class_docstr: Translates slice objects to concatenation along the first axis.
  
  This is a simple way to build up tensor quickly. There are two use cases.
  
  1. If the index expression contains comma separated tensors, then stack
     them along their first axis.
  2. If the index expression contains slice notation or scalars then create
     a 1-D tensor with a range indicated by the slice notation.
  
  If slice notation is used, the syntax ``start:stop:step`` is equivalent
  to ``mt.arange(start, stop, step)`` inside of the brackets. However, if
  ``step`` is an imaginary number (i.e. 100j) then its integer portion is
  interpreted as a number-of-points desired and the start and stop are
  inclusive. In other words ``start:stop:stepj`` is interpreted as
  ``mt.linspace(start, stop, step, endpoint=1)`` inside of the brackets.
  After expansion of slice notation, all comma separated sequences are
  concatenated together.
  
  Optional character strings placed as the first element of the index
  expression can be used to change the output. The strings 'r' or 'c' result
  in matrix output. If the result is 1-D and 'r' is specified a 1 x N (row)
  matrix is produced. If the result is 1-D and 'c' is specified, then a N x 1
  (column) matrix is produced. If the result is 2-D then both provide the
  same matrix result.
  
  A string integer specifies which axis to stack multiple comma separated
  tensors along. A string of two comma-separated integers allows indication
  of the minimum number of dimensions to force each entry into as the
  second integer (the axis to concatenate along is still the first integer).
  
  A string with three comma-separated integers allows specification of the
  axis to concatenate along, the minimum number of dimensions to force the
  entries to, and which axis should contain the start of the tensors which
  are less than the specified number of dimensions. In other words the third
  integer allows you to specify where the 1's should be placed in the shape
  of the tensors that have their shapes upgraded. By default, they are placed
  in the front of the shape tuple. The third argument allows you to specify
  where the start of the tensor should be instead. Thus, a third argument of
  '0' would place the 1's at the end of the tensor shape. Negative integers
  specify where in the new shape tuple the last dimension of upgraded tensors
  should be placed, so the default is '-1'.
  
  Parameters
  ----------
  Not a function, so takes no parameters
  
  
  Returns
  -------
  A concatenated tensor or matrix.
  
  See Also
  --------
  concatenate : Join a sequence of tensors along an existing axis.
  c_ : Translates slice objects to concatenation along the second axis.
  
  Examples
  --------
  >>> import mars.tensor as mt
  >>> mt.r_[mt.array([1,2,3]), 0, 0, mt.array([4,5,6])].execute()
  array([1, 2, 3, ..., 4, 5, 6])
  >>> mt.r_[-1:1:6j, [0]*3, 5, 6].execute()
  array([-1. , -0.6, -0.2,  0.2,  0.6,  1. ,  0. ,  0. ,  0. ,  5. ,  6. ])
  
  String integers specify the axis to concatenate along or the minimum
  number of dimensions to force entries into.
  
  >>> a = mt.array([[0, 1, 2], [3, 4, 5]])
  >>> mt.r_['-1', a, a].execute() # concatenate along last axis
  array([[0, 1, 2, 0, 1, 2],
         [3, 4, 5, 3, 4, 5]])
  >>> mt.r_['0,2', [1,2,3], [4,5,6]].execute() # concatenate along first axis, dim>=2
  array([[1, 2, 3],
         [4, 5, 6]])
  
  >>> mt.r_['0,2,0', [1,2,3], [4,5,6]].execute()
  array([[1],
         [2],
         [3],
         [4],
         [5],
         [6]])
  >>> mt.r_['1,2,0', [1,2,3], [4,5,6]].execute()
  array([[1, 4],
         [2, 5],
         [3, 6]])
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code: def __init__(self):
      AxisConcatenator.__init__(self, 0)
  
imports: import math
  import numpy
  from numpy.core.numerictypes import find_common_type
  from numpy.core.numeric import ScalarType
  from numpy.lib.index_tricks import ndindex
  from ..None import datasource
  from ..core import Tensor
  from ..base import ndim
  from ..merge import concatenate
label_desc: Use image node_id 1 to create a new RClass object from inherited base classes: AxisConcatenator with example: obj = RClass()