Io 如何从输入文件中引用列表

有很多关于如何在scheme中输入的手册，但我不知道如何使用它。 假设我们有一个简单的txt文件作为输入 a、 txt:

1 2 3

我正在调用它的标准io过程

(let ((p (open-input-file "a.txt")))
  (let f ((x (read p))) ; reading from file
    (if (eof-object? x) ; check for eof
    (begin
      (close-input-port p)
      '()
      )
   (cons x (f (read p))))))

输出：

"(1 2 3)

这是我正在寻找的东西，但如果我想对其进行标准的配对/列表操作，我没有得到我所期望的：

(let ((p (open-input-file "a.txt")))
  (let f ((x (read p))) ; reading from file
    (if (eof-object? x) ; check for eof
    (begin
      (close-input-port p)
      '()
      )
   (cdr 
      (cons x (f (read p)))))))

输出：

“（）

---

如何获取列表的其余部分？

请注意，要操作的列表是由

（let…

表达式构建的，即：

(let ((p ...)) ...)                       ;; => '(1 2 3)

(cdr (let ((p ...)) ...))                 ;; => '(2 3)

因此，要获得列表的“其余部分”，必须对整个let表达式应用

cdr

，即：

(let ((p ...)) ...)                       ;; => '(1 2 3)

(cdr (let ((p ...)) ...))                 ;; => '(2 3)

在let表达式外部应用

cdr

与在内部应用不同。当您在外部应用它时，let表达式首先被计算为一个列表，该列表

cdr

然后进行操作以获取其“rest”值。这相当于写：

(cdr '(1 2 3))                            ;; => '(2 3)

但是如果你在它的主体中应用它，那么在每次递归调用

f

时，其中一个

cons

单元格被递归地追加到列表中，你最终会将一个

cdr

链接到它，而不是像下面那样构建一个cons链：

(cons 1 (cons 2 (cons 3 '())))                            ;; => '(1 2 3)

(define p (open-input-file "a.txt"))

;; (let f (...) ...) is a named let,
;; and is equivalent to the function below.
(define (f x)
  (if (eof-object? x)
      '()
      (cons x (f (read p)))))

(f (read p))

(close-input-port p)

(f (read p))
=> (f 1)
=> (cons 1 (f (read p)))
=> (cons 1 (f 2))
=> (cons 1 (cons 2 (f (read p))))
=> (cons 1 (cons 2 (f 3)))
=> (cons 1 (cons 2 (cons 3 (f (read p)))))
=> (cons 1 (cons 2 (cons 3 (f eof))))
=> (cons 1 (cons 2 (cons 3 '())))    ;; => '(1 2 3)

(define (f x)
  (if (eof-object? x)
      '()
      (cdr (cons x (f (read p))))))

而是构建一个表单链：

(cdr (cons 1 (cdr (cons 2 (cdr (cons 3 '()))))))          ;; => '()

---

要了解这是如何发生的，最好从思想上考虑或写下程序的执行流程。这使您能够理解表达式的计算结果

例如，可以将第一个let表达式重写为等效函数，如下所示：

(cons 1 (cons 2 (cons 3 '())))                            ;; => '(1 2 3)

(define p (open-input-file "a.txt"))

;; (let f (...) ...) is a named let,
;; and is equivalent to the function below.
(define (f x)
  (if (eof-object? x)
      '()
      (cons x (f (read p)))))

(f (read p))

(close-input-port p)

(f (read p))
=> (f 1)
=> (cons 1 (f (read p)))
=> (cons 1 (f 2))
=> (cons 1 (cons 2 (f (read p))))
=> (cons 1 (cons 2 (f 3)))
=> (cons 1 (cons 2 (cons 3 (f (read p)))))
=> (cons 1 (cons 2 (cons 3 (f eof))))
=> (cons 1 (cons 2 (cons 3 '())))    ;; => '(1 2 3)

(define (f x)
  (if (eof-object? x)
      '()
      (cdr (cons x (f (read p))))))

它的执行流程如下所示：

(cons 1 (cons 2 (cons 3 '())))                            ;; => '(1 2 3)

(define p (open-input-file "a.txt"))

;; (let f (...) ...) is a named let,
;; and is equivalent to the function below.
(define (f x)
  (if (eof-object? x)
      '()
      (cons x (f (read p)))))

