Github user MechCoder commented on a diff in the pull request:

    https://github.com/apache/spark/pull/7854#discussion_r37826567
  
    --- Diff: python/pyspark/mllib/linalg/__init__.py ---
    @@ -461,32 +461,41 @@ def __init__(self, size, *args):
             self.size = int(size)
             """ Size of the vector. """
             assert 1 <= len(args) <= 2, "must pass either 2 or 3 arguments"
    -        if len(args) == 1:
    -            pairs = args[0]
    -            if type(pairs) == dict:
    -                pairs = pairs.items()
    -            pairs = sorted(pairs)
    -            self.indices = np.array([p[0] for p in pairs], dtype=np.int32)
    -            """ A list of indices corresponding to active entries. """
    -            self.values = np.array([p[1] for p in pairs], dtype=np.float64)
    -            """ A list of values corresponding to active entries. """
    +        if isinstance(args[0], bytes):
    +            assert isinstance(args[1], bytes), "values should be string 
too"
    +            if args[0]:
    +                self.indices = np.frombuffer(args[0], np.int32)
    +                self.values = np.frombuffer(args[1], np.float64)
    +            else:
    +                # np.frombuffer() doesn't work well with empty string in 
older version
    +                self.indices = np.array([], dtype=np.int32)
    +                self.values = np.array([], dtype=np.float64)
             else:
    -            if isinstance(args[0], bytes):
    -                assert isinstance(args[1], bytes), "values should be 
string too"
    -                if args[0]:
    -                    self.indices = np.frombuffer(args[0], np.int32)
    -                    self.values = np.frombuffer(args[1], np.float64)
    -                else:
    -                    # np.frombuffer() doesn't work well with empty string 
in older version
    -                    self.indices = np.array([], dtype=np.int32)
    -                    self.values = np.array([], dtype=np.float64)
    +            if len(args) == 1:
    +                args = args[0]
    +                if isinstance(args, dict):
    +                    args = args.items()
    +                args = list(zip(*args))
    +
    +            # Handle empty args case.
    +            if len(args) == 0:
    +                indices = []
    +                values = []
                 else:
    -                self.indices = np.array(args[0], dtype=np.int32)
    -                self.values = np.array(args[1], dtype=np.float64)
    -            assert len(self.indices) == len(self.values), "index and value 
arrays not same length"
    -            for i in xrange(len(self.indices) - 1):
    -                if self.indices[i] >= self.indices[i + 1]:
    -                    raise TypeError("indices array must be sorted")
    --- End diff --
    
    Ah, I often just look at the direct SparseVector initialization but not 
`Vectors.sparse`. hence I did not notice it.
    
    The SparseVector initialization should take constant time and one should 
expect the user should supply sorted indices (I remember another PR which was 
closed because it did a O(n) check)
    
    However, I do think that this should be documented somewhere clearly that 
the indices provided should be sorted.


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