Is there a way to do something like the following?
(format t "~{~va~}" '("aa" "bb" "cc") 4)
I need to iterate through a list. Each element of that list should be padded with a variable number of spaces (specified at runtime, so I cannot use "~4a").
Or more generally, is there a way to refer to a specific argument in the argument list of FORMAT?
By nesting format function, you can do what you want.
(format t (format nil "~~{~~~Aa~~}" 4) '("aa" "bb" "cc"))
;; returns: aa bb cc
Here the inner format directive:
The nil as first argument, format returns a string.
(format nil "~~{~~~Aa~~}" 4)
;; returns: "~{~4a~}" - and this is exactly what you want to give
;; to the outer `format` as second argument!
You can of course write a function for this:
(defun format-by-padding-over (lst padding)
(format t (format nil "~~{~~~Aa~~}" padding) lst))
And then:
(format-by-padding-over '("aa" "bb" "cc") 4)
;; aa bb cc
;; NIL
I learned this trick here from @Sylwester (many thanks!).
You could also interleave the list with repetitions of the padding:
(format t "~{~va~}"
(mapcan (lambda (element)
(list 4 element))
list))
You can build the format control string using nested format functions, but then you have to take care about escaping tildes. When working with regular expressions (using CL-PPCRE), one can define regular expressions using trees, like (:alternation #\\ #\*), which helps preventing bugs and headaches related to escaping special characters. The same can be done with format strings, using format-string-builder, available in Quicklisp:
(lambda (v)
(make-format-string `((:map () (:str ,v)))))
Returns a closure, which can be used to build format strings:
(funcall * 10)
=> "~{~10a~}"
