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Pandas Techniques For Data Manipulation:


Python is a great language for doing data analysis as it provides large number of data-centric python packages. Pandas is an open-source python library that is used for data manipulation and analysis. It provides many functions and methods to speed up the data analysis process.

Here , we discuss about some of the pandas techniques for data manipulation.


Importing Dataset and inspecting Dataframe.


In this tutorial , we are basically learning about reading a csv(Comma Separated Values) file into a dataframe and inspect the dataframe, learn about the data types of the columns values .

For this we created a csv file on our local computer named as emp_data.csv which contained the employee information of a company.


Reading csv file into dataframe:

First of all ,we import the pandas library.Pandas is usually imported under the 'pd' alias.we use as keyword to create an alias.

import pandas as pd

here a dataframe emp_df is created:

emp_df =pd.read_csv("C:\\Users\\DELL\\Desktop\\emp_data.csv")
emp_df.head()


emp_df.info()

This dataframe.info() method returns the data type of the dataframe columns.


Creating pandas datframe from dictionary of list:#


emps = {"Emp_name":["Ishan", "Gaurav", "ram", "Rohit"],
        "role": ["Data_science", "full stack", "front end", "ML and AI"]
        }
  
ddf = pd.DataFrame(emps)
ddf.head()

Here we created a dataframe ddf from dictionary of list using pandas.dataframe().


Sorting Dataframe:


For sorting the data frame in pandas, function sort_values() is used. Pandas sort_values() can sort the data frame in Ascending or Descending order.


For demostrating example we are adding a new column to the existing dataframe.


exp = pd.Series(['6', '8', '4', '3','2','7','8','9'])
emp_df['Experience_in_years'] = pd.to_numeric(exp)
emp_df.head(n=8)

Here we converted the object type of datframe column into integer type using pd.to_numeric() function.


Sorting in Ascending Order:

emp_dfsorted = emp_df.sort_values(by=["Experience_in_years"],ascending=True)
emp_dfsorted.head(n=8)

For sorting the pandas dataframe in ascending order we assign the name of the column to the by parameter . On the basis of that column value sorting is done.We assign True to ascending parameter to sort in ascending order. If we donot assign any value to ascending parameter it is by default sorted in ascending order.


Here we sorted the dataframe in the ascending order of employees experience using Experience_in_years column.


Sorting in Descending Order:


emp_dfsorted = emp_df.sort_values(by=["Experience_in_years"],ascending=False)
emp_dfsorted.head(n=8)

Here we sort the dataframe in descending order by assigning False value to the ascending parameter.



Sorting by multiple columns:


emp_dfsorted1 = emp_df.sort_values(by=['Emp_id', 'Experience_in_years'],ascending=[True,False])
emp_dfsorted1.head()

For sorting the dataframe by multiple columns we pass a list of columns to by parameter and a list of boolean values to ascending parameter.

The first item of the list in ascending parameter assigns to the first column of the list and so on.



Subsetting Dataframe:


Here are some operations by using which we can select a subset of a given dataframe:


Selecting single column:


For selecting a single column we use square bracket [ ] .If we use single square bracket , the output is a pandas series .

emp_exp = emp_df["Experience_in_years"]
print(emp_exp)
print(type(emp_exp))

If we use two square brackets for selecting column , the resukt is a pandas dataframe.

emp_expdf =emp_df[["Experience_in_years"]]
emp_expdf.head()


print(type(emp_expdf))

<class 'pandas.core.frame.DataFrame'>


Selecting Multiple columns:


For selecting multiple columns we can pass a list of column names inside the square bracket [] .

name_role = emp_df[["Name","Role"]]
name_role.head()

Selecting Specific rows from a dataframe:



Using Condition to subset rows:

For selecting specific rows ,we can put conditions within the brackets to select specific rows depending on the condition.

Experienced_employee = emp_df[emp_df['Experience_in_years'] > 4]
Experienced_employee.head()

Here we only get the rows having the Experience_in_years value greater than 4.


For this we can use plenty of other operators like <,=,<=, >= ,!= etc..




