sarray_mode.py

import graphlab as gl

def mode_sa(sa, single_mode=True):
    """Return a mode of sa, or all modes if there are several.

    single_mode: whether to return a single mode or an SArray of all modes (default: True)."""
    
    if len(sa) == 0:
        raise ValueError("Can't find mode(s) in empty SArray")
    
    sf = gl.SFrame({"value": sa})
    sf2 = sf.groupby("value", {"count": gl.aggregate.COUNT()})
    max_count_index = sf2["count"].argmax()

    if single_mode:
        return sf2[max_count_index]["value"]

    else:
        max_count = sf2[max_count_index]["count"]
        return sf2[sf2["count"] == max_count]["value"]


# Create an SArray with two modes (most-common elements: 2 and 3)
sa = gl.SArray([1, 2, 2, 3, 3])

# Find one of the modes
single_mode = mode_sa(sa) # returns 2

# Find all modes
all_modes = mode_sa(sa, single_mode=False)
# Returns
# dtype: int
# Rows: 2
# [2, 3]


# A faster (albeit maybe less accurate) way to find the mode value is using sa.sketch_summary().frequent_items() .
# There are two caveats to this approach:
# 1. won't work for very low-frequency mode values, and 
# 2. won't necessarily give the correct result if there are multiple likely candidates.

def sketch_mode_sa(sa, single_mode=True):
    """Fast (albeit less accurate) way to find the mode value(s) of SArray sa.

    single_mode: whether to return a single mode or an SArray of all modes (default: True)."""
    
    if len(sa) == 0:
        raise ValueError("Can't find mode(s) in empty SArray")
    
    frequent_items_sketch = sa.sketch_summary().frequent_items()
    modes_sketch = [k for (k, v) in frequent_items_sketch.iteritems()
                    if v == max(frequent_items_sketch.itervalues())] 
    return modes_sketch[0] if single_mode else modes_sketch

sketch_mode_sa(sa) # returns 2
sketch_mode_sa(sa, single_mode=False) # returns [2, 3]


# Both approaches should handle empty SArrays.
# The implementations above will simply raise a ValueError if `sa` is empty.
try:
    mode_sa(gl.SArray([]))
except ValueError:
    pass

try:
    sketch_mode_sa(gl.SArray([]))
except ValueError:
    pass