August 30, 2012
From the book a way to respond to a non-existent key in an assoc. array:

assert(aa["hello"] == "ciao");
// Key "hello" exists, therefore ignore the second argume
assert(aa.get("hello", "salute") == "ciao");
// Key "yo" doesn’t exist, return the second argument
assert(aa.get("yo", "buongiorno") == "buongiorno");

Should this work in multidimensional arrays?

aa.get("key1" "key2" "key2", "nonexistent") == "sometext"

Thanks for your time.
August 30, 2012
On Thu, Aug 30, 2012 at 7:18 PM, Paul <phshaffer@gmail.com> wrote:
> From the book a way to respond to a non-existent key in an assoc. array:
>
> assert(aa["hello"] == "ciao");
> // Key "hello" exists, therefore ignore the second argume
> assert(aa.get("hello", "salute") == "ciao");
> // Key "yo" doesn’t exist, return the second argument
> assert(aa.get("yo", "buongiorno") == "buongiorno");
>
> Should this work in multidimensional arrays?
>
> aa.get("key1" "key2" "key2", "nonexistent") == "sometext"

D multi-key associative arrays do not exist as such, they are associative arrays inside one another. When you write

int[string][string][double] aa;

you can use get() on aa, but only on its keys, which are of type double, whereas its values are of type int[string][string].

I guess an effect similar to what you're asking can be obtained by using a tuple as a key:

import std.typecons;
int[Tuple!(string,string,double)] aa;

auto p = aa.get(tuple("abc","def", 3.14), 0);  // 0 is the default value.
August 30, 2012
On Thursday, 30 August 2012 at 18:20:02 UTC, Philippe Sigaud wrote:
> On Thu, Aug 30, 2012 at 7:18 PM, Paul <phshaffer@gmail.com> wrote:
>> From the book a way to respond to a non-existent key in an assoc. array:
>>
>> assert(aa["hello"] == "ciao");
>> // Key "hello" exists, therefore ignore the second argume
>> assert(aa.get("hello", "salute") == "ciao");
>> // Key "yo" doesn’t exist, return the second argument
>> assert(aa.get("yo", "buongiorno") == "buongiorno");
>>
>> Should this work in multidimensional arrays?
>>
>> aa.get("key1" "key2" "key2", "nonexistent") == "sometext"
>
> D multi-key associative arrays do not exist as such, they are
> associative arrays inside one another. When you write
>
> int[string][string][double] aa;
>
> you can use get() on aa, but only on its keys, which are of type
> double, whereas its values are of type int[string][string].
>
> I guess an effect similar to what you're asking can be obtained by
> using a tuple as a key:
>
> import std.typecons;
> int[Tuple!(string,string,double)] aa;
>
> auto p = aa.get(tuple("abc","def", 3.14), 0);  // 0 is the default value.

my array is of the form string[string][string][string] abc;
August 30, 2012
On Thursday, 30 August 2012 at 18:29:28 UTC, Paul wrote:
> On Thursday, 30 August 2012 at 18:20:02 UTC, Philippe Sigaud wrote:
>> On Thu, Aug 30, 2012 at 7:18 PM, Paul <phshaffer@gmail.com> wrote:
>>> From the book a way to respond to a non-existent key in an assoc. array:
>>>
>>> assert(aa["hello"] == "ciao");
>>> // Key "hello" exists, therefore ignore the second argume
>>> assert(aa.get("hello", "salute") == "ciao");
>>> // Key "yo" doesn’t exist, return the second argument
>>> assert(aa.get("yo", "buongiorno") == "buongiorno");
>>>
>>> Should this work in multidimensional arrays?
>>>
>>> aa.get("key1" "key2" "key2", "nonexistent") == "sometext"
>>
>> D multi-key associative arrays do not exist as such, they are
>> associative arrays inside one another. When you write
>>
>> int[string][string][double] aa;
>>
>> you can use get() on aa, but only on its keys, which are of type
>> double, whereas its values are of type int[string][string].
>>
>> I guess an effect similar to what you're asking can be obtained by
>> using a tuple as a key:
>>
>> import std.typecons;
>> int[Tuple!(string,string,double)] aa;
>>
>> auto p = aa.get(tuple("abc","def", 3.14), 0);  // 0 is the default value.
>
> my array is of the form string[string][string][string] abc;

Maybe I'm not going about my project from the best angle?  Another problem I have is when I go to printout my array, being associative, it is not in the order I built it.  It would help greatly if I could print it in order.  Maybe I should look into this "tuple" thing more.  As well I would like to be able to check for the existence of a particular "key" quickly without setting up one those three-tier foreach iteration loops.
August 30, 2012
On Thursday, August 30, 2012 20:57:44 Paul wrote:
> Maybe I'm not going about my project from the best angle? Another problem I have is when I go to printout my array, being associative, it is not in the order I built it. It would help greatly if I could print it in order.

Hash tables (and an associative array is a hash table) are unordered, so you're not going to get them out in the order that you put them in or anything like that.

If you use std.container.RedBlackTree (which defaults to being a set but can be used as a map if you adjust its comparison predicate appropriately), then you have ordering, but it's ordered according to its predicate, not the insertion order.

I believe that if you want a map (be it ordered or unordered) to give you items back in the order that you inserted them, then a separate list is required (be it integrated into the container or something you do alongside it) where that list has the keys in the order that they were inserted into the container.

- Jonathan M Davis
August 30, 2012
On Thu, Aug 30, 2012 at 8:57 PM, Paul <phshaffer@gmail.com> wrote:

> Maybe I'm not going about my project from the best angle?  Another problem I have is when I go to printout my array, being associative, it is not in the order I built it.  It would help greatly if I could print it in order.

