I'm trying to write some function to sort this type of list:
((1 A) (2 E) (4 D))
I found the built-in function sort in Common Lisp, but I can't do what I want.
This should be the result:
'((1 A) (4 D) (2 E))
I want to sort the list by the second element of each lists, alphabetically.
This is what I've done:
(sort '((1 A) (4 D) (2 E)) #'char-lessp :key #'second)
Anyway, I would understand how to sort a list with a specific function, using :key'.
Thank you.
You should use:
(sort '((1 A) (4 E) (2 D)) #'string<= :key #'second)
The reason is that char-lessp compares characters, and A, E and D are not characters, but symbols. The function string< can used to compare their names, which are the strings "A", "E" and "D". For instance:
CL-USER> (string= 'symbol "SYMBOL")
T
CL-USER> (symbol-name 'symbol)
"SYMBOL"
Note that the comparison operators on strings have two versions, for case sensitive and case insensitive comparisons:
CL-USER> (string= "a" "A")
NIL
CL-USER> (string-equal "a" "A")
T
CL-USER> (string= 'a "a")
NIL
CL-USER> (string-equal 'a "a")
T
