Indexing, Slicing and Subsetting DataFrames in Python

Last updated on 2024-02-13 | Edit this page

Estimated time: 60 minutes

Overview

Questions

  • How can I access specific data within my data set?
  • How can Python and Pandas help me to analyse my data?

Objectives

  • Describe what 0-based indexing is.
  • Manipulate and extract data using column headings and index locations.
  • Employ slicing to select sets of data from a DataFrame.
  • Employ label and integer-based indexing to select ranges of data in a dataframe.
  • Reassign values within subsets of a DataFrame.
  • Create a copy of a DataFrame.
  • Query /select a subset of data using a set of criteria using the following operators: =, !=, >, <, >=, <=.
  • Locate subsets of data using masks.
  • Describe BOOLEAN objects in Python and manipulate data using BOOLEANs.

In lesson 01, we read a CSV into a Python pandas DataFrame. We learned:

  • how to save the DataFrame to a named object,
  • how to perform basic math on the data,
  • how to calculate summary statistics, and
  • how to create plots of the data.

In this lesson, we will explore ways to access different parts of the data using:

  • indexing,
  • slicing, and
  • subsetting.

Loading our data

We will continue to use the surveys dataset that we worked with in the last lesson. Let’s reopen and read in the data again:

PYTHON 

# Make sure pandas is loaded
import pandas as pd

# read in the survey csv
eebo_df = pd.read_csv("eebo.csv")

Indexing and Slicing in Python

We often want to work with subsets of a DataFrame object. There are different ways to accomplish this including: using labels (column headings), numeric ranges, or specific x,y index locations.

Selecting data using Labels (Column Headings)

We use square brackets [] to select a subset of an Python object. For example, we can select all data from a column named species_id from the surveys_df DataFrame by name. There are two ways to do this:

PYTHON 

# Method 1: select a 'subset' of the data using the column name
eebo_df['Place']

# Method 2: use the column name as an 'attribute'; gives the same output
eebo_df.Place

We can also create a new object that contains only the data within the Status column as follows:

PYTHON 

# creates an object, texts_species, that only contains the `status_id` column
texts_status = eebo_df['Status']

We can pass a list of column names too, as an index to select columns in that order. This is useful when we need to reorganize our data.

NOTE: If a column name is not contained in the DataFrame, an exception (error) will be raised.

PYTHON 

# select the author and EEBO columns from the DataFrame
eebo_df[['Author', 'EEBO']]

# what happens when you flip the order?
eebo_df[['EEBO', 'Author']]

#what happens if you ask for a column that doesn't exist?
eebo_df['Texts']

Extracting Range based Subsets: Slicing

REMINDER: Python Uses 0-based Indexing

Let’s remind ourselves that Python uses 0-based indexing. This means that the first element in an object is located at position 0. This is different from other tools like R and Matlab that index elements within objects starting at 1.

PYTHON 

# Create a list of numbers:
a = [1, 2, 3, 4, 5]

indexing diagramslicing diagram

Challenge - Extracting data

  1. What value does the code below return?

PYTHON 

a[0]
  1. How about this:

PYTHON 

a[5]

  1. In the example above, calling a[5] returns an error. Why is that?

  2. What about?

PYTHON 

a[len(a)]

Slicing Subsets of Rows in Python

Slicing using the [] operator selects a set of rows and/or columns from a DataFrame. To slice out a set of rows, you use the following syntax: data[start:stop]. When slicing in pandas the start bound is included in the output. The stop bound is one step BEYOND the row you want to select. So if you want to select rows 0, 1 and 2 your code would look like this:

PYTHON 

# select rows 0, 1, 2 (row 3 is not selected)
eebo_df[0:3]

The stop bound in Python is different from what you might be used to in languages like Matlab and R.

PYTHON 

# select the first 5 rows (rows 0, 1, 2, 3, 4)
eebo_df[:5]

# select the last element in the list
# (the slice starts at the last element,
# and ends at the end of the list)
eebo_df[-1:]

We can also reassign values within subsets of our DataFrame.

But before we do that, let’s look at the difference between the concept of copying objects and the concept of referencing objects in Python.

