Selecting a row of pandas series/dataframe by integer index
Question:
I am curious as to why df[2] is not supported, while df.ix[2] and df[2:3] both work.
In [26]: df.ix[2]
Out[26]:
A 1.027680
B 1.514210
C -1.466963
D -0.162339
Name: 2000-01-03 00:00:00
In [27]: df[2:3]
Out[27]:
A B C D
2000-01-03 1.02768 1.51421 -1.466963 -0.162339
I would expect df[2] to work the same way as df[2:3] to be consistent with Python indexing convention. Is there a design reason for not supporting indexing row by single integer?
Answers:
You can think DataFrame as a dict of Series. df[key] try to select the column index by key and returns a Series object.
However slicing inside of [] slices the rows, because it’s a very common operation.
You can read the document for detail:
http://pandas.pydata.org/pandas-docs/stable/indexing.html#basics
You can take a look at the source code .
DataFrame has a private function _slice() to slice the DataFrame, and it allows the parameter axis to determine which axis to slice. The __getitem__() for DataFrame doesn’t set the axis while invoking _slice(). So the _slice() slice it by default axis 0.
You can take a simple experiment, that might help you:
print df._slice(slice(0, 2))
print df._slice(slice(0, 2), 0)
print df._slice(slice(0, 2), 1)
echoing @HYRY, see the new docs in 0.11
http://pandas.pydata.org/pandas-docs/stable/indexing.html
Here we have new operators, .iloc to explicity support only integer indexing, and .loc to explicity support only label indexing
e.g. imagine this scenario
In [1]: df = pd.DataFrame(np.random.rand(5,2),index=range(0,10,2),columns=list('AB'))
In [2]: df
Out[2]:
A B
0 1.068932 -0.794307
2 -0.470056 1.192211
4 -0.284561 0.756029
6 1.037563 -0.267820
8 -0.538478 -0.800654
In [5]: df.iloc[[2]]
Out[5]:
A B
4 -0.284561 0.756029
In [6]: df.loc[[2]]
Out[6]:
A B
2 -0.470056 1.192211
[] slices the rows (by label location) only
you can loop through the data frame like this .
for ad in range(1,dataframe_c.size):
print(dataframe_c.values[ad])
To index-based access to the pandas table, one can also consider numpy.as_array option to convert the table to Numpy array as
np_df = df.as_matrix()
and then
np_df[i]
would work.
The primary purpose of the DataFrame indexing operator, [] is to select columns.
When the indexing operator is passed a string or integer, it attempts to find a column with that particular name and return it as a Series.
So, in the question above: df[2] searches for a column name matching the integer value 2. This column does not exist and a KeyError is raised.
The DataFrame indexing operator completely changes behavior to select rows when slice notation is used
Strangely, when given a slice, the DataFrame indexing operator selects rows and can do so by integer location or by index label.
df[2:3]
This will slice beginning from the row with integer location 2 up to 3, exclusive of the last element. So, just a single row. The following selects rows beginning at integer location 6 up to but not including 20 by every third row.
df[6:20:3]
You can also use slices consisting of string labels if your DataFrame index has strings in it. For more details, see this solution on .iloc vs .loc.
I almost never use this slice notation with the indexing operator as its not explicit and hardly ever used. When slicing by rows, stick with .loc/.iloc.
I would normally go for .loc/.iloc as suggested by Ted, but one may also select a row by transposing the DataFrame. To stay in the example above, df.T[2] gives you row 2 of df.
I am curious as to why df[2] is not supported, while df.ix[2] and df[2:3] both work.
In [26]: df.ix[2]
Out[26]:
A 1.027680
B 1.514210
C -1.466963
D -0.162339
Name: 2000-01-03 00:00:00
In [27]: df[2:3]
Out[27]:
A B C D
2000-01-03 1.02768 1.51421 -1.466963 -0.162339
I would expect df[2] to work the same way as df[2:3] to be consistent with Python indexing convention. Is there a design reason for not supporting indexing row by single integer?
You can think DataFrame as a dict of Series. df[key] try to select the column index by key and returns a Series object.
However slicing inside of [] slices the rows, because it’s a very common operation.
You can read the document for detail:
http://pandas.pydata.org/pandas-docs/stable/indexing.html#basics
You can take a look at the source code .
DataFrame has a private function _slice() to slice the DataFrame, and it allows the parameter axis to determine which axis to slice. The __getitem__() for DataFrame doesn’t set the axis while invoking _slice(). So the _slice() slice it by default axis 0.
You can take a simple experiment, that might help you:
print df._slice(slice(0, 2))
print df._slice(slice(0, 2), 0)
print df._slice(slice(0, 2), 1)
echoing @HYRY, see the new docs in 0.11
http://pandas.pydata.org/pandas-docs/stable/indexing.html
Here we have new operators, .iloc to explicity support only integer indexing, and .loc to explicity support only label indexing
e.g. imagine this scenario
In [1]: df = pd.DataFrame(np.random.rand(5,2),index=range(0,10,2),columns=list('AB'))
In [2]: df
Out[2]:
A B
0 1.068932 -0.794307
2 -0.470056 1.192211
4 -0.284561 0.756029
6 1.037563 -0.267820
8 -0.538478 -0.800654
In [5]: df.iloc[[2]]
Out[5]:
A B
4 -0.284561 0.756029
In [6]: df.loc[[2]]
Out[6]:
A B
2 -0.470056 1.192211
[] slices the rows (by label location) only
you can loop through the data frame like this .
for ad in range(1,dataframe_c.size):
print(dataframe_c.values[ad])
To index-based access to the pandas table, one can also consider numpy.as_array option to convert the table to Numpy array as
np_df = df.as_matrix()
and then
np_df[i]
would work.
The primary purpose of the DataFrame indexing operator, [] is to select columns.
When the indexing operator is passed a string or integer, it attempts to find a column with that particular name and return it as a Series.
So, in the question above: df[2] searches for a column name matching the integer value 2. This column does not exist and a KeyError is raised.
The DataFrame indexing operator completely changes behavior to select rows when slice notation is used
Strangely, when given a slice, the DataFrame indexing operator selects rows and can do so by integer location or by index label.
df[2:3]
This will slice beginning from the row with integer location 2 up to 3, exclusive of the last element. So, just a single row. The following selects rows beginning at integer location 6 up to but not including 20 by every third row.
df[6:20:3]
You can also use slices consisting of string labels if your DataFrame index has strings in it. For more details, see this solution on .iloc vs .loc.
I almost never use this slice notation with the indexing operator as its not explicit and hardly ever used. When slicing by rows, stick with .loc/.iloc.
I would normally go for .loc/.iloc as suggested by Ted, but one may also select a row by transposing the DataFrame. To stay in the example above, df.T[2] gives you row 2 of df.