Python Pandas - DataFrame

A Data frame is a two-dimensional data structure, i.e., data is aligned in a tabular fashion in rows and columns.
Potentially columns are of different types
Size – Mutable
Labeled axes (rows and columns)
Can Perform Arithmetic operations on rows and columns
Let us assume that we are creating a data frame with student’s data.
You can think of it as an SQL table or a spreadsheet data representation.
A pandas DataFrame can be created using the following constructor −
pandas.DataFrame( data, index, columns, dtype, copy)

The parameters of the constructor are as follows −
data
data takes various forms like ndarray, series, map, lists, dict, constants and also another DataFrame.
index
For the row labels, the Index to be used for the resulting frame is Optional Default np.arange(n) if no index is passed.
columns
For column labels, the optional default syntax is - np.arange(n). This is only true if no index is passed.
dtype
Data type of each column.
copy
This command (or whatever it is) is used for copying of data, if the default is False.
A pandas DataFrame can be created using various inputs like −
Lists
dict
Series
Numpy ndarrays
Another DataFrame
In the subsequent sections of this chapter, we will see how to create a DataFrame using these inputs.
A basic DataFrame, which can be created is an Empty Dataframe.
#import the pandas library and aliasing as pd
import pandas as pd
df = pd.DataFrame()
print df
Its output is as follows −
Empty DataFrame
Columns: []
Index: []

The DataFrame can be created using a single list or a list of lists.
import pandas as pd
data = [1,2,3,4,5]
df = pd.DataFrame(data)
print df
Its output is as follows −
     0
0    1
1    2
2    3
3    4
4    5

import pandas as pd
data = [['Alex',10],['Bob',12],['Clarke',13]]
df = pd.DataFrame(data,columns=['Name','Age'])
print df
Its output is as follows −
      Name      Age
0     Alex      10
1     Bob       12
2     Clarke    13

import pandas as pd
data = [['Alex',10],['Bob',12],['Clarke',13]]
df = pd.DataFrame(data,columns=['Name','Age'],dtype=float)
print df
Its output is as follows −
      Name     Age
0     Alex     10.0
1     Bob      12.0
2     Clarke   13.0

Note − Observe, the dtype parameter changes the type of Age column to floating point.
All the ndarrays must be of same length. If index is passed, then the length of the index should equal to the length of the arrays.
If no index is passed, then by default, index will be range(n), where n is the array length.
import pandas as pd
data = {'Name':['Tom', 'Jack', 'Steve', 'Ricky'],'Age':[28,34,29,42]}
df = pd.DataFrame(data)
print df
Its output is as follows −
      Age      Name
0     28        Tom
1     34       Jack
2     29      Steve
3     42      Ricky

Note − Observe the values 0,1,2,3. They are the default index assigned to each using the function range(n).
Let us now create an indexed DataFrame using arrays.
import pandas as pd
data = {'Name':['Tom', 'Jack', 'Steve', 'Ricky'],'Age':[28,34,29,42]}
df = pd.DataFrame(data, index=['rank1','rank2','rank3','rank4'])
print df
Its output is as follows −
         Age    Name
rank1    28      Tom
rank2    34     Jack
rank3    29    Steve
rank4    42    Ricky

Note − Observe, the index parameter assigns an index to each row.
List of Dictionaries can be passed as input data to create a DataFrame. The dictionary keys are by default taken as column names.
The following example shows how to create a DataFrame by passing a list of dictionaries.
import pandas as pd
data = [{'a': 1, 'b': 2},{'a': 5, 'b': 10, 'c': 20}]
df = pd.DataFrame(data)
print df
Its output is as follows −
    a    b      c
0   1   2     NaN
1   5   10   20.0

Note − Observe, NaN (Not a Number) is appended in missing areas.
The following example shows how to create a DataFrame by passing a list of dictionaries and the row indices.
import pandas as pd
data = [{'a': 1, 'b': 2},{'a': 5, 'b': 10, 'c': 20}]
df = pd.DataFrame(data, index=['first', 'second'])
print df
Its output is as follows −
        a   b       c
first   1   2     NaN
second  5   10   20.0

The following example shows how to create a DataFrame with a list of dictionaries, row indices, and column indices.
import pandas as pd
data = [{'a': 1, 'b': 2},{'a': 5, 'b': 10, 'c': 20}]

#With two column indices, values same as dictionary keys
df1 = pd.DataFrame(data, index=['first', 'second'], columns=['a', 'b'])

#With two column indices with one index with other name
df2 = pd.DataFrame(data, index=['first', 'second'], columns=['a', 'b1'])
print df1
print df2
Its output is as follows −
#df1 output
         a  b
first    1  2
second   5  10

#df2 output
         a  b1
first    1  NaN
second   5  NaN

Note − Observe, df2 DataFrame is created with a column index other than the dictionary key; thus, appended the NaN’s in place. Whereas, df1 is created with column indices same as dictionary keys, so NaN’s appended.
Dictionary of Series can be passed to form a DataFrame. The resultant index is the union of all the series indexes passed.
import pandas as pd

d = {'one' : pd.Series([1, 2, 3], index=['a', 'b', 'c']),
   'two' : pd.Series([1, 2, 3, 4], index=['a', 'b', 'c', 'd'])}

df = pd.DataFrame(d)
print df
Its output is as follows −
      one    two
a     1.0    1
b     2.0    2
c     3.0    3
d     NaN    4

