Enable/Disable specific constraints:

In below example we can understand how we can enable or disable a specific constraint whenever we need to do.

class rmode; 
  rand int a; 
  rand int b; 
  constraint ct_a { a == 50;} 
  constraint ct_b { b == 100;} 
endclass 

program main; 
  rmode rnd = new(); 
  initial begin 
    void'(rnd.randomize()); 
    $display(" 1 : a : %0d,  b : %0d ",rnd.a, rnd.b);
    
    rnd.ct_a.constraint_mode(0); //disable constraint ct_a
    void'(rnd.randomize()); 
    $display(" 2 : a : %0d,  b : %0d ",rnd.a, rnd.b);
    
    rnd.ct_a.constraint_mode(1); //enable constraint ct_a
    void'(rnd.randomize());
    $display(" 3 : a : %0d,  b : %0d ",rnd.a, rnd.b); 
  end 
endprogram

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