使用租用方案

问题描述

我想编写一个程序来查找Scheme中二次方程的根.我使用LET进行某些绑定.

I want to write a program to find the roots of the quadratic equation in Scheme. I used LET for certain bindings.

(define roots-with-let
  (λ (a b c)
    (let ((4ac (* 4 a c))
          (2a (* 2 a))
          (discriminant (sqrt ( - (* b b) (4ac)))))
      (cons ( / ( + (- b) discriminant) 2a)
            ( / ( - (- b) discriminant) 2a)))))

我用4ac定义了判别式，因为我不想使用(* 4 a c).即使我定义了(4ac (* 4 a c))，它也会给我这个错误:

I defined the discriminant with 4ac since I did not want (* 4 a c). Even though I have defined (4ac (* 4 a c)), it is giving me this error:

我的问题是如何评估(什么顺序)?如果我要在我的let中使用4ac，我应该再写一个内部let吗?有更好的方法吗?

My question is how is let evaluated (what order)? And if i want 4ac in my let should i write another inner let? Is there a better way to do this?

推荐答案

使用let*代替let.

let和let*之间的区别如下:

let*从左到右绑定变量.可以将较早的绑定用于右侧(或向下)的新绑定中.

let* binds variables from left to right. Earlier bindings can be used in new binding further to the right (or down).

let可以被视为简单lambda抽象的语法糖(或宏):

let on the other hand can be thought of as syntactic sugar (or macro) for simple lambda abstraction:

(let ((a exp1)
      (b exp2))
   exp)

等效于

((lambda (a b)
    exp)
 exp1 exp2)

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