Python Pandas - Series

Series is a one-dimensional labeled array capable of holding data of any type (integer, string, float, python objects, etc.). The axis labels are collectively called index.
A pandas Series can be created using the following constructor −
pandas.Series( data, index, dtype, copy)

The parameters of the constructor are as follows −
data
data takes various forms like ndarray, list, constants
index
Index values must be unique and hashable, same length as data. Default np.arrange(n) if no index is passed.
dtype
dtype is for data type. If None, data type will be inferred
copy
Copy data. Default False
A series can be created using various inputs like −
Array
Dict
Scalar value or constant
A basic series, which can be created is an Empty Series.
#import the pandas library and aliasing as pd
import pandas as pd
s = pd.Series()
print s
Its output is as follows −
Series([], dtype: float64)

If data is an ndarray, then index passed must be of the same length. If no index is passed, then by default index will be range(n) where n is array length, i.e., [0,1,2,3.... range(len(array))-1].
#import the pandas library and aliasing as pd
import pandas as pd
import numpy as np
data = np.array(['a','b','c','d'])
s = pd.Series(data)
print s
Its output is as follows −
0   a
1   b
2   c
3   d
dtype: object

We did not pass any index, so by default, it assigned the indexes ranging from 0 to len(data)-1, i.e., 0 to 3.
#import the pandas library and aliasing as pd
import pandas as pd
import numpy as np
data = np.array(['a','b','c','d'])
s = pd.Series(data,index=[100,101,102,103])
print s
Its output is as follows −
100  a
101  b
102  c
103  d
dtype: object

We passed the index values here. Now we can see the customized indexed values in the output.
A dict can be passed as input and if no index is specified, then the dictionary keys are taken in a sorted order to construct index. If index is passed, the values in data corresponding to the labels in the index will be pulled out.
#import the pandas library and aliasing as pd
import pandas as pd
import numpy as np
data = {'a' : 0., 'b' : 1., 'c' : 2.}
s = pd.Series(data)
print s
Its output is as follows −
a 0.0
b 1.0
c 2.0
dtype: float64

Observe − Dictionary keys are used to construct index.
#import the pandas library and aliasing as pd
import pandas as pd
import numpy as np
data = {'a' : 0., 'b' : 1., 'c' : 2.}
s = pd.Series(data,index=['b','c','d','a'])
print s
Its output is as follows −
b 1.0
c 2.0
d NaN
a 0.0
dtype: float64

Observe − Index order is persisted and the missing element is filled with NaN (Not a
Number).
If data is a scalar value, an index must be provided. The value will be repeated to match
the length of index
#import the pandas library and aliasing as pd
import pandas as pd
import numpy as np
s = pd.Series(5, index=[0, 1, 2, 3])
print s
Its output is as follows −
0  5
1  5
2  5
3  5
dtype: int64

Data in the series can be accessed similar to that in an ndarray.
Retrieve the first element. As we already know, the counting starts from zero for the array,
which means the first element is stored at zeroth position and so on.
import pandas as pd
s = pd.Series([1,2,3,4,5],index = ['a','b','c','d','e'])

#retrieve the first element
print s[0]
Its output is as follows −
1

Retrieve the first three elements in the Series. If a : is inserted in front of it, all items from that index onwards will be extracted. If two parameters (with : between them) is used, items between the two indexes (not including the stop index)
import pandas as pd
s = pd.Series([1,2,3,4,5],index = ['a','b','c','d','e'])

#retrieve the first three element
print s[:3]
Its output is as follows −
a  1
b  2
c  3
dtype: int64

Retrieve the last three elements.
import pandas as pd
s = pd.Series([1,2,3,4,5],index = ['a','b','c','d','e'])

#retrieve the last three element
print s[-3:]
Its output is as follows −
c  3
d  4
e  5
dtype: int64

A Series is like a fixed-size dict in that you can get and set values by index label.
Retrieve a single element using index label value.
import pandas as pd
s = pd.Series([1,2,3,4,5],index = ['a','b','c','d','e'])

#retrieve a single element
print s['a']
Its output is as follows −
1

Retrieve multiple elements using a list of index label values.
import pandas as pd
s = pd.Series([1,2,3,4,5],index = ['a','b','c','d','e'])

#retrieve multiple elements
print s[['a','c','d']]
Its output is as follows −
a  1
c  3
d  4
dtype: int64

If a label is not contained, an exception is raised.
import pandas as pd
s = pd.Series([1,2,3,4,5],index = ['a','b','c','d','e'])

#retrieve multiple elements
print s['f']
Its output is as follows −
...
KeyError: 'f'


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