Appending character to string in Common Lisp

Question

I have a character ch that I want to append to a string str. I realize you can concatenate strings like such:

(setf str (concatenate 'string str (list ch)))

But that seems rather inefficient. Is there a faster way to just append a single character?

Munsey

Answer 1

If the string has a fill-pointer and maybe is also adjustable.

Adjustable = can change its size.

fill-pointer = the content size, the length, can be less than the string size.

VECTOR-PUSH = add an element at the end and increment the fill-pointer.

VECTOR-PUSH-EXTEND = as VECTOR-PUSH, additionally resizes the array, if it is too small.

We can make an adjustable string from a normal string:

CL-USER 32 > (defun make-adjustable-string (s)
               (make-array (length s)
                           :fill-pointer (length s)
                           :adjustable t
                           :initial-contents s
                           :element-type (array-element-type s)))
MAKE-ADJUSTABLE-STRING

VECTOR-PUSH-EXTEND then can add characters to the end of the string. Here we add the character #\! to the end of the string "Lisp". The result string then is "Lisp!":

CL-USER 33 > (let ((s (make-adjustable-string "Lisp")))
               (vector-push-extend #\! s)
               s)
"Lisp!"

Motorboating

Answer 2

If you want to extend a single string multiple times, it is often quite performant to use with-output-to-string, writing to the stream it provides. Be sure to use write or princ etc. (instead of format) for performance.

Delve

or the obscure alternative, make sure that the FORMAT string can be compiled. – Motorboating