How do I subtract 1 from the second element of the list (3 6)?
(setq x '(3 6))
(- (nth 1 x) 1)
The above quote does the math but does not return a modified list.
2 Answers
Using the built-in cl-lib package, you can write the following:
(eval-when-compile
(require 'cl-lib))
(cl-decf (nth 1 x))
Alternatively, in vanilla Elisp:
(setf (nth 1 x) (1- (nth 1 x)))
Be careful, though, to avoid modifying quoted constant lists such as '(3 6) in-place, as doing so modifies the constant globally and may cause the code to behave differently in separate runs.
Instead, make sure to operate on a new list each time, e.g. via:
(setq x (list 3 6))
-
1@Basil You actually hijacked low-level answer to transform it into a high-level one. You should have written your own answer.– TobiasFeb 11, 2020 at 14:56
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1@Tobias I did not change how low- or high-level the answer is. I merely extended and corrected the answer to distinguish between
cl-lib(not the obsoletecl) and vanilla Elisp, and added a warning against destructive modifications. If you or the author are opposed to these additions, you are free to revert my suggestions. Writing a separate answer for these extensions would be redundant and off-topic.– BasilFeb 11, 2020 at 15:00 -
1@Basil:
cl.elis not obsolete. Some features of that library are obsolete. See CL: Obsolete Features.– DrewFeb 11, 2020 at 16:13 -
1FWIW, I agree that @Basil did the right thing (a right thing). He by no means hijacked anything, IMO.– DrewFeb 11, 2020 at 16:15
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1@Drew You're right,
cl.elis not officially obsolete, but its use is discouraged in favour ofcl-lib.elwhere practical. See(cl) Organization.– BasilFeb 11, 2020 at 16:43
Just tinkering, you can use car and cdr too. I'm sure someone will comment on whether this is good or bad.
(let ((v 99) (p 3) (l (list 1 2 3 4)))
(setcar (nthcdr p l) (+ (car (nthcdr p l)) v))
l)
;; => (1 2 3 103)
-is a pure arithmetic function which accepts numbers and returns a number. It knows nothing about where the arguments came from and does not modify any stored data. If you care about the result of the arithmetic operation then you should store it back in the list, e.g. using the convenient macros listed in the answer(s) below. Be careful, though, not to destructively modify quoted constant lists such as'(3 6)in-place; make a copy of the list usingcopy-sequencefirst or allocate a new one using(list 3 6)instead.