3.5.3 Emacs Lisp forms as formulas

It is also possible to write a formula in Emacs Lisp. This can be useful for string manipulation and control structures, if Calc’s functionality is not enough.

If a formula starts with a single-quote followed by an opening parenthesis, then it is evaluated as a Lisp form. The evaluation should return either a string or a number. Just as with Calc formulas, you can specify modes and a printf format after a semicolon.

With Emacs Lisp forms, you need to be conscious about the way field references are interpolated into the form. By default, a reference is interpolated as a Lisp string (in double-quotes) containing the field. If you provide the ‘N’ mode switch, all referenced elements are numbers—non-number fields will be zero—and interpolated as Lisp numbers, without quotes. If you provide the ‘L’ flag, all fields are interpolated literally, without quotes. For example, if you want a reference to be interpreted as a string by the Lisp form, enclose the reference operator itself in double-quotes, like ‘"$3"’. Ranges are inserted as space-separated fields, so you can embed them in list or vector syntax.

Here are a few examples—note how the ‘N’ mode is used when we do computations in Lisp:

‘'(concat (substring $1 1 2) (substring $1 0 1) (substring $1 2))’

Swap the first two characters of the content of column 1.

‘'(+ $1 $2);N’

Add columns 1 and 2, equivalent to Calc’s ‘$1+$2’.

‘'(apply '+ '($1..$4));N’

Compute the sum of columns 1 to 4, like Calc’s ‘vsum($1..$4)’.