Note − Observe, for the series one, there is no label ‘d’ passed, but in the result, for the d label, NaN is appended with NaN.
Let us now understand column selection, addition, and deletion through examples.
We will understand this by selecting a column from the DataFrame.
import pandas as pd

d = {'one' : pd.Series([1, 2, 3], index=['a', 'b', 'c']),
   'two' : pd.Series([1, 2, 3, 4], index=['a', 'b', 'c', 'd'])}

df = pd.DataFrame(d)
print df ['one']
Its output is as follows −
a     1.0
b     2.0
c     3.0
d     NaN
Name: one, dtype: float64

We will understand this by adding a new column to an existing data frame.
import pandas as pd

d = {'one' : pd.Series([1, 2, 3], index=['a', 'b', 'c']),
   'two' : pd.Series([1, 2, 3, 4], index=['a', 'b', 'c', 'd'])}

df = pd.DataFrame(d)

# Adding a new column to an existing DataFrame object with column label by passing new series

print ("Adding a new column by passing as Series:")
df['three']=pd.Series([10,20,30],index=['a','b','c'])
print df

print ("Adding a new column using the existing columns in DataFrame:")
df['four']=df['one']+df['three']

print df
Its output is as follows −
Adding a new column by passing as Series:
     one   two   three
a    1.0    1    10.0
b    2.0    2    20.0
c    3.0    3    30.0
d    NaN    4    NaN

Adding a new column using the existing columns in DataFrame:
      one   two   three    four
a     1.0    1    10.0     11.0
b     2.0    2    20.0     22.0
c     3.0    3    30.0     33.0
d     NaN    4     NaN     NaN

Columns can be deleted or popped; let us take an example to understand how.
# Using the previous DataFrame, we will delete a column
# using del function
import pandas as pd

d = {'one' : pd.Series([1, 2, 3], index=['a', 'b', 'c']), 
   'two' : pd.Series([1, 2, 3, 4], index=['a', 'b', 'c', 'd']), 
   'three' : pd.Series([10,20,30], index=['a','b','c'])}

df = pd.DataFrame(d)
print ("Our dataframe is:")
print df

# using del function
print ("Deleting the first column using DEL function:")
del df['one']
print df

# using pop function
print ("Deleting another column using POP function:")
df.pop('two')
print df
Its output is as follows −
Our dataframe is:
      one   three  two
a     1.0    10.0   1
b     2.0    20.0   2
c     3.0    30.0   3
d     NaN     NaN   4

Deleting the first column using DEL function:
      three    two
a     10.0     1
b     20.0     2
c     30.0     3
d     NaN      4

Deleting another column using POP function:
   three
a  10.0
b  20.0
c  30.0
d  NaN

We will now understand row selection, addition and deletion through examples. Let us begin with the concept of selection.
Rows can be selected by passing row label to a loc function.
import pandas as pd

d = {'one' : pd.Series([1, 2, 3], index=['a', 'b', 'c']), 
   'two' : pd.Series([1, 2, 3, 4], index=['a', 'b', 'c', 'd'])}

df = pd.DataFrame(d)
print df.loc['b']
Its output is as follows −
one 2.0
two 2.0
Name: b, dtype: float64

The result is a series with labels as column names of the DataFrame. And, the Name of the series is the label with which it is retrieved.
Rows can be selected by passing integer location to an iloc function.
import pandas as pd

d = {'one' : pd.Series([1, 2, 3], index=['a', 'b', 'c']),
   'two' : pd.Series([1, 2, 3, 4], index=['a', 'b', 'c', 'd'])}

df = pd.DataFrame(d)
print df.iloc[2]
Its output is as follows −
one   3.0
two   3.0
Name: c, dtype: float64

Multiple rows can be selected using ‘ : ’ operator.
import pandas as pd

d = {'one' : pd.Series([1, 2, 3], index=['a', 'b', 'c']), 
   'two' : pd.Series([1, 2, 3, 4], index=['a', 'b', 'c', 'd'])}

df = pd.DataFrame(d)
print df[2:4]
Its output is as follows −
   one  two
c  3.0    3
d  NaN    4

Add new rows to a DataFrame using the append function. This function will append the rows at the end.
import pandas as pd

df = pd.DataFrame([[1, 2], [3, 4]], columns = ['a','b'])
df2 = pd.DataFrame([[5, 6], [7, 8]], columns = ['a','b'])

df = df.append(df2)
print df
Its output is as follows −
   a  b
0  1  2
1  3  4
0  5  6
1  7  8

Use index label to delete or drop rows from a DataFrame. If label is duplicated, then multiple rows will be dropped.
If you observe, in the above example, the labels are duplicate. Let us drop a label and will see how many rows will get dropped.
import pandas as pd

df = pd.DataFrame([[1, 2], [3, 4]], columns = ['a','b'])
df2 = pd.DataFrame([[5, 6], [7, 8]], columns = ['a','b'])

df = df.append(df2)

# Drop rows with label 0
df = df.drop(0)

print df
Its output is as follows −
  a b
1 3 4
1 7 8

In the above example, two rows were dropped because those two contain the same label 0.

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