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4551 :: How to iterate over a seq[seq[int]] to keep only columns?

• cag104 (orginal) [2019-01-13T10:46:45+01:00] view original

  I have this seq[seq[int]] object:

  
  @[@[2, 4, 1], @[2, 0, 2], @[1, 1, 2], @[0, 0, 0]]
  Run

  I want to iterate over each column so that I can perform a function on the entire column. So the output should be:

  
  column = @[2, 2, 1, 0]
  column = @[4, 0, 1, 0]
  column = @[1, 2, 2, 0]
  Run

  How do I do this?

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• Stromberg (orginal) [2019-01-13T11:41:44+01:00] view original

  This works, however I'm looking forward to see what others come up with.

  let rows = @[@[2, 4, 1], @[2, 0, 2], @[1, 1, 2], @[0, 0, 0]]

  for index in rows[0].low .. rows[0].high:

  var column: seq[int]

  for row in rows:

  column.add(row[index])

  echo("column = ", column)

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• BigEpsilon (orginal) [2019-01-13T13:45:09+01:00] view original

  If you want to work with matrices I advice you to work with a library like Arraymancer instead of doing it by hand.

  It will be easier and more efficient.

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• miran (orginal) [2019-01-13T14:14:27+01:00] view original

  If you want to work with matrices I advice you to work with a library like ...

  ... Neo

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• cag104 (orginal) [2019-01-13T20:31:39+01:00] view original

  This is for coding homework where we are not allowed to use external packages. Otherwise yes I agree!

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• cag104 (orginal) [2019-01-13T20:33:13+01:00] view original

  This works perfectly, thank you! I manages to simply transpose the matrix to get what I wanted, but this makes your code snippet was very helpful.

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• lux (orginal) [2019-01-14T20:04:11+01:00] view original

  I'm a newbie but I think it is very 'Nim' to make an iterator:

  iterator columns*[T](mat: seq[seq[T]]): seq[T] =
    var col = newSeq[T](mat.len)
    for c in 0..<mat[0].len:
      for r in 0..<mat.len:
        col[r] = mat[r][c]
      yield col
  Run

  and then you can do:

  let v = @[@[2, 4, 1], @[2, 0, 2], @[1, 1, 2], @[0, 0, 0]]
    for i in columns(v):
      echo i
  
  
  @[2, 2, 1, 0]
  @[4, 0, 1, 0]
  @[1, 2, 2, 0]
  Run

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