Associative arrays reorganize themselves to allow for fast insertion/retrieval. They do not make any promise concerning order or printing.

> Maybe I should look into this "tuple" thing more.  As well I would like to be able to check for the existence of a particular "key" quickly without setting up one those three-tier foreach iteration loops.

A tuple is just a grouping of values together, its just a handy predefined struct-maker in std.typecons. In your case, its equivalent to defining

struct MyKey { string a,b,c}

string[MyKey] aa;

to fill it:  aa[MyKey("abc","def","ghi")] = "my value";

It depends whether you need intermediate associative arrays (aa["abc"], aa["abc"]["def"]) or not. It's like a 2D/3D array: do you want access to lines / arrays or just to individual elements. In your case, if you just want access to individual elements, using a key grouping all your key strings under one struct might be interesting: it means just one query in the AA.

To check for a key:
auto wanted = MyKey("abc", "def", "ghi");

if (auto p = wanted in aa)   // p !is null, => the key is in the
associative array
{
    // use *p
}
August 30, 2012
On Thu, Aug 30, 2012 at 9:30 PM, Jonathan M Davis <jmdavisProg@gmx.com> wrote:

> I believe that if you want a map (be it ordered or unordered) to give you items back in the order that you inserted them, then a separate list is required (be it integrated into the container or something you do alongside it) where that list has the keys in the order that they were inserted into the container.

Yes, that's what I wanted to propose. Group an AA and a standard, dynamic, array. The array is just filled with the key, when you assign a new key/value pair. To query the structure, use the AA. To print the structure in order, iterate on this array of keys and query the AA accordingly.

Of course, there is a downside:

* when you discard a key/value pair, your key array must be iterated
to find the now-discarded key and recreated after the key.
* when you re-assign an already defined key, you might want to put the
key in front/back of the array once more.

Pro: in order printing/iteration.
Con: slow discarding, slow re-assigning.
August 30, 2012
On Thursday, August 30, 2012 21:40:34 Philippe Sigaud wrote:
> On Thu, Aug 30, 2012 at 9:30 PM, Jonathan M Davis <jmdavisProg@gmx.com>
wrote:
> > I believe that if you want a map (be it ordered or unordered) to give you
> > items back in the order that you inserted them, then a separate list is
> > required (be it integrated into the container or something you do
> > alongside
> > it) where that list has the keys in the order that they were inserted into
> > the container.
> 
> Yes, that's what I wanted to propose. Group an AA and a standard, dynamic, array. The array is just filled with the key, when you assign a new key/value pair. To query the structure, use the AA. To print the structure in order, iterate on this array of keys and query the AA accordingly.
> 
> Of course, there is a downside:
> 
> * when you discard a key/value pair, your key array must be iterated
> to find the now-discarded key and recreated after the key.
> * when you re-assign an already defined key, you might want to put the
> key in front/back of the array once more.
> 
> Pro: in order printing/iteration.
> Con: slow discarding, slow re-assigning.

Yeah, though I expect that there are ways to make it a less of a problem. For instance, you could make it so that the hash table's value was really a tuple of the value and the index into the array or vector holding the keys in insertion order (or use a second hash table to hold the indices). Then when you remove an item, you use the index from the value in the table and set that element in the array to a value indicating that it's empty (so that it's skipped when iterating over it). Then removing items is as fast as inserting them. The problem that that causes of course is that the list of inserted keys keeps growing, but as long as it doesn't make rehashing too expensive, you could adjust the array (and the indices in the hash table) when you rehash (and possibly after a certain number of removals as well in case the table gets items removed often enough that rehashing isn't ever necessary). That may or may not be the best solution, but I expect that it's problem that's already been explored and probably has some good existing solutions somewhere.

In general though, you just don't care about insertion order into a hash table, which takes care of the whole problem.

- Jonathan M Davis
August 30, 2012
On Thursday, 30 August 2012 at 19:40:44 UTC, Philippe Sigaud wrote:
> On Thu, Aug 30, 2012 at 9:30 PM, Jonathan M Davis <jmdavisProg@gmx.com> wrote:

> Yes, that's what I wanted to propose. Group an AA and a standard,
> dynamic, array. The array is just filled with the key, when you assign
> a new key/value pair. To query the structure, use the AA. To print the
> structure in order, iterate on this array of keys and query the AA
> accordingly.

So one array like-  aa[MyKey("abc","def","ghi")] = "my value";

and another like- string[] da; da[99]="abc"~","~"def"~","~"ghi";
or maybe-          MyKey[] da; da[99]=MyKey("abc","def","ghi");

This thread has been quite helpful already.  Thanks to all.
August 30, 2012
On Thu, Aug 30, 2012 at 10:24 PM, Paul <phshaffer@gmail.com> wrote:
>
> So one array like-  aa[MyKey("abc","def","ghi")] = "my value";
>
> and another like- string[] da; da[99]="abc"~","~"def"~","~"ghi"; or maybe-          MyKey[] da; da[99]=MyKey("abc","def","ghi");

The latter, if you group your keys, I think. You can just push the new value at the array's end, as if it was a stack:


struct MyKey(string k1,k2,k3);
string[MyKey] aa;
MyKey[] da;

// pushing "value" with key ("abc","def","ghi"):

MyKey k = MyKey("abc","def","ghi");
aa[k] = "value";
da ~= k; // append k to da

Jonathan's advice of recording the key's index (in the array) with the value is interesting, but then you need a special value in the array to indicate a discarded key. Here the recent thread about Option!T comes to mind... but I don't know what you're trying the achieve, so let's not go into complicated stuff right now.


> This thread has been quite helpful already.  Thanks to all

You're welcome. Note that your need of having a structure which is both associative and ordered is, if not unheard-of, at least somewhat uncommon.
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