The following code excerpt will add up a column of numbers and yield results that are several decimal points long. Could anyone please give an example of how to convert the results into dollars and cents, rounding to the second decimal -- i.e., `1.555`

should round *up* to `1.56`

; and `1.554`

should round *down* to `1.55`

. Also, I'd like to insert comma separators every three digits to the left of the decimal point -- e.g., `1124412.555`

should be converted to `1,124,412.56`

.

```
(let ((sum 0))
(while (re-search-forward "[0-9]*\\.?[0-9]+" nil t)
(setq sum (+ sum (string-to-number (match-string 0)))))
(insert "\n\nTotal Hours: " (format "%s" sum ))
(insert "\n\nTotal Fee for Services Rendered: " (format "%s" (* 250 sum)))
(insert "\n\nOne-third of total fee: " (format "%s" (/ (* 250 sum) 3))))
2.0
0.2
0.1
4.75
4.0
6.5
0.1
Total Hours: 17.650000000000002
Total Fee for Services Rendered: 4412.500000000001
One-third of total fee: 1470.8333333333337
```

Based upon the answers provided by @Drew and @abo-abo, the following is a revised draft that now appears to be working correctly:

```
08884.75585
78774.1235
6.545
Total Hours: 87,665.42
Total Fee: $21,916,356.09
One-Third: $7,305,452.03
(let ((sum 0))
(while (re-search-forward "[0-9]*\\.?[0-9]+" nil t)
(setq sum (+ sum (string-to-number (match-string 0)))))
(setq total-hours (group-number (number-conversion (format "%s" sum ))))
(setq services-rendered (group-number (number-conversion (format "%s" (* 250 sum)))))
(setq one-third (group-number (number-conversion (format "%s" (/ (* 250 sum) 3)))))
(insert "\n\nTotal Hours: " total-hours)
(insert "\n\nTotal Fee: $" services-rendered)
(insert "\n\nOne-Third: $" one-third) )
;; @abo-abo
(defun number-conversion (str)
(let ((x (read str)))
(format "%0.2f" (* 0.01 (round (* 100 x)))) ))
;; http://www.emacswiki.org/emacs/ElispCookbook#toc23
(defun group-number (num &optional size char)
"Format NUM as string grouped to SIZE with CHAR."
;; Based on code for `math-group-float' in calc-ext.el
(let* ((size (or size 3))
(char (or char ","))
(str (if (stringp num)
num
(number-to-string num)))
(pt (or (string-match "[^0-9a-zA-Z]" str) (length str))))
(while (> pt size)
(setq str (concat (substring str 0 (- pt size))
char
(substring str (- pt size)))
pt (- pt size)))
str))
```

I believe abo-abo's `(* 0.01 (round (* 100 x)))`

can lead to surprises in corner cases (because 0.01 cannot be represented exactly in the internal floating point representation). A simpler solution for the rounding is:

```
(format "%0.2f" x)
```