<div dir="ltr"><div>I have a specific need for it. I need 2 functions working on maps which allow me to step over all the key/value pairs in a map without making an iterator.</div><div><br></div><div>What I need is a maps:first(Map) and then a maps:next(Key, Map) which takes a map and a key and gives me the next key/value pair WITHOUT an iterator. All I have is the map and a key.and cannot keep an iterator. Yes, it could be done by generating an iterator each time and stepping down it but not efficiently. The order would be irrelevant just so long as I can be just to get all the keys as long as the map is not updated while stepping over it. If the map is updated then all bets are off.</div><div><br></div><div>This is the only thing which stops me from using maps inside Luel to represent Lua tables and it is most annoying.</div><div><br></div><div>Robert</div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, 14 Aug 2019 at 09:52, Andreas Schultz <<a href="mailto:andreas.schultz@travelping.com">andreas.schultz@travelping.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div dir="ltr">Am Di., 13. Aug. 2019 um 20:18 Uhr schrieb Oliver Bollmann <<a href="mailto:oliver.bollmann@t-online.de" target="_blank">oliver.bollmann@t-online.de</a>>:<br></div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF">
<p>Hi,<br>
</p>
<p>i guess <b>we</b> do all the same,</p>
<p>1) processes with propery Y, i have about 1-10 million processes<br>
2) gen_server to manage these processes in maps,dict,lists for
example a pool with in,out,leave,delete<br>
3) monitoring these processes<br>
</p>
<p>Dilemma: <br>
</p>
<p>If process P died i have to delete it from data structure using
in 2)<br>
<br>
- maps, dict very fast; <br>
lists -> List -- [Item] slow!<br>
OK, i could use a sorted list and and the subtract would be
faster, but insert is then slower!<br>
And subtract need an algorithm!<br>
<br>
- i do not want a special process, only one random or the first
one from data structure using in 2)<br>
here is the place where i need <b>first_from_data_structure</b><br>
<b> </b>lists very fast: first defines it itself ->
[First|Rest] = List,<br>
but using maps or dict there is not first, at all!</p></div></blockquote><div><br></div><div>Are you sure a maps first is really what you want? Maps leys are not ordered (beyond MAP_SMALL_MAP_LIMIT, which is 32), but they are also not random.</div><div><br></div><div>As long as your map has less elements than MAP_SMALL_MAP_LIMIT it will behave like a ordered list, beyond that it will use a hash for ordering.</div><div>That means that for a given set of keys the ordering will be stable (but not sorted). So when you take the first element of a map and put it back again (based on key), it will be the new first element again.</div><div><br></div><div>IMHO the only use case for you maps:first function is something that tries to distribute new requests/elements across the elements of a map. Your maps:first function coupled with the constant ordering of map keys would lead to a distribution that is highly skewed towards a small number of keys (or even just one key).</div><div><br></div><div>Now, a maps:nth/2 function that works like list:nth/2 is another story. You could use</div><div><br></div><div> maps:nth(rand:uniform(maps:size(Map)), Map)</div><div><br></div><div>to get a somewhat evenly distributed key/element from a map.</div><div>Implementing this function currently requires you to convert the map to a list first. For large maps having a maps:nth/2 should yield better performance.</div><div><br></div><div>Regards</div><div>Andreas</div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div bgcolor="#FFFFFF">
<p>The question is: How can i get a random or first item from maps
or dict?</p>
<p>Hope that helps!</p>
<p>Oliver <br>
</p>
<p><br>
</p>
<div class="gmail-m_-3065800988767471310gmail-m_-547880167734857521moz-cite-prefix">On 13.08.19 15:41, Lukas Larsson wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div dir="ltr"><br>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Tue, Aug 13, 2019 at 3:34
PM Oliver Bollmann <<a href="mailto:oliver.bollmann@t-online.de" target="_blank">oliver.bollmann@t-online.de</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF">
<p>I often need <a class="gmail-m_-3065800988767471310gmail-m_-547880167734857521gmail-m_-4002335776611155796moz-txt-link-freetext">maps:first(Map)</a>, is there
plan for it?</p>
</div>
</blockquote>
<div><br>
</div>
<div>No, there is currently no plan for it, as I cannot think
of a use case when it would be useful, but that might just
be poor imagination on my part.</div>
<div><br>
</div>
<div>What do you use <a class="gmail-m_-3065800988767471310gmail-m_-547880167734857521moz-txt-link-freetext">maps:first</a> for?</div>
<div> </div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF">
<p>My current implementation is: </p>
<pre style="background-color:rgb(255,255,255);color:rgb(0,0,0);font-family:Menlo">first(<span style="color:rgb(102,14,122)">Map</span>) ->
<span style="color:rgb(0,0,128);font-weight:bold">case </span><a class="gmail-m_-3065800988767471310gmail-m_-547880167734857521gmail-m_-4002335776611155796moz-txt-link-freetext">maps:next(maps:iterator(</a><span style="color:rgb(102,14,122)">Map</span>)) <span style="color:rgb(0,0,128);font-weight:bold">of
</span><span style="color:rgb(0,0,128);font-weight:bold"> </span>{<span style="color:rgb(102,14,122)">Key</span>,<span style="color:rgb(102,14,122)">Val</span>,<span style="color:rgb(102,14,122)">_</span>} -> {<span style="color:rgb(102,14,122)">Key</span>,<span style="color:rgb(102,14,122)">Val</span>};
none -> none
<span style="color:rgb(0,0,128);font-weight:bold">end.
Create </span><span style="color:rgb(0,0,128);font-weight:bold"><span style="color:rgb(0,0,128);font-weight:bold">the iterator</span> always a new map?
</span></pre>
<pre class="gmail-m_-3065800988767471310gmail-m_-547880167734857521gmail-m_-4002335776611155796moz-signature" cols="72">--
Grüße
Oliver Bollmann</pre>
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<pre class="gmail-m_-3065800988767471310gmail-m_-547880167734857521moz-signature" cols="72">--
Grüße
Oliver Bollmann</pre>
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</blockquote></div><br clear="all"><div><br></div>-- <br><div dir="ltr" class="gmail-m_-3065800988767471310gmail_signature"><p><span style="font-family:verdana,geneva,sans-serif;font-size:10pt">Andreas Schultz</span></p>
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