数值模式匹配(Numerical Pattern Matching)

 如果您担心做某事的成本，那就去衡量一下。 如果你无法衡量差异，那么差异可能无关紧要（如果重要的话，你就可以衡量它）。 
 
 要回答更具体的问题，变量没有任何内在成本。 在CPU上实际执行的代码中不存在变量。 它们是源级构造，并且编译器没有义务在源代码变量之间创建某种一对一映射，例如，执行push指令，或使用的寄存器或其他任何东西。 代码中的单个变量可以对应于使用多个不同的寄存器（编译器可能会选择不时地移动它），或者几个不同的变量可能最终共享同一个寄存器，因为它们从未在同时。 
 
 编译器所做的许多事情之一是确定变量的生命周期。 何时首次使用，何时使用？ 当你声明它或它超出范围时，无关紧要。 
 
 编译器通常使用的转换将您的代码放入更适合优化的形式，即将每个赋值转换为一个新的，唯一的变量，基本上与第二个示例类似。 这就是编译器喜欢使用代码的方式，因为现在它可以清楚地看到依赖关系。 在第一种情况下，看起来整个函数中必须存在相同的变量，并且可以重用这两个操作。 在第二种情况下，我们可以很容易地看到它们是独立的，独立的变量，并且我们可以看到它们的生命周期不重叠（在声明rateNode之后不使用rateNode ）。 
 
 两个版本都可能生成完全相同的代码，但第二个版本更接近于优化编译器内部对代码所做的操作。 

If you're worried about the cost of doing something, then measure it. If you can't measure a difference, then the difference can not possibly matter (if it mattered, you'd be able to measure it).
 
To answer your more specific question, a variable does not have any intrinsic cost. Variables do not exist in the code that is actually executed on the CPU. They are a source-level construct, and the compiler is not obliged to create some kind of 1-to-1 mapping between source code variables and, say, push instructions executed, or registers used, or anything else. A single variable in your code can correspond to the use of several different registers (the compiler might choose to move it around from time to time), or several different variables might all end up sharing the same register because they're never used at the same time.
 
One of the many things a compiler does is determine the lifetime of a variable. When is it first used, and when is it last used? It doesn't matter when you declared it, or when it goes out of scope. 
 
A transformation commonly used by compilers to put your code into a form more amenable to optimizations is to turn every assignment into a new, unique, variable, basically like in your second example. That's how the compiler prefers to work with the code, because now it can see the dependencies clearly. In the first case, it looks as if the same variable has to exist throughout the function, and be reused for both operations. In the second case, we can easily see that they're separate, independent, variables, and we can see that their lifetimes do not overlap (idNode is not used after the declaration of rateNode).
 
Both versions will likely produce the exact same code, but the second version is closer to what optimizing compilers do to the code internally.