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In [5]: format(123456789,',d')
Out[5]: '123,456,789'

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>
(define (format1000 n)
  (setq s (reverse (string n)))
  (setq acc "")
  (for (i 0 (dec (length s)))
    (when (and (> i 0) (zero? (% i 3)) (number? (int (s i))))
      (setq acc (append acc ",")))
    (setq acc (append acc (s i))))
  (reverse acc))
> (format1000 123456789)
"123,456,789"
> (format1000 -123456789)
"-123,456,789"

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(define (explode* xs (width 1) (bool nil) (acc '()))
  "Works like `explode`, but operates from right to left."
  (if (empty? xs)
      acc
      (< (length xs) width)
      (if bool acc (cons xs acc))
      (explode* (slice xs 0 (- width)) width bool
                (cons (slice xs (- width) width) acc))))

(define (humanize-integer integr (sep ","))
  (string (sgn integr "-" "" "")
          (join (explode* (string (abs integr)) 3) sep)))

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> (humanize-integer 123456789)
(humanize-integer 123456789)
"123,456,789"
> (humanize-integer -123456789)
(humanize-integer -123456789)
"-123,456,789"

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> (format "%'d" 123456789)
"123,456,789"

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(define (format1000 n)
	(append (sgn n "-" "" "") (reverse (join (explode (reverse (string (abs n))) 3) ","))))
(format1000 123456789)
(format1000 -123456789)

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(define (format1000 n)
   (append (and (find {^(.*)(\..*)} (string (abs n)) 1) (append (sgn n "-" "" "") (reverse (join (explode (reverse $1) 3) ",")))) (or $2 "")))