[mercury-users] Converting List To List Of Tuples

Raphael Collet rco at missioncriticalit.com
Fri Dec 17 19:44:01 AEDT 2010


Dear user,

I suggest you to have a look at the modules map, pair and assoc_list. 
With a map, you will be able to build the frequency map.  Start from an 
empty map, then increment the frequency of each character you find in 
the string.  The predicate string.foldl/4 is useful to iterate over all 
the characters of the string.

To convert the map into a list of pairs, you may simply use

     ListOfPairs = map.to_assoc_list(FrequencyMap)

In your case, the result will be of type list(pair(char, int)).  Note 
that the pair is not strictly a tuple, but a more specific type for a 
tuple of exactly two elements.

The last point you need is to sort the list with list.sort/3.  Simply 
define a comparison predicate to compare two values of type pair(char, 
int): order them by their second components.  Something like:

:- pred compare_freq(pair(K, int), pair(K, int), comparison_result).
:- mode compare_freq(in, in, out) is det.

compare_freq(X1 - F1, X2 - F2, Result) :- compare(Result, F1, F2).

The predicate compare/3 and its associated types is defined in the 
module builtin.


I hope this will help you.

Cheers,
Raphael


On 12/17/2010 05:22 AM, win1for at yahoo.com wrote:
> Hello,
>
>
> I am just a total beginner in mercury and finding it hard to solve this
> problem. I want to convert a list to a list of tupples sorted from
> smaller to higher frequenties. Eg:
>
> |string.to_char_list("this is a test")  becomes
>
> [{'a', 1}, {'e', 1}, {'h', 1}, {'i', 2}, {' ', 3}, {'s', 3}, {'t', 3}]
>
> OR
>
> [3,2,1,2,1,1,2]  becomes
>
> [{3, 1}, {1, 3}, {2, 3}]
> |
>
> You can see that the list of tuples are sorted from smaller to higher
> frequenties.
>
> I am asking if someone can help me to solve this or point me to a
> tutorial where i can find more tips to do it.
>
>
> Thanks for your reply.
>
>

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