DataFrame.where()

DataFrame.where(cond, other=nan, inplace=False, axis=None, level=None, try_cast=False, raise_on_error=True) [source]

Return an object of same shape as self and whose corresponding entries are from self where cond is True and otherwise are from other.

Parameters:

Parameters:	cond : boolean NDFrame, array or callable If cond is callable, it is computed on the NDFrame and should return boolean NDFrame or array. The callable must not change input NDFrame (though pandas doesn?t check it). New in version 0.18.1. A callable can be used as cond. other : scalar, NDFrame, or callable If other is callable, it is computed on the NDFrame and should return scalar or NDFrame. The callable must not change input NDFrame (though pandas doesn?t check it). New in version 0.18.1. A callable can be used as other. inplace : boolean, default False Whether to perform the operation in place on the data axis : alignment axis if needed, default None level : alignment level if needed, default None try_cast : boolean, default False try to cast the result back to the input type (if possible), raise_on_error : boolean, default True Whether to raise on invalid data types (e.g. trying to where on strings)
Returns:	wh : same type as caller

cond : boolean NDFrame, array or callable

If cond is callable, it is computed on the NDFrame and should return boolean NDFrame or array. The callable must not change input NDFrame (though pandas doesn?t check it).

New in version 0.18.1.

A callable can be used as cond.

other : scalar, NDFrame, or callable

If other is callable, it is computed on the NDFrame and should return scalar or NDFrame. The callable must not change input NDFrame (though pandas doesn?t check it).

New in version 0.18.1.

A callable can be used as other.

inplace : boolean, default False

Whether to perform the operation in place on the data

axis : alignment axis if needed, default None

level : alignment level if needed, default None

try_cast : boolean, default False

try to cast the result back to the input type (if possible),

raise_on_error : boolean, default True

Whether to raise on invalid data types (e.g. trying to where on strings)

Returns:

wh : same type as caller

Notes

The where method is an application of the if-then idiom. For each element in the calling DataFrame, if cond is True the element is used; otherwise the corresponding element from the DataFrame other is used.

The signature for DataFrame.where() differs from numpy.where(). Roughly df1.where(m, df2) is equivalent to np.where(m, df1, df2).

For further details and examples see the where documentation in indexing.

Examples

>>> s = pd.Series(range(5))
>>> s.where(s > 0)
0    NaN
1    1.0
2    2.0
3    3.0
4    4.0

>>> df = pd.DataFrame(np.arange(10).reshape(-1, 2), columns=['A', 'B'])
>>> m = df % 3 == 0
>>> df.where(m, -df)
   A  B
0  0 -1
1 -2  3
2 -4 -5
3  6 -7
4 -8  9
>>> df.where(m, -df) == np.where(m, df, -df)
      A     B
0  True  True
1  True  True
2  True  True
3  True  True
4  True  True
>>> df.where(m, -df) == df.mask(~m, -df)
      A     B
0  True  True
1  True  True
2  True  True
3  True  True
4  True  True

Links:

http://pandas.pydata.org/pandas-docs/version/0.19.2/generated/pandas.DataFrame.where.html

doc_Pandas

2017-01-12 04:46:51

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