4 ms·

The common-lisp.net page for ITERATE has this example for finding the longest list in a list of lists: (iterate (for elt in list-of-lists) (findin

The common-lisp.net page for ITERATE has this example for finding the longest list in a list of lists: (iterate (for elt in list-of-lists) (finding elt maximizing (length elt))) But what's wrong with this? (reduce (lambda (a b) (if (> (length b) (length a)) b a)) list-of-lists) Okay, you have to do a little more work if you only want to take the length of each list once instead of twice. But if you don't care about that, this is quite elegant and introduces no new vocabulary. (Unless the lists are very long, calling LENGTH twice isn't anywhere near twice the cost of calling it once, because of cache effects.)

lispm 12y ago
The reduce code is just not very declarative. I have to inspect the code and i need to have a mental model of REDUCE (which is not trivial). What is b and what is a? I need a mental model of ITERATE too, but the declarative wording gives hints what it is supposed to do. Code like above is simply less readable. There are lots of uses of REDUCE which are simple to understand. Code like above, less so - where I need to write a special purpose function. Another problem is that you need to think about the corner cases: (reduce (lambda (a b) (if (> (length b) (length a)) b a)) '()) -> Error Also you can see that the REDUCE version is slower. Usually there will be a function call overhead - unless the compiler has some optimization for that in some optimize level. Also 'LENGTH' is called multiple times on the longest list so far. You could work around that in REDUCE ... In ITERATE this is not needed: it calls LENGTH only once per list item.
- ScottBurson 12y ago
  You always need to think about the corner cases. What is the longest element in an empty list of lists? It might be better to get an error in such a case than to have an empty list returned, since that's not an element of the input list, in violation of one possible interpretation of the function's specification. > Code like above is simply less readable. I write code like this all the time, and have for 35 years. I think it's perfectly readable.

lispm 12y ago

The reduce code is just not very declarative. I have to inspect the code and i need to have a mental model of REDUCE (which is not trivial). What is b and what is a? I need a mental model of ITERATE too, but the declarative wording gives hints what it is supposed to do. Code like above is simply less readable. There are lots of uses of REDUCE which are simple to understand. Code like above, less so - where I need to write a special purpose function. Another problem is that you need to think about the corner cases: (reduce (lambda (a b) (if (> (length b) (length a)) b a)) '()) -> Error Also you can see that the REDUCE version is slower. Usually there will be a function call overhead - unless the compiler has some optimization for that in some optimize level. Also 'LENGTH' is called multiple times on the longest list so far. You could work around that in REDUCE ... In ITERATE this is not needed: it calls LENGTH only once per list item.

ScottBurson 12y ago
You always need to think about the corner cases. What is the longest element in an empty list of lists? It might be better to get an error in such a case than to have an empty list returned, since that's not an element of the input list, in violation of one possible interpretation of the function's specification. > Code like above is simply less readable. I write code like this all the time, and have for 35 years. I think it's perfectly readable.