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

win1for at yahoo.com win1for at yahoo.com
Fri Dec 17 21:56:56 AEDT 2010

```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 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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