Python Pandas Tutorial
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Python Pandas - Iteration
Python Pandas - Iteration
The behavior of basic iteration over Pandas objects depends on the type. When iterating over a Series, it is regarded as array-pke, and basic iteration produces the values. Other data structures, pke DataFrame and Panel, follow the dict-pke convention of iterating over the keys of the objects.
In short, basic iteration (for i in object) produces −
Series − values
DataFrame − column labels
Panel − item labels
Iterating a DataFrame
Iterating a DataFrame gives column names. Let us consider the following example to understand the same.
import pandas as pd
import numpy as np
N=20
df = pd.DataFrame({
A : pd.date_range(start= 2016-01-01 ,periods=N,freq= D ),
x : np.pnspace(0,stop=N-1,num=N),
y : np.random.rand(N),
C : np.random.choice([ Low , Medium , High ],N).topst(),
D : np.random.normal(100, 10, size=(N)).topst()
})
for col in df:
print col
Its output is as follows −
A C D x y
To iterate over the rows of the DataFrame, we can use the following functions −
iteritems() − to iterate over the (key,value) pairs
iterrows() − iterate over the rows as (index,series) pairs
itertuples() − iterate over the rows as namedtuples
iteritems()
Iterates over each column as key, value pair with label as key and column value as a Series object.
import pandas as pd import numpy as np df = pd.DataFrame(np.random.randn(4,3),columns=[ col1 , col2 , col3 ]) for key,value in df.iteritems(): print key,value
Its output is as follows −
col1 0 0.802390 1 0.324060 2 0.256811 3 0.839186 Name: col1, dtype: float64 col2 0 1.624313 1 -1.033582 2 1.796663 3 1.856277 Name: col2, dtype: float64 col3 0 -0.022142 1 -0.230820 2 1.160691 3 -0.830279 Name: col3, dtype: float64
Observe, each column is iterated separately as a key-value pair in a Series.
iterrows()
iterrows() returns the iterator yielding each index value along with a series containing the data in each row.
import pandas as pd import numpy as np df = pd.DataFrame(np.random.randn(4,3),columns = [ col1 , col2 , col3 ]) for row_index,row in df.iterrows(): print row_index,row
Its output is as follows −
0 col1 1.529759 col2 0.762811 col3 -0.634691 Name: 0, dtype: float64 1 col1 -0.944087 col2 1.420919 col3 -0.507895 Name: 1, dtype: float64 2 col1 -0.077287 col2 -0.858556 col3 -0.663385 Name: 2, dtype: float64 3 col1 -1.638578 col2 0.059866 col3 0.493482 Name: 3, dtype: float64
Note − Because iterrows() iterate over the rows, it doesn t preserve the data type across the row. 0,1,2 are the row indices and col1,col2,col3 are column indices.
itertuples()
itertuples() method will return an iterator yielding a named tuple for each row in the DataFrame. The first element of the tuple will be the row’s corresponding index value, while the remaining values are the row values.
import pandas as pd
import numpy as np
df = pd.DataFrame(np.random.randn(4,3),columns = [ col1 , col2 , col3 ])
for row in df.itertuples():
print row
Its output is as follows −
Pandas(Index=0, col1=1.5297586201375899, col2=0.76281127433814944, col3=- 0.6346908238310438) Pandas(Index=1, col1=-0.94408735763808649, col2=1.4209186418359423, col3=- 0.50789517967096232) Pandas(Index=2, col1=-0.07728664756791935, col2=-0.85855574139699076, col3=- 0.6633852507207626) Pandas(Index=3, col1=0.65734942534106289, col2=-0.95057710432604969, col3=0.80344487462316527)
Note − Do not try to modify any object while iterating. Iterating is meant for reading and the iterator returns a copy of the original object (a view), thus the changes will not reflect on the original object.
import pandas as pd import numpy as np df = pd.DataFrame(np.random.randn(4,3),columns = [ col1 , col2 , col3 ]) for index, row in df.iterrows(): row[ a ] = 10 print df
Its output is as follows −
col1 col2 col3
0 -1.739815 0.735595 -0.295589
1 0.635485 0.106803 1.527922
2 -0.939064 0.547095 0.038585
3 -1.016509 -0.116580 -0.523158
Observe, no changes reflected.
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