VLSI DV Interview Puzzles · All levels

Zero Weight Means Never Chosen

Given this dist, what is P(x==0) and P(x==4)?

Puzzle

Difficulty: Medium · Puzzle 6 of 6 · Topic: Distribution Puzzles (`dist`, `:=` vs `:/`)

Given this dist, what is P(x==0) and P(x==4)?

Code

systemverilog

class zero_weight;
  rand int unsigned x;
  constraint c {
    x dist { 0 := 0, [1:3] := 1, [4:5] :/ 2 };
  }
endclass

Hint

Translate [4:5] :/ 2 into per-value weights first.

Step-by-step solution

diagram

1) x=0 has weight 0, so it is effectively excluded.
2) x=1,2,3 each get weight 1.
3) [4:5] :/ 2 gives each of 4 and 5 weight 1.
4) Total non-zero weight=5.
5) P(x==0)=0 and P(x==4)=1/5.

Answer

Answer: x==0 is impossible (0%), and P(x==4)=20%.

Why candidates get it wrong

Some candidates think zero weight means 'rare' instead of 'never selected.'

Interviewer follow-up

Would adding x inside {[0:5]} change these probabilities?

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