Associative array/Creation

Associative array/Creation
You are encouraged to solve this task according to the task description, using any language you may know.

In this task, the goal is to create an associative array.

Maintenance

This category lists articles requiring maintenance of some sort. Please read the directions for the category and apply them.

C++

Compiler: g++ 4.0.2

 #include <map>
 #include <string>
 #include <iostream>
 #include <ostream>
 
 int main()
 {
   // This is an associative array which maps strings to ints
   typedef std::map<std::string, int> colormap_t;
   colormap_t colormap;
   
   // First, populate it with some values
   colormap["red"] = 0xff0000;
   colormap["green"] = 0x00ff00;
   colormap["blue"] = 0x0000ff;
   colormap["my favourite color"] = 0x00ffff;
   
   // then, get some values out
   int color = colormap["green"]; // color gets 0x00ff00
   color = colormap["black"]; // accessing unassigned values assigns them to 0
   
   // get some value out without accidentally inserting new ones
   colormap_t::iterator i = colormap.find("green");
   if (i == colormap.end())
   {
     std::cerr << "color not found!\n";
   }
   else
   {
     color = i->second;
   }
   
   // Now I changed my mind about my favourite color, so change it
   colormap["my favourite color"] = 0x337733;
   
   // print out all defined colors
   for (colormap_t::iterator i = colormap.begin(); i != colormap.end(); ++i)
     std::cerr << "colormap[\"" << i->first << "\"] = 0x" << std::hex << i->second << "\n";
 }

C#

Map in .NET 1.x

 System.Collections.HashTable map = new System.Collections.HashTable();
 map["key1"] = "foo";

Map in .NET 2.0

 Dictionary<string, string> map = new Dictionary<string,string>();
 map[ "key1" ] = "foo";

Java

Defining a Map

 Map<String, Integer> map = new HashMap<String, Integer>();
 map.put("foo", 5);
 map.put("bar", 10);
 map.put("baz", 15);

Retreiving a value

 map.get("foo") // => 5
 map.get("invalid") // => null

Iterate over keys

 for (String key: map.keySet()) 
    System.out.println(key);

Iterate over values

  for (int value: map.values())
     System.out.println(value);

Iterate over key,values

  for (Map.Entry<String, Integer> entry: map.entrySet())
     System.out.println(entry.getKey() + " => " + entry.getValue());

OCaml

A simple idiom to create a hash table mapping strings to integers:

 let hash = Hashtbl.create 0;;
 List.iter (fun (key, value) -> Hashtbl.add hash key value)
   ["foo", 5; "bar", 10; "baz", 15];;

To retrieve a value:

 let bar = Hashtbl.find hash "bar";; (* bar = 5 *)

To retrieve a value, returning a default if the key is not found:

 let quux = try Hashtbl.find hash "quux" with Not_found -> some_value;;

PHP

 $array = array();
 $array['foo'] = 'bar';
 $array['bar'] = 'foo';
 
 echo($array['foo']); // bar
 echo($array['moo']); // Undefined index

 //alternative (inline) way
 $array2 = array('fruit'=>'apple, 'price'=>12.96, 'colour'=>'green');

Iterate over key/value

 foreach($array as $key => $value)
 {
    echo('Key: '.$key.' Value: '.$value);
 }

Perl

Interpeter: Perl

Defining a Hash

# using => key does not need to be quoted unless it contains special chars
my %hash = (
  key1 => 'val1',
  'key-2' => 2,
  three => -238.83,
  4 => 'val3',
);

# using , both key and value need to be quoted if containing something non-numeric in nature
my %hash = (
  'key1', 'val1',
  'key-2', 2,
  'three', -238.83,
  4, 'val3',
);

Defining a HashRef

my $hashref = {
  key1 => 'val1',
  'key-2' => 2,
  three => -238.83,
  4 => 'val3',
}

Using a Hash

print $hash{'key1'};

$hash{'key1'} = 'val1';

@hash{'key1', 'three'} = ('val1', -238.83);

Using a HashRef

print $hash->{'key1'};

$hash->{'key1'} = 'val1';

@hash->{'key1', 'three'} = ('val1', -238.83);

Python

In Python, hashes are called dictionaries.

 # empty dictionary
 d = {}
 d['spam'] = 1
 d['eggs'] = 2  

 # dictionaries with two keys
 d1 = {'spam': 1, 'eggs': 2}
 d2 = dict(spam=1, eggs=2)

 # dictionaries from tuple list
 d1 = dict([('spam', 1), ('eggs', 2)])
 d2 = dict(zip(['spam', 'eggs'], [1, 2]))

 # iterating over keys
 for key in d:
   print key, d[key]

 # iterating over (key, value) pairs
 for key, value in d.iteritems():
   print key, value

Create a generic mapping function that applys a callback to elements in a list:

Ruby

A hash object that returns nil for unknown keys

 hash={}
 hash[666]='devil'
 hash[777]  # => nil
 hash[666]  # => 'devil'

A hash object that returns 'unknown key' for unknown keys

 hash=Hash.new('unknown key')
 hash[666]='devil'
 hash[777]  # => 'unknown key'
 hash[666]  # => 'devil'

A hash object that returns "unknown key #{key}" for unknown keys

 hash=Hash.new{|h,k|h[k]="unknown key #{k}"}
 hash[666]='devil'
 hash[777]  # => 'unknown key 777'
 hash[666]  # => 'devil'