pandas.DataFrame Query

A pandas.DataFrame Operation is a Pandas query (a python-based tabular data structure query) for a pandas.DataFrame structure.

• Context:
  • It can be a pandas.DataFrame.join Query.
  • …
• Example(s)
  • Query an array:
    • dataValue = df.loc[indexValue,['col2']][0]
    • seriesRecord = df.iloc[row=7]
    • df[ ['colK','colL'] ][(df.colI=='valX') and (df.colJ=='valY')]
    • df[df['colX'].str.contains("strS")]
    • mask=df['count'] > 2
df[mask]
    • df.textItem.apply(lambda s: s.split(' ')).str.len() # token count .
    • df.groupby(["colX","colY"]).count(), Group By Query.
    • DataFrame({'count' : df.groupby(["colX","colY"]).size()}).reset_index()
    • DataFrame({'count' : df.groupby(["colX","colY"]).size()}).reset_index().query('(count>4321)')
    • g=df.groupby(['col1'])
g.count().sort('col2', ascending=False)
g.filter(lambda x: x['col1'].count() > minCount) # Roll-Up Query
    • srs_tokenCount = df.col2.apply(lambda x: pd.value_counts(x.lower().split(" "))).sum(axis = 0)
  • Query an array's structure:
    • rows, cols = df.shape
    • rows = len(df.index)
  • Query an array's metadata:
    • df.columns
    • df.dtypes
  • Iterate over an array.
    • for index, row in df.iterrows():
    print row['colY'], row['colX']
  • Selection by Label
    • dfl = pd.DataFrame(np.random.randn(5,4), columns=list('ABCD'), index=pd.date_range('20130101',periods=5))
dfl.loc['20130102':'20130104']
  • Selection by Position
    • df1 = pd.DataFrame(np.random.randn(6,4), index=list(range(0,12,2)), columns=list(range(0,8,2)))
df1.iloc[1:5, 2:4]
    • Selection by Callable
  • df1 = pd.DataFrame(np.random.randn(6, 4), index=list('abcdef'), columns=list('ABCD'))
df1.loc[lambda df: df.A > 0, :]
df1.iloc[:, lambda df: [0, 1]]
• Counter-Example(s)
  • a pandas.Series Query, on a pandas.Series.
  • a numpy.ndarray Query, on a numpy.ndarray.
  • a SciPy Sparse Array Query.
  • a Python Array Query.
  • a Python List Query.
  • a Perl Associative Array Query.
  • a Perl Array Query.
  • a SQL Table Query.
  • an R DataFrame Query.
• See: pandas.DataFrame Attribute, Pandas DataFrame Operation.

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References

• http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.query.html

2016

• http://pandas.pydata.org/pandas-docs/stable/indexing.html#selection-by-label
  • dfl = pd.DataFrame(np.random.randn(5,4), columns=list('ABCD'), index=pd.date_range('20130101',periods=5))
dfl.loc['20130102':'20130104']
• http://pandas.pydata.org/pandas-docs/stable/indexing.html#selection-by-position
  • df1 = pd.DataFrame(np.random.randn(6,4), index=list(range(0,12,2)), columns=list(range(0,8,2)))
df1.iloc[1:5, 2:4]
• http://pandas.pydata.org/pandas-docs/stable/indexing.html#selection-by-callable
  • df1 = pd.DataFrame(np.random.randn(6, 4), index=list('abcdef'), columns=list('ABCD'))
df1.loc[lambda df: df.A > 0, :]
df1.iloc[:, lambda df: [0, 1]]

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