How do I append a character to the end of a String

Question

From my understanding Strings are vectors of type character. So far my attempts have been unfruitful

(vector-push #\a "Hol")
;; "Hol" Is not of type vector and simple-array

Is the literal "Hol" a constant vector in the same manner that '(1 2 3) is not the same as (list 1 2 3)? Should I just create a vector of characters explicitly and add characters to it?

Answer 1

You've got the right diagnosis. It's not even a matter of literal data (e.g., (list 1 2 3) vs. '(1 2 3)) versus modifiable data, though. It's whether the vector has a fill pointer. The documentation for vector-push and vector-push-extend say that the vector argument is a vector with a fill pointer. You'll get a similar error with non-literal arrays that don't have a fill pointer, as in:

(let ((v (make-array 3)))
  (vector-push nil v))

All you need to do is make sure that you create the vector with a fill pointer, and that it big enough to hold the things you push in:

(let ((v (make-array 2 :fill-pointer 0)))
  (print v)
  (vector-push 'x v)
  (print v)
  (vector-push 'y v)
  (print v)
  (vector-push 'z v)
  (print v))

vector-push doesn't adjust the array, so you get not z in the vector from the final vector-push:

#() 
#(X) 
#(X Y) 
#(X Y) 

If you make the vector adjustable, and use vector-push-extend, you can get a bigger array:

(let ((v (make-array 2 :adjustable t :fill-pointer 0)))
  (print v)
  (vector-push 'x v)
  (print v)
  (vector-push 'y v)
  (print v)
  (vector-push-extend 'z v)
  (print v))

#() 
#(X) 
#(X Y) 
#(X Y Z) 

Using an element type of character, you'll be doing this with strings:

(let ((v (make-array 2 :element-type 'character :adjustable t :fill-pointer 0)))
  (print v)
  (vector-push #\x v)
  (print v)
  (vector-push #\y v)
  (print v)
  (vector-push-extend #\z v)
  (print v))

"" 
"x" 
"xy" 
"xyz"