Copying Objects vs Referencing Objects in Python

Let’s start with an example:

PYTHON 

# using the 'copy() method'
true_copy_eebo_df = eebo_df.copy()

# using '=' operator
ref_eebo_df = eebo_df

You might think that the code ref_eebo_df = eebo_df creates a fresh distinct copy of the surveys_df DataFrame object. However, using the = operator in the simple statement y = x does not create a copy of our DataFrame. Instead, y = x creates a new variable y that references the same object that x refers to. To state this another way, there is only one object (the DataFrame), and both x and y refer to it.

In contrast, the copy() method for a DataFrame creates a true copy of the DataFrame.

Let’s look at what happens when we reassign the values within a subset of the DataFrame that references another DataFrame object:

 # Assign the value `0` to the first three rows of data in the DataFrame
 ref_eebo_df[0:3] = 0
 ```

Let's try the following code:

 ```
# ref_eebo_df was created using the '=' operator
 ref_eebo_df.head()

 # surveys_df is the original dataframe
 eebo_df.head()

What is the difference between these two dataframes?

When we assigned the first 3 columns the value of 0 using the ref_surveys_df DataFrame, the surveys_df DataFrame is modified too. Remember we created the reference ref_survey_df object above when we did ref_survey_df = surveys_df. Remember surveys_df and ref_surveys_df refer to the same exact DataFrame object. If either one changes the object, the other will see the same changes to the reference object.

To review and recap:

  • Copy uses the dataframe’s copy() method

    true_copy_eebo_df = eebo_df.copy()

  • A Reference is created using the = operator

    PYTHON 

    ref_eebo_df = eebo_df

Okay, that’s enough of that. Let’s create a brand new clean dataframe from the original data CSV file.

PYTHON 

eebo_df = pd.read_csv("eebo.csv")

Slicing Subsets of Rows and Columns in Python

We can select specific ranges of our data in both the row and column directions using either label or integer-based indexing. Columns can be selected either by their name, or by the index of their location in the dataframe. Rows can only be selected by their index.

  • loc is primarily label based indexing. Integers may be used but they are interpreted as a label.
  • iloc is primarily integer based indexing

To select a subset of rows and columns from our DataFrame, we can use the iloc method. For example, we can select month, day and year (columns 2, 3 and 4 if we start counting at 1), like this:

PYTHON 

# iloc[row slicing, column slicing]
eebo_df.iloc[0:3, 1:4]

which gives the output

         EEBO    VID                       STC
0  99850634.0  15849  STC 1000.5; ESTC S115415
1  99842408.0   7058   STC 10000; ESTC S106695
2  99844302.0   9101   STC 10002; ESTC S108645

Notice that we asked for a slice from 0:3. This yielded 3 rows of data. When you ask for 0:3, you are actually telling Python to start at index 0 and select rows 0, 1, 2 up to but not including 3.

Let’s explore some other ways to index and select subsets of data:

PYTHON 

# select all columns for rows of index values 0 and 10
eebo_df.loc[[0, 10], :]

# what does this do?
eebo_df.loc[0, ['Author', 'Title', 'Status']]

# What happens when you type the code below?
eebo_df.loc[[0, 10, 149], :]

NOTE: Labels must be found in the DataFrame or you will get a KeyError.

Indexing by labels loc differs from indexing by integers iloc. With iloc, the start bound and the stop bound are inclusive. When using loc instead, integers can also be used, but the integers refer to the index label and not the position. For example, using loc and select 1:4 will get a different result than using iloc to select rows 1:4.

We can also select a specific data value using a row and column location within the DataFrame and iloc indexing:

PYTHON 

# Syntax for iloc indexing to finding a specific data element
dat.iloc[row, column]

In this iloc example,

PYTHON 

eebo_df.iloc[2, 6]

gives the output

'1528'

Remember that Python indexing begins at 0. So, the index location [2, 6] selects the element that is 3 rows down and 7 columns over in the DataFrame.