(f (read p))

(close-input-port p)

(f (read p))
=> (f 1)
=> (cons 1 (f (read p)))
=> (cons 1 (f 2))
=> (cons 1 (cons 2 (f (read p))))
=> (cons 1 (cons 2 (f 3)))
=> (cons 1 (cons 2 (cons 3 (f (read p)))))
=> (cons 1 (cons 2 (cons 3 (f eof))))
=> (cons 1 (cons 2 (cons 3 '())))    ;; => '(1 2 3)

(define (f x)
  (if (eof-object? x)
      '()
      (cdr (cons x (f (read p))))))

但是如果您将

cdr

应用于递归调用，如下所示：

(cons 1 (cons 2 (cons 3 '())))                            ;; => '(1 2 3)

(define p (open-input-file "a.txt"))

;; (let f (...) ...) is a named let,
;; and is equivalent to the function below.
(define (f x)
  (if (eof-object? x)
      '()
      (cons x (f (read p)))))

(f (read p))

(close-input-port p)

(f (read p))
=> (f 1)
=> (cons 1 (f (read p)))
=> (cons 1 (f 2))
=> (cons 1 (cons 2 (f (read p))))
=> (cons 1 (cons 2 (f 3)))
=> (cons 1 (cons 2 (cons 3 (f (read p)))))
=> (cons 1 (cons 2 (cons 3 (f eof))))
=> (cons 1 (cons 2 (cons 3 '())))    ;; => '(1 2 3)

(define (f x)
  (if (eof-object? x)
      '()
      (cdr (cons x (f (read p))))))

然后执行将更改为以下内容：

(f (read p))
=> (f 1)
=> (cdr (cons 1 (f (read p))))
=> (cdr (cons 1 (f 2)))
=> (cdr (cons 1 (cdr (cons 2 (f (read p))))))
=> (cdr (cons 1 (cdr (cons 2 (f 3)))))
=> (cdr (cons 1 (cdr (cons 2 (cdr (cons 3 (f (read p))))))))
=> (cdr (cons 1 (cdr (cons 2 (cdr (cons 3 (f eof)))))))
=> (cdr (cons 1 (cdr (cons 2 (cdr (cons 3 '()))))))
=> (cdr (cons 1 (cdr (cons 2 '()))))
=> (cdr (cons 1 '()))
=> '()

---

请注意，要操作的列表是由

（let…

表达式构造的，即：

(let ((p ...)) ...)                       ;; => '(1 2 3)

(cdr (let ((p ...)) ...))                 ;; => '(2 3)

因此，要获得列表的“其余部分”，必须对整个let表达式应用

cdr

，即：

(let ((p ...)) ...)                       ;; => '(1 2 3)

(cdr (let ((p ...)) ...))                 ;; => '(2 3)

在let表达式外部应用

cdr

与在内部应用不同。当您在外部应用它时，let表达式首先被计算为一个列表，该列表

cdr

然后进行操作以获取其“rest”值。这相当于写：

(cdr '(1 2 3))                            ;; => '(2 3)

但是如果你在它的主体中应用它，那么在每次递归调用

f

时，其中一个

cons

单元格被递归地追加到列表中，你最终会将一个

cdr

链接到它，而不是像下面那样构建一个cons链：

(cons 1 (cons 2 (cons 3 '())))                            ;; => '(1 2 3)

(define p (open-input-file "a.txt"))

;; (let f (...) ...) is a named let,
;; and is equivalent to the function below.
(define (f x)
  (if (eof-object? x)
      '()
      (cons x (f (read p)))))

(f (read p))

(close-input-port p)

(f (read p))
=> (f 1)
=> (cons 1 (f (read p)))
=> (cons 1 (f 2))
=> (cons 1 (cons 2 (f (read p))))
=> (cons 1 (cons 2 (f 3)))
=> (cons 1 (cons 2 (cons 3 (f (read p)))))
=> (cons 1 (cons 2 (cons 3 (f eof))))
=> (cons 1 (cons 2 (cons 3 '())))    ;; => '(1 2 3)

(define (f x)
  (if (eof-object? x)
      '()
      (cdr (cons x (f (read p))))))

而是构建一个表单链：

(cdr (cons 1 (cdr (cons 2 (cdr (cons 3 '()))))))          ;; => '()

