Question about Ord for lists and multi-dimensional lists
Hi all, I'm having trouble finding resources explaining how Ord for two lists works. I've created a public gist to explain what I mean. https://gist.github.com/iduhetonas/9318958 Could anybody shed some light on what I'm seeing? Regards, Tyler Huffman
On 03/03/14 05:33, Tyler Huffman wrote:
Hi all,
I'm having trouble finding resources explaining how Ord for two lists works.
I've created a public gist to explain what I mean.
https://gist.github.com/iduhetonas/9318958
Could anybody shed some light on what I'm seeing?
Regards, Tyler Huffman
_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners
It does element-wise comparisons and in case one of the lists is empty, the other one is considered greater (unless they are both empty). So: Prelude> max [1] [2] Here 2 is bigger than 1 so we get the result straight away Prelude> max [1,2] [1] Here the first element is equal so we compare the rest of lists: [2] and []. As the second list is empty, we return the first list. Prelude> max [2,1] [1] No problem at all here, 2 is greater than 1 so we just return the first list on the first comparison. Prelude> max [1,2] [1,2,4] This is exactly the same case as ‘max [1, 2] [1]’: we first compare the 1s, find they are equal and compare the rest of the list. Same with 2s. Then the base case is ‘max [] [4]’ so the second list is treated as greater. Prelude> max [1,2,4] [1,2] Same as above but with arguments switched. I believe that the law here is ‘max x y ≡ max y x’. Prelude> max [[1,2,3],[2,3,4]] [[1,2],[2,3],[2,4]] This is just like all our other examples except one more level of nesting. We first end up comparing the first elements of the argument (so we call out ‘max [1, 2, 3] [1, 2]’, and then check each element of these lists. 1 and 2 are equal and we reach the ‘max [3] []’ case: the first list was greater so our whole call returns ‘[[1, 2, 3], [2, 3, 4]]’. I hope that sheds some light. -- Mateusz K.
Awesome, that helps a lot.
Thanks a bunch, Mateusz!
Regards,
Tyler Huffman
On Sun, Mar 2, 2014 at 10:44 PM, Mateusz Kowalczyk
On 03/03/14 05:33, Tyler Huffman wrote:
Hi all,
I'm having trouble finding resources explaining how Ord for two lists works.
I've created a public gist to explain what I mean.
https://gist.github.com/iduhetonas/9318958
Could anybody shed some light on what I'm seeing?
Regards, Tyler Huffman
_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners
It does element-wise comparisons and in case one of the lists is empty, the other one is considered greater (unless they are both empty).
So: Prelude> max [1] [2]
Here 2 is bigger than 1 so we get the result straight away
Prelude> max [1,2] [1]
Here the first element is equal so we compare the rest of lists: [2] and []. As the second list is empty, we return the first list.
Prelude> max [2,1] [1]
No problem at all here, 2 is greater than 1 so we just return the first list on the first comparison.
Prelude> max [1,2] [1,2,4]
This is exactly the same case as ‘max [1, 2] [1]’: we first compare the 1s, find they are equal and compare the rest of the list. Same with 2s. Then the base case is ‘max [] [4]’ so the second list is treated as greater.
Prelude> max [1,2,4] [1,2]
Same as above but with arguments switched. I believe that the law here is ‘max x y ≡ max y x’.
Prelude> max [[1,2,3],[2,3,4]] [[1,2],[2,3],[2,4]]
This is just like all our other examples except one more level of nesting. We first end up comparing the first elements of the argument (so we call out ‘max [1, 2, 3] [1, 2]’, and then check each element of these lists. 1 and 2 are equal and we reach the ‘max [3] []’ case: the first list was greater so our whole call returns ‘[[1, 2, 3], [2, 3, 4]]’.
I hope that sheds some light.
-- Mateusz K. _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners
On Mon, Mar 3, 2014 at 12:33 PM, Tyler Huffman
Could anybody shed some light on what I'm seeing?
This is the standard lex order you see in dictionaries. The word "a" is followed by "aardvark" which is in turn followed by "an". Similarly [1] is followed by [1,2] which is in turn followed by [1,2,4]. (It's possible to override the order.) -- Kim-Ee
participants (3)
-
Kim-Ee Yeoh -
Mateusz Kowalczyk -
Tyler Huffman