在 Common Lisp 中，如何格式化浮点并指定分组、群聊和小数分隔符

假设我有浮点数1234.9

我想将其格式化为1.234,90

是否有格式指令组合？~D，可以处理分组和群聊，仅处理整数。~F根本不处理分组。据我所知，没有人可以改变小数点. to ,

我看到的唯一解决方案是使用~D对于整数部分数字分组并将其与,和小数部分。还有更好的想法吗？

您可以定义一个用波浪线斜杠调用的函数，大多数其他答案已经完成了，但是为了获得类似于 ~F 的输出，但注入了逗号字符，并替换了小数点，我认为它是最好调用 get ~F 产生的输出，然后修改它并将其写入字符串。这是一种使用实用程序来做到这一点的方法注入逗号以指定的间隔向字符串添加逗号字符。这是指令函数：

(defun print-float (stream arg colonp atp
                    &optional
                      (point-char #\.)
                      (comma-char #\,)
                      (comma-interval 3))
  "A function for printing floating point numbers, with an interface
suitable for use with the tilde-slash FORMAT directive.  The full form
is 

    ~point-char,comma-char,comma-interval/print-float/

The point-char is used in place of the decimal point, and defaults to
#\\.  If : is specified, then the whole part of the number will be
grouped in the same manner as ~D, using COMMA-CHAR and COMMA-INTERVAL.
If @ is specified, then the sign is always printed."
  (let* ((sign (if (minusp arg) "-" (if (and atp (plusp arg)) "+" "")))
         (output (format nil "~F" arg))
         (point (position #\. output :test 'char=))
         (whole (subseq output (if (minusp arg) 1 0) point))
         (fractional (subseq output (1+ point))))
    (when colonp
      (setf whole (inject-comma whole comma-char comma-interval)))
    (format stream "~A~A~C~A"
            sign whole point-char fractional)))

这里有些例子：

(progn 
  ;; with @ (for sign) and : (for grouping)
  (format t "~','.2@:/print-float/ ~%" 12345.6789) ;=> +1.23.45,679

  ;; with no @ (no sign) and : (for grouping)
  (format t "~'.'_3:/print-float/ ~%" 12345.678)   ;=>  12_345.678

  ;; no @ (but sign, since negative) and : (for grouping)
  (format t "~'.'_3:/print-float/ ~%" -12345.678)  ;=> -12_345.678

  ;; no @ (no sign) and no : (no grouping)
  (format t "~'.'_3@/print-float/ ~%" 12345.678))  ;=> +12345.678 (no :)

以下是来自的示例coredump-的答案 https://stackoverflow.com/a/35018021/1281433，这实际上帮助我发现了负数的错误：

CL-USER> (loop for i in '(1034.34 -223.12 -10.0 10.0 14 324 1020231)
            do (format t "~','.:/print-float/~%" i))
1.034,34
-223,12
-10,0
10,0
14,0
324,0
1.020.231,0
NIL

Here's 注入逗号，举一些例子：

(defun inject-comma (string comma-char comma-interval)
  (let* ((len (length string))
         (offset (mod len comma-interval)))
    (with-output-to-string (out)
      (write-string string out :start 0 :end offset)
      (do ((i offset (+ i comma-interval)))
          ((>= i len))
        (unless (zerop i)
          (write-char comma-char out))
        (write-string string out :start i :end (+ i comma-interval))))))

(inject-comma "1234567" #\, 3)
;;=> "1,234,567"

(inject-comma "1234567" #\. 2)
;;=> "1.23.45.67"