---

要了解这是如何发生的，最好从思想上考虑或写下程序的执行流程。这使您能够理解表达式的计算结果

例如，可以将第一个let表达式重写为等效函数，如下所示：

(cons 1 (cons 2 (cons 3 '())))                            ;; => '(1 2 3)

(define p (open-input-file "a.txt"))

;; (let f (...) ...) is a named let,
;; and is equivalent to the function below.
(define (f x)
  (if (eof-object? x)
      '()
      (cons x (f (read p)))))

(f (read p))

(close-input-port p)

(f (read p))
=> (f 1)
=> (cons 1 (f (read p)))
=> (cons 1 (f 2))
=> (cons 1 (cons 2 (f (read p))))
=> (cons 1 (cons 2 (f 3)))
=> (cons 1 (cons 2 (cons 3 (f (read p)))))
=> (cons 1 (cons 2 (cons 3 (f eof))))
=> (cons 1 (cons 2 (cons 3 '())))    ;; => '(1 2 3)

(define (f x)
  (if (eof-object? x)
      '()
      (cdr (cons x (f (read p))))))

它的执行流程如下所示：

(cons 1 (cons 2 (cons 3 '())))                            ;; => '(1 2 3)

(define p (open-input-file "a.txt"))

;; (let f (...) ...) is a named let,
;; and is equivalent to the function below.
(define (f x)
  (if (eof-object? x)
      '()
      (cons x (f (read p)))))

(f (read p))

(close-input-port p)

(f (read p))
=> (f 1)
=> (cons 1 (f (read p)))
=> (cons 1 (f 2))
=> (cons 1 (cons 2 (f (read p))))
=> (cons 1 (cons 2 (f 3)))
=> (cons 1 (cons 2 (cons 3 (f (read p)))))
=> (cons 1 (cons 2 (cons 3 (f eof))))
=> (cons 1 (cons 2 (cons 3 '())))    ;; => '(1 2 3)

(define (f x)
  (if (eof-object? x)
      '()
      (cdr (cons x (f (read p))))))

但是如果您将

cdr

应用于递归调用，如下所示：

(cons 1 (cons 2 (cons 3 '())))                            ;; => '(1 2 3)

(define p (open-input-file "a.txt"))

;; (let f (...) ...) is a named let,
;; and is equivalent to the function below.
(define (f x)
  (if (eof-object? x)
      '()
      (cons x (f (read p)))))

(f (read p))

(close-input-port p)

(f (read p))
=> (f 1)
=> (cons 1 (f (read p)))
=> (cons 1 (f 2))
=> (cons 1 (cons 2 (f (read p))))
=> (cons 1 (cons 2 (f 3)))
=> (cons 1 (cons 2 (cons 3 (f (read p)))))
=> (cons 1 (cons 2 (cons 3 (f eof))))
=> (cons 1 (cons 2 (cons 3 '())))    ;; => '(1 2 3)

(define (f x)
  (if (eof-object? x)
      '()
      (cdr (cons x (f (read p))))))

然后执行将更改为以下内容：

(f (read p))
=> (f 1)
=> (cdr (cons 1 (f (read p))))
=> (cdr (cons 1 (f 2)))
=> (cdr (cons 1 (cdr (cons 2 (f (read p))))))
=> (cdr (cons 1 (cdr (cons 2 (f 3)))))
=> (cdr (cons 1 (cdr (cons 2 (cdr (cons 3 (f (read p))))))))
=> (cdr (cons 1 (cdr (cons 2 (cdr (cons 3 (f eof)))))))
=> (cdr (cons 1 (cdr (cons 2 (cdr (cons 3 '()))))))
=> (cdr (cons 1 (cdr (cons 2 '()))))
=> (cdr (cons 1 '()))
=> '()

---

太多了！我需要它就行了。只是一个简单的问题。为什么我不能在let中以同样的方式进行cdr？无论如何，我都会将其分配给p。@Adam查看我上面的编辑，以了解为什么在计算列表的表达式之外应用

cdr

与在其主体内应用

cdr

不同。谢谢！我需要它就行了。只是一个简单的问题。为什么我不能在let中以同样的方式进行cdr？无论如何，我都会将其分配给p。@Adam请参阅我上面的编辑，以了解为什么在对列表求值的表达式外部应用

cdr

与在其主体中应用

cdr

不同。