I don't understand this SV randomization randc behaviour - system-verilog

Question

This is code for transaction class,
class transaction;
//declaring the transaction items
randc bit [3:0] a;
randc bit [3:0] b;
bit [6:0] c;
function void display(string name);
$display("-------------------------");
$display("- %s ",name);
$display("-------------------------");
$display("- a = %0d, b = %0d",a,b);
$display("- c = %0d",c);
$display("-------------------------");
endfunction
endclass
And this is code for generator class,
class generator;
rand transaction trans;
int repeat_count;
mailbox gen2driv;
event ended;
function new(mailbox gen2driv);
this.gen2driv = gen2driv;
endfunction
task main();
repeat(repeat_count) begin
trans = new();
if( !trans.randomize() ) $fatal("Gen:: trans randomization failed");
trans.display("[ Generator ]");
gen2driv.put(trans);
end
-> ended;
endtask
endclass
The value of repeat_count that I passed is 10, and here's the output:
- a = 2, b = 0
- c = 0
- a = 1, b = 9
- c = 0
- a = 9, b = 9
- c = 0
- a = 7, b = 15
- c = 0
- a = 10, b = 15
- c = 0
- a = 3, b = 1
- c = 0
- a = 13, b = 12
- c = 0
- a = 1, b = 9
- c = 0
- a = 7, b = 5
- c = 0
- a = 3, b = 15
- c = 0
But, values during randomization are not showing cyclic repetition. It is repeating itself before all possible value can occur for variables a and b.

Answer 1

Move the transaction constructor out of the repeat loop. Change:
repeat(repeat_count) begin
trans = new();
to:
trans = new();
repeat(repeat_count) begin
It seems new resets the initial random permutation of the range values each time it is called.
Here is a self-contained, runnable example which demonstrates the issue (and the fix):
class transaction;
randc bit [3:0] a; // 16 values: 0-15
function void display;
$display("a=%0d", a);
endfunction
endclass
class generator;
rand transaction trans;
task main;
//trans = new(); // Un-comment this line for fix
repeat (16) begin
trans = new(); // Comment out this line for fix
if (!trans.randomize()) $fatal(1, "trans randomization failed");
trans.display();
end
endtask
endclass
module tb;
generator gen = new();
initial gen.main();
endmodule

Answer 2

Yeah exactly to what #toolic said...initializing with new operator inside the repeat loop would create a new space every time the loop runs and thus the newly created object (transaction in your case) has no trace of the values that has been previously exercised. Thus giving out a random number making randc of no use. It though works normal with ' rand' keyword.