Iterating over strings

Question

I have two strings of the same length which differ in exactly one character and I want a string of all the characters which are pairwise equal. So basically something like this which evaluates to a string instead of a list of characters:

(loop for a across "abcd"
      for b across "abce"
      when (char= a b) collect a)

Although performance isn't an issue here, I found it cumbersome to have a (coerce ... 'string) around it.

So I came up with something like

(loop with result = ""
      for a across "abcd"
      for b across "abce"
      when (char= a b)
        do (setf result (concatenate 'string result (string a)))
      finally (return result))

which does the job but looks not very elegant to me.

(map 'string (lambda (a b) (when (char= a b) a)) "abcd" "abce")

looks nicer but is not working because NIL is not a character when aand bare not equal.

Is there a more elegant idiom to iterate over a string and get a string back?

Answer 1

Another possibility is to use mismatch as in the comment of David Hodge:

CL-USER> (defun f(a b)
           (let ((pos (mismatch a b)))
             (concatenate 'string (subseq a 0 pos) (subseq a (1+ pos)))))
F
CL-USER> (f "abcdefg" "abcxefg")
"abcefg"

Answer 2

(loop with result = ""
      for a across "abcd"
      for b across "abce"
      when (char= a b)
        do (setf result (concatenate 'string result (string a)))
      finally (return result))

Repeated concatenate are not a that good idea for longer strings.

Alternatives:

Loop into a list and coercing to a string

CL-USER 3 > (loop for a across "abcd"
                  and b across "abce"
                  when (char= a b) collect a into list
                  finally (return (coerce list 'string)))
"abc"

Using stream and converting it to a string

CL-USER 4 > (with-output-to-string (*standard-output*)
              (loop for a across "abcd"
                    and b across "abce"
                    when (char= a b) do (write-char a)))
"abc"

Using an adjustable string

CL-USER 5 > (loop with string = (make-array 0
                                            :element-type 'character
                                            :adjustable t
                                            :fill-pointer 0)
                  for a across "abcd"
                  for b across "abce"
                  when (char= a b) do (vector-push-extend a string)
                  finally (return string))
"abc"