foreach Inline Constraint on Dynamic Array

Puzzle

Difficulty: Medium · Puzzle 3 of 6 · Topic: randomize() with Puzzles

Does this randomize call have one solution or many? State the exact resulting n and data.

Code

class payload_req;
  rand int unsigned n;
  rand byte data[];
  constraint c_base {
    n inside {[2:4]};
    data.size() == n;
  }
endclass

payload_req r = new();
assert(r.randomize() with {
  n == 3;
  foreach (data[i]) data[i] == i;
});

Hint

n fixes size first; foreach fixes each element exactly.

Step-by-step solution

1) Inline constraint fixes n=3.
2) data.size()==n gives size 3.
3) foreach imposes data[0]=0, data[1]=1, data[2]=2.
4) Exactly one legal assignment exists.

Answer

Answer: Deterministic outcome: n=3 and data='{0,1,2}; success probability is effectively 100% because the set is satisfiable and unique.

Why candidates get it wrong

Some assume foreach introduces randomness by itself; equality in foreach can make the result fully deterministic.

Interviewer follow-up

How would you tweak foreach to keep n=3 but allow 8 distinct data combinations?

Related topics

• Back to randomize() with Puzzles

• Randomization & Constraint Puzzles section