Indexing, Slicing and Subsetting DataFrames in Python

Overview

Teaching: 20 min
Exercises: 20 min

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 the first episode of this lesson, we read a CSV file into a pandas’ DataFrame. We learned how to:

save a DataFrame to a named object,
perform basic math on data,
calculate summary statistics, and
create plots based on the data we loaded into pandas.

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 lobsters dataset that we worked with in the last episode. Let’s reopen and read in the data again:

# Make sure pandas is loaded
import pandas as pd

# Read in the lobsters CSV
lobsters_df = pd.read_csv("data/lobsters_data.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 a Python object. For example, we can select all data from a column named site from the lobsters_df DataFrame by name. There are two ways to do this:

# TIP: use the .head() method we saw earlier to make output shorter
# Method 1: select a 'subset' of the data using the column name
lobsters_df['year']

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

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

# Creates an object, data_year, that only contains the `year` column
data_year = lobsters_df['year']

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.

# Select the year and site columns from the DataFrame
lobsters_df[['year', 'site']]

# What happens when you flip the order?
lobsters_df[['site', 'year']]

# What happens if you ask for a column that doesn't exist?
lobsters_df['year']

Python tells us what type of error it is in the traceback, at the bottom it says KeyError: 'YEAR' which means that YEAR is not a valid column name (nor a valid key in the related Python data type dictionary).

Reminder

The Python language and its modules (such as Pandas) define reserved words that should not be used as identifiers when assigning objects and variable names. Examples of reserved words in Python include Boolean values True and False, operators and, or, and not, among others. The full list of reserved words for Python version 3 is provided at https://docs.python.org/3/reference/lexical_analysis.html#identifiers.

When naming objects and variables, it’s also important to avoid using the names of built-in data structures and methods. For example, a list is a built-in data type. It is possible to use the word ‘list’ as an identifier for a new object, for example list = ['apples', 'oranges', 'bananas']. However, you would then be unable to create an empty list using list() or convert a tuple to a list using list(sometuple).

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.

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

indexing diagram slicing diagram

Challenge - Extracting data
What value does the code below return?
a[0]
How about this:
a[5]
In the example above, calling a[5] returns an error. Why is that?
What about?
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:

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

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

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

# Select the last element in the list
# (the slice starts at the last element, and ends at the end of the list)
lobsters_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:

# Using the 'copy() method'
true_copy_df = lobsters_df.copy()

# Using the '=' operator
ref_df = lobsters_df

You might think that the code ref_df = lobsters_df creates a fresh distinct copy of the lobsters_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_df[0:3] = 0

Let’s try the following code:

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

# lobsters_df is the original dataframe
lobsters_df.head()

What is the difference between these two dataframes?

