I like to track my expenses in Org tables. For example:

```#+tblname: daily-expenses
,| Expense | Category |
,|---------+----------|
,|       2 | Food     |
,|       1 | House    |
,|      10 | Clothes  |
,|       3 | Food     |
,|       5 | House    |
```

At the of the month, I want to see how much was spent in each category. I wrote the following Babel function to calculate this:

```#+name: group-categories
#+begin_src emacs-lisp :var table=daily-expenses :colnames nil
,  (defun category-sums (catcol valcol)
,    "Sum each of the categories in an Org table.
,
,  \(fn CATEGORYCOLUMN VALUECOLUMN)"
,    (let (gcats
,          result)
,      (dolist (row table)
,        (let ((cat (nth (1- catcol) row)))
,          (when (not (member cat gcats))
,            (push cat gcats))))
,      (dolist (gcat gcats)
,        (let ((sum 0))
,          (dolist (row table)
,            (let ((val (nth (1- valcol) row))
,                  (cat (nth (1- catcol) row)))
,              (when (equal gcat cat)
,                (incf sum val))))
,          (push (list gcat sum) result)))
,      result))
,  (category-sums 2 1)
#+end_src
```

Here is an example of the output:

```#+results: group-categories
,| Category | Expense |
,|----------+---------|
,| Food     |       5 |
,| House    |       6 |
,| Clothes  |      10 |
```

Dan Davison provided an alternate way of doing this using an R function:

```#+begin_src R :var tab=daily-expenses :colnames yes
,  attach(tab)
,  aggregate(Expense ~ Category, FUN=sum)
#+end_src
```

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