D Programming - Associative Arrays

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Associative arrays have an index that is not necessarily an integer, and can be sparsely populated. The index for an associative array is called the Key, and its type is called the KeyType.

Associative arrays are declared by placing the KeyType within the [ ] of an array declaration. A simple example for associative array is shown below.

Live Demo 
import std.stdio;

void main () { 
   int[string] e;      // associative array b of ints that are  
   
   e["test"] = 3; 
   writeln(e["test"]); 
   
   string[string] f; 
   
   f["test"] = "Tuts"; 
   writeln(f["test"]); 
   
   writeln(f);  
   
   f.remove("test"); 
   writeln(f); 
}

When the above code is compiled and executed, it produces the following result −

3 
Tuts 
["test":"Tuts"] 
[]

Initializing Associative Array

A simple initialization of associative array is shown below.

Live Demo 
import std.stdio;

void main () { 
   int[string] days = 
      [ "Monday" : 0, 
         "Tuesday" : 1, 
         "Wednesday" : 2, 
         "Thursday" : 3, 
         "Friday" : 4, 
         "Saturday" : 5, 
         "Sunday" : 6 ]; 
   writeln(days["Tuesday"]);    
}

When the above code is compiled and executed, it produces the following result −

1

Properties of Associative Array

Here are the properties of an associative array −

Sr.No. Property & Description
1

.sizeof

Returns the size of the reference to the associative array; it is 4 in 32-bit builds and 8 on 64-bit builds.
2

.length

Returns number of values in the associative array. Unlike for dynamic arrays, it is read-only.
3

.dup

Create a new associative array of the same size and copy the contents of the associative array into it.
4

.keys

Returns dynamic array, the elements of which are the keys in the associative array.
5

.values

Returns dynamic array, the elements of which are the values in the associative array.
6

.rehash

Reorganizes the associative array in place so that lookups are more efficient. rehash is effective when, for example, the program is done loading up a symbol table and now needs fast lookups in it. Returns a reference to the reorganized array.
7

.byKey()

Returns a delegate suitable for use as an Aggregate to a ForeachStatement which will iterate over the keys of the associative array.
8

.byValue()

Returns a delegate suitable for use as an Aggregate to a ForeachStatement which will iterate over the values of the associative array.
9

.get(Key key, lazy Value defVal)

Looks up key; if it exists returns corresponding value else evaluates and returns defVal.
10

.remove(Key key)

Removes an object for key.

Example

An example for using the above properties is shown below.

Live Demo 
import std.stdio;

void main () { 
   int[string] array1;

   array1["test"] = 3; 
   array1["test2"] = 20; 
   
   writeln("sizeof: ",array1.sizeof); 
   writeln("length: ",array1.length); 
   writeln("dup: ",array1.dup);  
   array1.rehash; 
   
   writeln("rehashed: ",array1);  
   writeln("keys: ",array1.keys); 
   writeln("values: ",array1.values);
   
   foreach (key; array1.byKey) { 
      writeln("by key: ",key); 
   }

   foreach (value; array1.byValue) { 
      writeln("by value ",value); 
   }

   writeln("get value for key test: ",array1.get("test",10)); 
   writeln("get value for key test3: ",array1.get("test3",10));  
   array1.remove("test"); 
   writeln(array1); 
}

When the above code is compiled and executed, it produces the following result −

sizeof: 8                                                                          
length: 2                                                                          
dup: ["test":3, "test2":20]                                                        
rehashed: ["test":3, "test2":20]                                                   
keys: ["test", "test2"]                                                            
values: [3, 20]                                                                    
by key: test                                                                       
by key: test2                                                                      
by value 3                                                                         
by value 20                                                                        
get value for key test: 3                                                          
get value for key test3: 10                                                        
["test2":20]
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