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digitalmars.D.learn - Finding the index of the maximum value in an associative array

reply Lyle <lylereadchallen gmail.com> writes:
Hi,

I have an associative array of type int[ulong] and I'm trying to 
get the index of the maximum value, like this:

int[ulong] aa = [1UL: 2000,
                  2UL: 5000,
                  5UL: 1000];

writeln(aa.maxIndex); // should print 2

Can anyone help me out?

Many thanks,
Lyle
May 30 2017
next sibling parent reply cym13 <cpicard openmailbox.org> writes:
On Tuesday, 30 May 2017 at 17:57:04 UTC, Lyle wrote:

 Hi,

 I have an associative array of type int[ulong] and I'm trying 
 to get the index of the maximum value, like this:

 int[ulong] aa = [1UL: 2000,
                  2UL: 5000,
                  5UL: 1000];

 writeln(aa.maxIndex); // should print 2

 Can anyone help me out?

 Many thanks,
 Lyle


Simple enough: get all key/values pairs in the AA, and ask for 
the element which is the maximum relatively to the value, then 
take its key.

     auto maxIndex(int[ulong] aa) {
         import std.algorithm;

         return aa.byKeyValue
                  .maxElement!(x => x.value)
                  .key;
     }
May 30 2017
parent Lyle <lylereadchallen gmail.com> writes:
On Tuesday, 30 May 2017 at 18:05:02 UTC, cym13 wrote:

 On Tuesday, 30 May 2017 at 17:57:04 UTC, Lyle wrote:

 Hi,

 I have an associative array of type int[ulong] and I'm trying 
 to get the index of the maximum value, like this:

 int[ulong] aa = [1UL: 2000,
                  2UL: 5000,
                  5UL: 1000];

 writeln(aa.maxIndex); // should print 2

 Can anyone help me out?

 Many thanks,
 Lyle


 Simple enough: get all key/values pairs in the AA, and ask for 
 the element which is the maximum relatively to the value, then 
 take its key.

     auto maxIndex(int[ulong] aa) {
         import std.algorithm;

         return aa.byKeyValue
                  .maxElement!(x => x.value)
                  .key;
     }


Thank you! I used this solution in the end. I had looked at the 
.byKeyValue and .byPair methods but they didn't seem to work - 
turns out I was using an old DMD version!
May 30 2017
prev sibling parent reply "H. S. Teoh via Digitalmars-d-learn" <digitalmars-d-learn puremagic.com> writes:
On Tue, May 30, 2017 at 05:57:04PM +0000, Lyle via Digitalmars-d-learn wrote:

 Hi,
 
 I have an associative array of type int[ulong] and I'm trying to get
 the index of the maximum value, like this:
 
 int[ulong] aa = [1UL: 2000,
                  2UL: 5000,
                  5UL: 1000];
 
 writeln(aa.maxIndex); // should print 2
 
 Can anyone help me out?


[...]

Try this:

	void main() {
		import std.algorithm.iteration : fold;
		import std.array : byPair;
		import std.stdio : writeln;

		int[ulong] aa = [1UL: 2000,
				2UL: 5000,
				5UL: 1000];

		writeln(aa.byPair
			  .fold!((a,b) => a[1] < b[1] ? b : a)[0]);
	}

Note that an associative array really isn't the best data structure if
you'll be performing this operation frequently, because you'll be
iterating over the entire contents each time. You may want to consider
using a sorted container like a RedBlackTree in std.container or
something similar instead.


T

-- 
Ignorance is bliss... until you suffer the consequences!
May 30 2017
parent John Colvin <john.loughran.colvin gmail.com> writes:
On Tuesday, 30 May 2017 at 18:05:07 UTC, H. S. Teoh wrote:

 On Tue, May 30, 2017 at 05:57:04PM +0000, Lyle via 
 Digitalmars-d-learn wrote:

 Hi,
 
 I have an associative array of type int[ulong] and I'm trying 
 to get the index of the maximum value, like this:
 
 int[ulong] aa = [1UL: 2000,
                  2UL: 5000,
                  5UL: 1000];
 
 writeln(aa.maxIndex); // should print 2
 
 Can anyone help me out?


 [...]

 Try this:

 	void main() {
 		import std.algorithm.iteration : fold;
 		import std.array : byPair;
 		import std.stdio : writeln;

 		int[ulong] aa = [1UL: 2000,
 				2UL: 5000,
 				5UL: 1000];

 		writeln(aa.byPair
 			  .fold!((a,b) => a[1] < b[1] ? b : a)[0]);
 	}


writeln(aa.byPair.maxElement!(p => p[1])[0]);

or with https://github.com/dlang/phobos/pull/5436 :

writeln(aa.byPair.maxElement!(p => p.value).key);
Jun 01 2017