SystemVerilog Dynamic Array

A dynamic array is an unpacked array whose size can be set or changed at run time, and hence is quite different from a static array where the size is pre-determined during declaration of the array. The default size of a dynamic array is zero until it is set by the new() constructor.

Syntax

A dynamic array dimensions are specified by the empty square brackets [ ].

	[data_type] [identifier_name]  [];

	bit [7:0] 	stack []; 		// A dynamic array of 8-bit vector
	string 		names []; 		// A dynamic array that can contain strings 

The new() function is used to allocate a size for the array and initialize its elements if required.

Dynamic Array Example

module tb;
	// Create a dynamic array that can hold elements of type int
	int 	array []; 			
	
	initial begin
		// Create a size for the dynamic array -> size here is 5
		// so that it can hold 5 values
		array = new [5]; 			
      
		// Initialize the array with five values
		array = '{31, 67, 10, 4, 99};
      
		// Loop through the array and print their values
		foreach (array[i]) 
			$display ("array[%0d] = %0d", i, array[i]);
	end
endmodule

ncsim> run
array[0] = 31
array[1] = 67
array[2] = 10
array[3] = 4
array[4] = 99
ncsim: *W,RNQUIE: Simulation is complete.

Dynamic Array Methods

module tb;
	// Create a dynamic array that can hold elements of type string
	string 	fruits []; 			
	
	initial begin
		// Create a size for the dynamic array -> size here is 5
		// so that it can hold 5 values
      	fruits = new [3]; 			
      
		// Initialize the array with five values
      	fruits = '{"apple", "orange", "mango"};
      
      	// Print size of the dynamic array
		$display ("fruits.size() = %0d", fruits.size());
      
		// Empty the dynamic array by deleting all items
		fruits.delete();
		$display ("fruits.size() = %0d", fruits.size());
	end
endmodule

ncsim> run
fruits.size() = 3
fruits.size() = 0
ncsim: *W,RNQUIE: Simulation is complete.

How to add new items to a dynamic array ?

Many times we may need to add new elements to an existing dynamic array without losing its original contents. Since the new() operator is used to allocate a particular size for the array, we also have to copy the old array contents into the new one after creation.

	int array [];
	array = new [10];
	
	// This creates one more slot in the array, while keeping old contents
	array = new [array.size() + 1] (array); 	

Copying dynamic array example

module tb;
	// Create two dynamic arrays of type int
	int array [];
	int id [];
  
	initial begin
		// Allocate 5 memory locations to "array" and initialize with values
		array = new [5];
		array = '{1, 2, 3, 4, 5};
      
		// Point "id" to "array"
		id = array;
      
		// Display contents of "id"
		$display ("id = %p", id);
      
		// Grow size by 1 and copy existing elements to the new dyn.Array "id"
		id = new [id.size() + 1] (id);
      
		// Assign value 6 to the newly added location [index 5]
		id [id.size() - 1] = 6;
      
		// Display contents of new "id"
		$display ("New id = %p", id);
      
		// Display size of both arrays
		$display ("array.size() = %0d, id.size() = %0d", array.size(), id.size());
	end
endmodule

run -all;
ncsim> run
id = '{1, 2, 3, 4, 5}
New id = '{1, 2, 3, 4, 5, 6}
array.size() = 5
, id.size() = 6
ncsim: *W,RNQUIE: Simulation is complete.