Using Square brackets to subset rows:

We can only select rows using square brackets if we specify a slice, like 1:4. Here we are using the integer indexes of the rows .


print(emp_df[1:4])

Here the integer before colon is inclusive and that after the colon is exclusive.



Selecting Both rows and columns:

In above methods , it was not possible to select specific rows and columns combined so the loc and iloc operators are needed. The portion before the comma specifies the rows we would like to choose and the part after the comma specifies the columns we like to choose.


Using loc operator:
data_scientist_df = emp_df.loc[emp_df["Role"] == "Data Scientist", "Name"]
data_scientist_df.head()

Here we select the rows having role value equal to Data Scientist and

select the Name column.


Using iloc operator:

The iloc operator allows us to subset pandas DataFrames based on their position or index.

any_two = emp_df.iloc[[1, 2], [0, 1]]
any_two.head()

Here we are selecting the rows having rows index 1 and 2. And the columns having column index 0 and 1.


Selecting all rows and only some columns:

all_rows = emp_df.iloc[:, [0, 1]]
all_rows.head()

for selecting all rows we pass colon : And for selecting specific columns we pass a list of column index.


subsetting a regular sequence of specific rows and columns:

any_rows = emp_df.iloc[0:4 , 0:2]
any_rows.head()

For this we specify the slice to indicate the rows and columns.The end of the slice is exclusive in both the slices.


Interating over rows of dataframe:


For iterating over rows in dataframe , we make use of the three functions iteritems(), iterrows(), itertuples().


Using iterrows():


Iterate over DataFrame rows as (index, Series) pairs.

To iterate over rows of a Pandas DataFrame, we use DataFrame.iterrows() function which returns an iterator yielding index and row data for each row.


In this example we are iterating through dataframe rows using Python For Loop and iterrows() function.

for index, row in emp_df.iterrows():
    print(index,row)
    


Using iteritems():


Iterate over (column name, Series) pairs.

This method iterates over (column name, Series) pairs. When this method applied to the DataFrame, it iterates over the DataFrame columns and returns a tuple which consists of column name and the content as a Series.

for key, value in emp_df.iteritems():
    print(key, value)
    print()
    

Using itertuples():

Iterate over DataFrame rows as namedtuples.

In order to iterate over rows, we apply a function itertuples() this function return a tuple for each row in the DataFrame. The first tuple element represents row index while other element represents row values.



for t in emp_df.itertuples() :
    print(t)
    print()


Dropping Duplicates:


Pandas drop_duplicates() method helps in removing duplicates from the data frame.

For demonstrating the examples on drop_duplicates() function we created a dataframe called games_df , that is about the games played by different countries in different years and no of medals won by them.


games_df= pd.read_csv("C:\\Users\\DELL\\Desktop\\games.csv")
games_df.head()

Now adding a new row that contain entire duplicate values.

games_df.loc[len(games_df.index)] = ['India',2015, 12, 24] 
print(games_df)

Here first and the last rows are same.


Removing rows with all duplicate values

games_df.drop_duplicates()

From above result we can see that the rows having all the values same is dropped.Thus, dataframe.drop_duplicates() method is used to drop the rows having entire duplicate values.


To remove duplicates on specific column we use subset:

games_df.drop_duplicates(subset=['Country'])

Here all the rows with repeated country names in the Country column are dropped.


This drops the rows having duplicate Country column name and Year column name.

games_df.drop_duplicates(subset=['Country','Year'])

To remove duplicates and keep first or last occurrences, we use keep.


games_df.drop_duplicates(subset=['Country'], keep='last')

It removes duplicates in the Country column and kepp the last occurence.


In above result the last row is kept while the first row is removed.


games_df.drop_duplicates(subset=['Country'], keep='first')

If we assign keep equal to False.Both the same rows with same country name are dropped.

games_df.drop_duplicates(subset=['Country'], keep= False)

Modifying actual dataframe:


If we assign inplace parameter to True the actual dataframe remains changed.

games_df.drop_duplicates(subset=['Country'], keep='first',inplace= True)
games_df



The link to the notebook in the github repo is here.

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