Challenge - Range

  1. Given the three range indicies below, what do you expect to get back? Does it match what you actually get back?
  • eebo_df[0:1]
  • eebo_df[:4]
  • eebo_df[:-1]

Subsetting Data using Criteria

We can also select a subset of our data using criteria. For example, we can select all rows that have a status value of Free:

PYTHON 

eebo_df.loc[eebo_df["Terms"].str.contains("sermon", na=False)]

Which produces the following output:

PYTHON 

        TCP      EEBO  ...  Page Count   Place
23   A00156  99851064  ...           8  London
27   A00164  99851065  ...           7  London
113  A00426  99857357  ...          72  London
141  A00510  99852090  ...          48  London

[4 rows x 11 columns]

Or we can select all rows with a page length greater than 100:

PYTHON 

eebo_df[eebo_df["Page Count"] > 100]

We can define sets of criteria too:

PYTHON 

eebo_df[(eebo_df.Date >= 1500) & (eebo_df.Date <= 1550)]

Python Syntax Cheat Sheet

Use can use the syntax below when querying data by criteria from a DataFrame. Experiment with selecting various subsets of the “surveys” data.

  • Equals: ==
  • Not equals: !=
  • Greater than, less than: > or <
  • Greater than or equal to >=
  • Less than or equal to <=

Challenge - Advanced Queries

  1. Select a subset of rows in the eebo_df DataFrame that contain data from the year 1540 and that contain page count values less than or equal to 8. How many rows did you end up with? What did your neighbor get?

  2. You can use the isin command in Python to query a DataFrame based upon a list of values as follows. Notice how the indexing relies on a reference to the dataframe being indexed. Think about the order in which the computer must evaluate these statements.

PYTHON 

eebo_df[
        eebo_df['Date'].isin([listGoesHere])
        ]

Use the isin function to find all books written by Robert Aylett and Robert Aytoun. How many are there?

  1. Experiment with other queries. Create a query that finds all rows with a Page Count value > or equal to 1.

  2. The ~ symbol in Python can be used to return the OPPOSITE of the selection that you specify in Python. It is equivalent to is not in. Write a query that selects all rows with Date NOT equal to 1500 or 1600 in the eebo data.

Using masks to identify a specific condition

A mask can be useful to locate where a particular subset of values exist or don’t exist - for example, NaN, or “Not a Number” values. To understand masks, we also need to understand BOOLEAN objects in Python.

Boolean values include True or False. For example,

PYTHON 

# set x to 5
x = 5

# what does the code below return?
x > 5

# how about this?
x == 5

When we ask Python what the value of x > 5 is, we get False. This is because the condition,x is not greater than 5, is not met since x is equal to 5.

To create a boolean mask:

  • Set the True / False criteria (e.g. values > 5 = True)
  • Python will then assess each value in the object to determine whether the value meets the criteria (True) or not (False).
  • Python creates an output object that is the same shape as the original object, but with a True or False value for each index location.

Let’s try this out. Let’s identify all locations in the survey data that have null (missing or NaN) data values. We can use the isnull method to do this. The isnull method will compare each cell with a null value. If an element has a null value, it will be assigned a value of True in the output object.

PYTHON 

pd.isnull(eebo_df)

A snippet of the output is below:

PYTHON 

         TCP   EEBO    VID    STC  Status  Author   Date  Title  Terms  Pages
0      False  False  False  False   False   False  False  False   True  False
1      False  False  False  False   False   False  False  False  False  False
2      False  False  False  False   False   False  False  False  False  False
3      False  False  False  False   False   False  False  False  False  False

[149 rows x 11 columns]

To select the rows where there are null values, we can use the mask as an index to subset our data as follows:

PYTHON 

# To select just the rows with NaN values, we can use the 'any()' method
eebo_df[
        pd.isnull(eebo_df).any(axis=1)
        ]

Note that the weight column of our DataFrame contains many null or NaN values. We will explore ways of dealing with this in Lesson 03.

We can run isnull on a particular column too. What does the code below do?

PYTHON 

# what does this do?
empty_authors = eebo_df[
                        pd.isnull(eebo_df['Author'])
                        ]['Author']
print(empty_authors)

Let’s take a minute to look at the statement above. We are using the Boolean object pd.isnull(eebo_df['Author']) as an index to eebo_df. We are asking Python to select rows that have a NaN value of author.

Challenge - Putting it all together

  1. Create a new DataFrame that only contains titles with status values that are not from London. Assign each status value in the new DataFrame to a new value of ‘x’. Determine the number of null values in the subset.

  2. Create a new DataFrame that contains only observations that are of status free and where page count values are greater than 100.