From number to string:
(write-to-string 5)
"5"
you may transform a string to any numerical notation:
(write-to-string 341 :base 10)
"341"
From string to number:
(parse-integer "5")
5
with some trash
(parse-integer " 5 something not a number" :junk-allowed t)
5
Or use this:
(read-from-string "23 absd")
23