Foreach Loop

System Verilog provides the foreach loop as a convenient way to iterate through arrays and multi-dimensional arrays. Unlike traditional loops like for, which require explicit index management, the foreach loop simplifies the process by automatically iterating over each element of an array. This feature is especially useful in verification environments and hardware modeling where array operations like dynamic, associative, and multi-dimensional arrays are common.

Syntax:

foreach (array_name[index]) begin
    // Code block using array_name[index]
end
  • array_name: The name of the array to iterate over.
  • index: The loop variable representing the current index.

Example 1: Iterating Over a One-Dimensional Array

module foreach_example1;
    logic [7:0] arr [0:9];  // Declare an array of 10 elements

    initial begin
        // Initialize the array with values equal to their indices
      foreach (arr[i]) begin
            arr[i] = i;
        end

        // Use foreach to display the array values
        foreach (arr[i]) begin
            $display("arr[%0d] = %0d", i, arr[i]);
        end
    end
endmodule
arr[0] = 0
arr[1] = 1
arr[2] = 2
arr[3] = 3
arr[4] = 4
arr[5] = 5
arr[6] = 6
arr[7] = 7
arr[8] = 8
arr[9] = 9

Example 2: Iterating Over a Multi-Dimensional Array

module foreach_example2;
    logic [3:0] matrix [0:2][0:2];  // Declare a 3x3 matrix

    initial begin
        // Initialize the matrix
        foreach (matrix[i, j]) begin
            matrix[i][j] = i * j;  // Set each element to row*column
        end

        // Display the matrix
        foreach (matrix[i, j]) begin
            $display("matrix[%0d][%0d] = %0d", i, j, matrix[i][j]);
        end
    end
endmodule

Explanation:

  • The foreach loop uses multiple indices (i and j) to iterate over the rows and columns of the matrix.
  • This simplifies handling of multi-dimensional arrays without needing nested loops.
matrix[0][0] = 0
matrix[0][1] = 0
matrix[0][2] = 0
matrix[1][0] = 0
matrix[1][1] = 1
matrix[1][2] = 2
matrix[2][0] = 0
matrix[2][1] = 2
matrix[2][2] = 4

Example 3: Iterating Over Associative Arrays

module foreach_example3;
    typedef int unsigned key_t;
    typedef logic [7:0] value_t;

    value_t assoc_array[key_t];  // Declare an associative array

    initial begin
        // Populate the associative array
        assoc_array[1] = 8'hAA;
        assoc_array[3] = 8'hBB;
        assoc_array[5] = 8'hCC;

        // Use foreach to iterate over the associative array
        foreach (assoc_array[key]) begin
            $display("assoc_array[%0d] = %h", key, assoc_array[key]);
        end
    end
endmodule

Explanation:

  • The foreach loop iterates over all valid keys in the associative array.
  • The loop variable key automatically takes the value of each key in the array.
assoc_array[1] = aa
assoc_array[3] = bb
assoc_array[5] = cc

Features of the foreach Loop

  1. Automatic Index Management:
    • The foreach loop handles indices automatically, reducing the risk of off-by-one errors.
  2. Supports All Array Types:
    • Works seamlessly with fixed-size arrays, dynamic arrays, associative arrays, and queues.
  3. Multi-Dimensional Support:
    • Simplifies operations on multi-dimensional arrays by handling multiple indices.
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