When we assigned the first 3 columns the value of 0 using the ref_df DataFrame, the lobsters_df DataFrame is modified too. Remember we created the reference ref_df object above when we did ref_df = lobsters_df. Remember lobsters_df and ref_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_df = lobsters_df.copy()
```
A Reference is created using the = operator
```
ref_df = lobsters_df
```

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

lobsters_df = pd.read_csv("data/lobsters_data.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.

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:

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

which gives the output

   site  transect replicate
IVEE         1         A
IVEE         5         B
IVEE         5         B

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:

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

# What does this do?
lobsters_df.loc[0, ['site', 'size_mm', 'year']]

# What happens when you type the code below?
lobsters_df.loc[[0, 10, 35549], :]

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 loc, both the start bound and the stop bound are inclusive. When using loc, integers can 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:

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

In this iloc example,

lobsters_df.iloc[2, 4]

gives the output

86.0

Remember that Python indexing begins at 0. So, the index location [2, 4] selects the element that is 3 rows down and 4 columns over in the DataFrame.

Challenge - Range

What happens when you execute:

lobster_df[0:1]

lobster_df[:4]

lobster_df[:-1]

What happens when you call:

lobster_df.iloc[0:4, 1:4]

lobster_df.loc[0:4, 1:4]

How are the two commands different?

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 year value of 2002:

lobsters_df[lobsters_df.year == 2021]

Which produces the following output:

       record_id  site  transect replicate  size_mm  year  month        date
      9805  IVEE         7         B     70.0  2021      8  2021-08-23
      9806  IVEE         8         B     71.0  2021      8  2021-08-23
      9807  IVEE         8         B     74.0  2021      8  2021-08-23
      9808  IVEE         8         C     56.0  2021      8  2021-08-23
      9809  IVEE         8         C     68.0  2021      8  2021-08-23
...          ...   ...       ...       ...      ...   ...    ...         ...
    10699  NAPL         2         C    101.0  2021      8  2021-08-26
    10700  NAPL         2         D     75.0  2021      8  2021-08-26
    10701  NAPL         2         D    136.0  2021      8  2021-08-26
    10702  NAPL         2         C     85.0  2021      8  2021-08-26
    10703  NAPL         1         C     66.0  2021      8  2021-08-26

[899 rows x 8 columns]

Or we can select all rows that do not contain the year 2021:

lobsters_df[lobsters_df.year != 2021]

We can define sets of criteria too:

lobsters_df[(lobsters_df.year <= 2021) & (lobsters_df.year >= 2018)]

Python Syntax Cheat Sheet

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

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

Challenge - Queries
Select a subset of rows in the lobsters_df DataFrame that contain data from the year 2012 and that contain size values less than or equal to 80. How many rows did you end up with? What did your neighbor get?
You can use the isin command in Python to query a DataFrame based upon a list of values as follows:
lobsters_df[lobsters_df['site'].isin([listGoesHere])]
Use the isin function to find all plots that contain a site value in the “lobsters” DataFrame. How many records contain these values?

Experiment with other queries. Create a query that finds all rows with a size value > or equal to 100.

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 replicate NOT equal to ‘A’ or ‘B’ in the “lobsters” 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,

# Set x to 5
x = 5

# What does the code below return?
x > 5

# How about this?
x == 5

When we ask Python whether x is greater than 5, it returns False. This is Python’s way to say “No”. Indeed, the value of x is 5, and 5 is not greater than 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 lobsters 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.

pd.isnull(lobsters_df)

A snippet of the output is below:

       record_id   site  transect  replicate  size_mm   year  month   date
        False  False     False      False     True  False  False  False
        False  False     False      False    False  False  False  False
        False  False     False      False    False  False  False  False
        False  False     False      False    False  False  False  False
        False  False     False      False     True  False  False  False

[10703 rows x 8 columns]

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

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

Note that the size_mm column of our DataFrame contains many null or NaN values. We will explore ways of dealing with this in the next episode on Data Types and Formats.

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

# What does this do?
empty_size = lobsters_df[pd.isnull(lobsters_df['size_mm'])]['size_mm']
print(empty_size)

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

Challenge - Putting it all together

Create a new DataFrame that only contains observations with size values that are not integers. Assign each size value in the new DataFrame to a new value of ‘x’. Determine the number of null values in the subset.

Create a new DataFrame that contains only observations where weight values are greater than 40 mm. Create a stacked bar plot of average size by plot with site values stacked for each plot.

Key Points

In Python, portions of data can be accessed using indices, slices, column headings, and condition-based subsetting.

Python uses 0-based indexing, in which the first element in a list, tuple or any other data structure has an index of 0.

Pandas enables common data exploration steps such as data indexing, slicing and conditional subsetting.

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Data Analysis and Visualization in Python

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Indexing, Slicing and Subsetting DataFrames in Python

Overview

Loading our data

Indexing and Slicing in Python

Selecting data using Labels (Column Headings)

Reminder

Extracting Range based Subsets: Slicing

Reminder

Challenge - Extracting data

Slicing Subsets of Rows in Python

Copying Objects vs Referencing Objects in Python

Slicing Subsets of Rows and Columns in Python

Challenge - Range

Subsetting Data using Criteria

Python Syntax Cheat Sheet

Challenge - Queries

Using masks to identify a specific condition

Challenge - Putting it all together

Key Points

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