System Verilog Constraints: Get a random prime number less than given input

SystemVerilog
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1

Hi,

I am trying to write a code in System verilog to “get a random prime number less than the given input”?

Can someone help me with the logic for this, or how to use constraints here?

2

In reply to Akhil Mehta:

See prime numbers constraint | Verification Academy

3

In reply to dave_59:

How to add the constraint of ‘less than input’?

4

In reply to Akhil Mehta:

The example I gave you shows how to get primes less that 200. change 200 to your input,

5

In reply to Akhil Mehta:

module test;
class prime_number;
  int limit;
  rand int a[];
  constraint abc {a.size==limit; }
  constraint cba { foreach(a[i])
    a[i]==prime(i);}
 
  function int prime( int g);
    if(g <= 1)return 2;
    for(int i=2;i<g;i++)
      if(g%i==0)
          return 2; //if it is not a prime number ,returning 2 which is one of prime
      prime=g;
    endfunction
  function void post_randomize();
    a=a.unique;
  endfunction
endclass
 
prime_number pri;
 
initial
  begin
    pri=new;
    pri.limit = 290;
    void'(pri.randomize);
    $display("%p",pri);
  end
endmodule
6

In reply to Shubhabrata:

In reply to Akhil Mehta:

module test;
class prime_number;
int limit;
rand int a[];
constraint abc {a.size==limit; }
constraint cba { foreach(a[i])
a[i]==prime(i);}
function int prime( int g);
if(g <= 1)return 2;
for(int i=2;i<g;i++)
if(g%i==0)
return 2; //if it is not a prime number ,returning 2 which is one of prime
prime=g;
endfunction
function void post_randomize();
a=a.unique;
endfunction
endclass
prime_number pri;
initial
begin
pri=new;
pri.limit = 290;
void'(pri.randomize);
$display("%p",pri);
end
endmodule

Can someone please explain how below part working in above code ?

for(int i=2;i<g;i++)
      if(g%i==0)
          return 2; //if it is not a prime number ,returning 2 which is one of prime
      prime=g;

Thanks in advance!!

7

In reply to Harshad:

The function prime(arg) is returning the argument passed into it if the argument is a prime number, otherwise it returns 2. This assumes the limit is going to be 2 or greater. Then post_randomize() eliminates all the duplicate 2’s.

This might be a clearer way to write the class

class prime_number;
  int limit;
  rand int a[];
  constraint abc {a.size==limit; limit > 1; }
  constraint cba { foreach(a[i])
    if (prime(i))
      a[i] == i;
    else
      a[i] == 2;
                 }
  function bit prime( int g);
    if(g <= 1) return 0; // not a prime
    for(int i=2;i<g;i++)
      if(g%i==0)
          return 0; // not a prime
      return 1; // is a prime
    endfunction
  function void post_randomize();
    a=a.unique; // gert rid of duplicate 2's
  endfunction
endclass
8

In reply to dave_59:

Thanks Dave ! but still I confused that how below part is execute

for(int i=2;i<g;i++)
      if(g%i==0)
          return 0; // not a prime
      return 1; // is a prime

My understanding is, suppose if g=3 and i=2 then 3%2 is not equal to zero because remainder is 1 and function will return 0(not prime) but here g=3 is a prime number so this is wrong

9

In reply to Harshad:
When g=3, the for-loop iterates once with i=2. 3%2 does not equal 0, so the for-loop terminates executing return 1 (is a prime).

10

In reply to dave_59:

Thanks Dave! understood…