# [mercury-users] Converting List To List Of Tuples

Raphael Collet rco at missioncriticalit.com
Fri Dec 17 22:33:05 AEDT 2010

```Dear anonymous user,

What you ask for looks a lot like a homework...  I ain't going to do the
homework for you.  Here are a few hints, though.

The function map.init returns an empty map.  That's where you should
start from.

The predicate map.set/4 (and the function map.set/3) allows you to set
the value associated to a key in the map.  In your case, this could be
the current frequency of a character.

The predicate map.search/3 looks up for a given key, and return the
corresponding value.  You can use this to retrieve the frequency of a
character.  The typical use-case is:

( map.search(Map, Key, Value) ->
% do something with the value
;
% no value for key, do something else
)

Cheers,
Raphael

On 12/17/2010 11:56 AM, win1for at yahoo.com wrote:
>
> Please am a total beginner can you give me an example of how to use the map.
>
> --- On *Fri, 12/17/10, Raphael Collet /<rco at missioncriticalit.com>/* wrote:
>
>
>     From: Raphael Collet <rco at missioncriticalit.com>
>     Subject: Re: [mercury-users] Converting List To List Of Tuples
>     To: mercury-users at csse.unimelb.edu.au
>     Cc: "win1for at yahoo.com" <win1for at yahoo.com>
>     Date: Friday, December 17, 2010, 3:44 AM
>
>     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.
>
>
>
>     Cheers,
>     Raphael
>
>
>     On 12/17/2010 05:22 AM, win1for at yahoo.com
>     </mc/compose?to=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.
>      >
>      >
>      >
>      >
>
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