Lesson 02 · Parsing a puzzle string

Parsing a puzzle string

Puzzles travel as an 81-character string - one character per cell, a digit for a clue and a dot or zero for a blank. Today you turn that string into a grid of 81 numbers, the input every solver call starts from.

The goal

Parse an 81-character puzzle string into a grid of 81 integers, 0 for a blank.

Start here - the target

TO DO

Scenario: A puzzle string becomes a grid of numbers

Giventhe 81-character string "003020600900305001001806400008102900700000008006708200002609500800203009005010300"

Whenit is parsed into a grid of 81 integers

Thencell 0 is 0 (a blank), cell 2 is 3, and exactly 32 cells are non-zero clues

Andboth "." and "0" parse to the blank value 0, so a cell is either 0 or a digit 1 through 9

Background

A Sudoku puzzle is almost always written as a single line of 81 characters, read row by row: the first 9 characters are the top row, the next 9 the second row, and so on down to the bottom-right corner at position 80. A filled square is its digit 1 to 9; an empty square is written as either a . or a 0, and both mean the same thing - blank. Accepting both spellings up front means you can paste puzzles from any source without massaging them first.

Storing the board as an array of 81 small integers, with 0 standing for empty, keeps everything downstream simple: a clue is a value you must not contradict, a blank is a 0 waiting to be filled. Counting the non-zero cells is a quick sanity check that the parse worked - this classic puzzle has 32 given clues.

Make it work

// one character per cell; '1'..'9' -> that digit, '.' or '0' -> 0
func Parse(s string) [81]int {
  var g [81]int
  for i := 0; i < 81; i++ {
    c := s[i]
    if c >= '1' && c <= '9' { g[i] = int(c - '0') }
    // anything else stays 0 (blank)
  }
  return g
}

CheckpointDONE

You can load an 81-character puzzle string into a grid of numbers. Commit and stop here.