Scala algorithm: Check word in grid (depth-first search)

Published

Algorithm goal

Determine whether a specified word exists contiguously a grid. The rules are: letters cannot be re-used, you can only go left, right, up, or down. For example, 'ALGO' is a word in the following grid:

ALGQ
WGRQ
WORQ
SWOD

There is the same problem but with a stack-safe Stack Safetysolution, at: CheckWordInGridStackSafe

Test cases in Scala

assert(
  wordInGridDepthFirst(
    Array(
      Array('A', 'L', 'G', 'Q'),
      Array('W', 'G', 'R', 'Q'),
      Array('W', 'O', 'R', 'Q'),
      Array('S', 'W', 'O', 'D')
    ),
    "ALGO"
  )
)
assert(
  !wordInGridDepthFirst(
    Array(
      Array('A', 'L', 'E', 'Q'),
      Array('W', 'G', 'R', 'Q'),
      Array('W', 'L', 'R', 'Q'),
      Array('S', 'W', 'O', 'D')
    ),
    "ALGLE"
  )
)
assert(
  wordInGridDepthFirst(
    Array(
      Array('A', 'L', 'E', 'Q'),
      Array('W', 'G', 'R', 'Q'),
      Array('W', 'L', 'E', 'Q'),
      Array('S', 'W', 'O', 'D')
    ),
    "ALGLE"
  )
)

Algorithm in Scala

31 lines of Scala (compatible versions 2.13 & 3.0), showing how concise Scala can be!

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Explanation

For each index in the grid, we generate a point, and then during the downward traversal, we include points we have already seen in a separate Set. This allows us to not touch points we already have touched.

For a more stack-safe solution, see: CheckWordInGridStackSafe (Stack Safety). (this is © from www.scala-algorithms.com)

Scala concepts & Hints

  1. Def Inside Def

    A great aspect of Scala is being able to declare functions inside functions, making it possible to reduce repetition.

    def exampleDef(input: String): String = {
  def surroundInputWith(char: Char): String = s"$char$input$char"
  surroundInputWith('-')
}

assert(exampleDef("test") == "-test-")

    It is also frequently used in combination with Tail Recursion.

  2. Range

    The (1 to n) syntax produces a "Range" which is a representation of a sequence of numbers.

    assert((1 to 5).toString == "Range 1 to 5")

assert((1 to 5).reverse.toString() == "Range 5 to 1 by -1")

assert((1 to 5).toList == List(1, 2, 3, 4, 5))

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Fully unit-tested, with explanations and relevant concepts; new algorithms published about once a week.

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  10. Check word in grid (depth-first search)
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Explore the 22 most useful Scala concepts

To save you going through various tutorials, we cherry-picked the most useful Scala concepts in a consistent form.

  1. Class Inside Class
  2. Class Inside Def
  3. Collect
  4. Def Inside Def
  5. Drop, Take, dropRight, takeRight
  6. foldLeft and foldRight
  7. For-comprehension
  8. Lazy List
  9. Option Type
  10. Ordering
  11. Partial Function
  12. Pattern Matching
  13. Range
  14. scanLeft and scanRight
  15. Sliding / Sliding Window
  16. Stack Safety
  17. State machine
  18. Tail Recursion
  19. Type Class
  20. Variance
  21. View
  22. Zip

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