Associative Arrays

What is an Associative Array? #

An Associative Array is a one-dimensional unpacked array in SystemVerilog where elements are indexed using non-consecutive values such as integers, strings, or even objects. Unlike fixed or dynamic arrays, storage is allocated only when an element is used, making associative arrays ideal for sparse data structures.

Syntax:

data_type array_name [index_type];

Example:

bit [31:0] mem[int];           // Integer-indexed associative array
string str_arr[string];        // String-indexed associative array
class C;                       // Class-based index
C obj;
bit [7:0] class_arr[C];        // Associative array indexed by class object

Differences from Regular Arrays

Built-in Methods

SystemVerilog provides special methods for traversing and querying associative arrays.

Example: Using Associative Array with Integer Index

module assoc;
  bit [31:0] mem[*];      // Associative array with integer index
  bit [31:0] i;

  initial begin
    // Store values at non-contiguous indices
    mem[0] = 32'hFFAAFFAA;
    mem[3] = 32'hFFAAFFAA;
    mem[5] = 32'hFFAAFFAA;
    mem[7] = 32'hFFAAFFAA;

    $display("Size of array: %0d", mem.num());

    // First index
    mem.first(i);
    $display("First index used: %0d", i);

    // Next index
    mem.next(i);
    $display("Next index: %0d", i);

    // Last index
    mem.last(i);
    $display("Last index used: %0d", i);

    // Previous index
    mem.prev(i);
    $display("Previous index: %0d", i);

    // Delete all elements
    mem.delete();
    $display("Size after delete: %0d", mem.num());

    // Check existence
    $display("Element exists? %0d", mem.exists(i));
  end
endmodule

Output:

Size of array:     4
First index used:  0
Next index:        3
Last index used:   7
Previous index:    5
Size after delete: 0
Element exists?    0

Summary