codeacademy的便笺簿告诉我，eval是邪恶的。是否有其他方法可以在不使用eval的情况下执行相同的操作？

This code works as a calculator, but the scratch pad at codeacademy tells me that eval is evil. Is there another way to do the same thing without using eval?

var calculate = prompt("Enter problem");
alert(eval(calculate));

推荐答案

eval 将字符串输入计算为JavaScript，巧合的是JavaScript支持计算并理解 1 + 1 ，这使其适合作为计算器。

eval evaluates the string input as JavaScript and coincidentally JavaScript supports calculations and understands 1+1, which makes it suitable as a calculator.

如果您不想使用 eval ，这很好，您必须自己解析该字符串，最后自己进行计算（不是完全是你自己）。看看这个数学处理器，它可以满足您的需求。

If you don't want to use eval, which is good, you have to parse that string yourself and, finally, do the computation yourself (not exactly yourself though). Have a look at this math processor, which does what you want.

基本上你做的是：

1. 用char读取输入字符串char（有这种问题的话）仍然可能）


2. 构建您想要执行的操作树


3. 在字符串的末尾，您评估树并进行一些计算

例如，您有1 + 2/3，这可以评估到以下数据结构：

For example you have "1+2/3", this could evaluate to the following data structure:

     "+"
     /  \
   "1"  "/"
       /   \
     "2"   "3"

然后你可以遍历从上到下的结构并进行计算。
首先你有+，左边有一个，右边有一些表情，
所以你必须首先评估该表达式。所以你去了/节点，它有两个数字子节点。知道了，你现在可以计算 2/3 并用结果替换整个/节点。现在你可以再次上升并计算 + 节点的结果： 1 + 0.66 。现在你用结果替换那个节点，你剩下的就是表达式的结果。

You could then traverse that structure from top to bottom and do the computations. At first you've got the "+", which has a 1 on the left side and some expression on the right side, so you have to evaluate that expression first. So you go to the "/" node, which has two numeric children. Knowing that, you can now compute 2/3 and replace the whole "/" node with the result of that. Now you can go up again and compute the result of the "+" node: 1 + 0.66. Now you replace that node with the result and all you've got left is the result of the expression.

一些关于代码如何看待的伪代码：

Some pseudo code on how this might look in your code:

calculation(operator, leftValue, rightValue):
   switch operator {
      case '+': return leftValue + rightValue
      case '-': return 42
   }

action(node):
   node.value = calculation(node.operator, action(node.left) action(node.right))

您可能已经注意到，树是这样设计的它尊重运营商优先权的方式。 / 的级别低于 + ，这意味着它首先得到评估。

As you might have noticed, the tree is designed in such a way that it honors operator precedence. The / has a lower level than the +, which means it get's evaluated first.

但是你详细说明了这一点，基本上就是这样。

However you do this in detail, that's basically the way to go.

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