VLSI DV Interview Puzzles · All levels

String Number Parsing Stops Early

What values do `atoi` and `atohex` return here?

Puzzle

Difficulty: Medium · Puzzle 6 of 6 · Topic: String and Casting Puzzles

What values do `atoi` and `atohex` return here?

Code

systemverilog

module p6;
  string s1 = "1234";
  string s2 = "12x3";
  string s3 = "1f";
  int d1, d2, h1;
  initial begin
    d1 = s1.atoi();
    d2 = s2.atoi();
    h1 = s3.atohex();
    $display("d1=%0d d2=%0d h1=%0d", d1, d2, h1);
  end
endmodule

Hint

`atoi` reads decimal digits from start and stops at first non-digit. `atohex` interprets hex digits.

Step-by-step solution

diagram

1) `"1234".atoi()` consumes all digits -> 1234.
2) `"12x3".atoi()` stops at `x`, so value is 12.
3) `"1f".atohex()` parses as hex 0x1F -> 31.

Answer

Answer: d1=1234, d2=12, h1=31

Why candidates get it wrong

Interviewees often assume parse fails completely when non-digit appears mid-string.

Interviewer follow-up

What does `"x123".atoi()` return, and why?

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