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In elisp, the if statement logic only allows me an if case and else case.
(if (< 3 5)
; if case
(foo)
; else case
(bar))but what if I want to do an else-if? Do I need to put a new if statement inside the else case? It just seems a little messy.
Since the parts of (if test-expression then-expression else-expression) an else if would be to nest a new if as the else-expression:
(if test-expression1
then-expression1
(if test-expression2
then-expression2
else-expression2))In other languages the else if is usually on the same level. In lisps we have cond for that. Here is the exact same with a cond:
(cond (test-expression1 then-expression1)
(test-expression2 then-expression2)
(t else-expression2))Note that an expression can be just that. Any expression so often these are like (some-test-p some-variable) and the other expressions usually are too. It's very seldom they are just single symbols to be evaluated but it can be for very simple conditionals.
cond" is probably the most idiomatic solution. Sometimes, things are clearer with nested ifs, but that is so seldom that "use cond" is a sensible first approach.
– Vatine
Dec 6 '16 at 14:51
(if (red? o) (if (square? o) 'red-square 'red-round) (if (square? o) 'blue-square 'blue-round)) you get away with far less tests than (cond ((and (red? o) (square? o)) 'red-square) ((red? o) 'red-round) ((square? o) 'blue-square) (t 'blue-round)). For an average of 3.25 tests per time vs 2 and with the cond the code is slightly more messy to follow. I never use cond if I don't use any of the extra features over if. eg. (if test then else) I never write as cond
– Sylwester
Dec 6 '16 at 15:23
if (or multiple when, wrapped in an or, with if inside) is clearer than a cond.
– Vatine
Dec 6 '16 at 16:08
cond– jkiiski Nov 19 '16 at 15:27