Hands-on practical: Accessing Specific Data Subsets

Accessing specific data subsets in Pandas DataFrames is achieved through primary methods such as selecting columns by name, label-based indexing with .loc, positional indexing with .iloc, and filtering data using boolean conditions. Practical exercises will solidify understanding and build confidence in applying these techniques.

First, ensure you have Pandas imported. Typically, we import it under the alias pd.

import pandas as pd
import numpy as np # We might use numpy for creating data later

Next, let's create a sample DataFrame to work with. We'll make a small dataset representing information about different types of fruits.

# Create the data dictionary
data = {
    'Fruit': ['Apple', 'Banana', 'Orange', 'Grape', 'Strawberry', 'Blueberry'],
    'Color': ['Red', 'Yellow', 'Orange', 'Purple', 'Red', 'Blue'],
    'Price': [1.2, 0.5, 0.8, 4.5, 3.0, 5.5],
    'Quantity': [100, 150, 80, 200, 120, 90]
}

# Define meaningful index labels
index_labels = ['F01', 'F02', 'F03', 'F04', 'F05', 'F06']

# Create the DataFrame
inventory_df = pd.DataFrame(data, index=index_labels)

# Display the DataFrame
print("Original Fruit Inventory DataFrame:")
print(inventory_df)

This will output:

Original Fruit Inventory DataFrame:
         Fruit   Color  Price  Quantity
F01      Apple     Red    1.2       100
F02     Banana  Yellow    0.5       150
F03     Orange  Orange    0.8        80
F04      Grape  Purple    4.5       200
F05 Strawberry     Red    3.0       120
F06  Blueberry    Blue    5.5        90

Now, let's practice selecting data from inventory_df.

Task 1: Selecting Columns

Select the Fruit column. Remember, accessing a single column returns a Pandas Series.

# Select the 'Fruit' column
fruit_column = inventory_df['Fruit']
print("\nSelecting the 'Fruit' column (returns a Series):")
print(fruit_column)

Selecting the 'Fruit' column (returns a Series):
F01         Apple
F02        Banana
F03        Orange
F04         Grape
F05    Strawberry
F06     Blueberry
Name: Fruit, dtype: object

Now, select both the Fruit and Price columns. Accessing multiple columns returns a DataFrame. Note the use of double square brackets [[]].

# Select 'Fruit' and 'Price' columns
fruit_price_df = inventory_df[['Fruit', 'Price']]
print("\nSelecting 'Fruit' and 'Price' columns (returns a DataFrame):")
print(fruit_price_df)

Selecting 'Fruit' and 'Price' columns (returns a DataFrame):
         Fruit  Price
F01      Apple    1.2
F02     Banana    0.5
F03     Orange    0.8
F04      Grape    4.5
F05 Strawberry    3.0
F06  Blueberry    5.5

Task 2: Selecting with .loc (Label-Based)

Select the row corresponding to the label F03 (Orange).

# Select row with label 'F03'
orange_row = inventory_df.loc['F03']
print("\nSelecting row with label 'F03' using .loc:")
print(orange_row)

Selecting row with label 'F03' using .loc:
Fruit       Orange
Color       Orange
Price          0.8
Quantity        80
Name: F03, dtype: object

Select the Price for the fruit with label F05 (Strawberry).

# Select the 'Price' for label 'F05'
strawberry_price = inventory_df.loc['F05', 'Price']
print(f"\nPrice of fruit 'F05' using .loc: {strawberry_price}")

Price of fruit 'F05' using .loc: 3.0

Select the rows for labels F02 to F04 (inclusive) and only the Fruit and Quantity columns.

# Select rows 'F02' through 'F04', columns 'Fruit' and 'Quantity'
subset_loc = inventory_df.loc['F02':'F04', ['Fruit', 'Quantity']]
print("\nSelecting rows 'F02' to 'F04' and specific columns using .loc:")
print(subset_loc)

Selecting rows 'F02' to 'F04' and specific columns using .loc:
      Fruit  Quantity
F02  Banana       150
F03  Orange        80
F04   Grape       200

Notice how .loc includes the end label in the slice (F04 is included).

Task 3: Selecting with .iloc (Position-Based)

Select the third row (which corresponds to Orange, at index position 2, as indexing starts from 0).

# Select the third row (index position 2)
third_row_iloc = inventory_df.iloc[2]
print("\nSelecting the third row (index 2) using .iloc:")
print(third_row_iloc)

Selecting the third row (index 2) using .iloc:
Fruit       Orange
Color       Orange
Price          0.8
Quantity        80
Name: F03, dtype: object

Select the value at the intersection of the 4th row (index 3) and the 2nd column (index 1). This should be the color 'Purple'.

# Select value at row index 3, column index 1
value_iloc = inventory_df.iloc[3, 1]
print(f"\nValue at row index 3, column index 1 using .iloc: {value_iloc}")

Value at row index 3, column index 1 using .iloc: Purple

Select the first three rows (indices 0, 1, 2) and the first two columns (indices 0, 1).

# Select the first 3 rows and first 2 columns
subset_iloc = inventory_df.iloc[0:3, 0:2] # Note: 3 and 2 are exclusive here
print("\nSelecting first 3 rows and first 2 columns using .iloc:")
print(subset_iloc)

Selecting first 3 rows and first 2 columns using .iloc:
     Fruit   Color
F01   Apple     Red
F02  Banana  Yellow
F03  Orange  Orange

Remember that slicing with .iloc excludes the end position, similar to standard Python list slicing.

Task 4: Conditional Selection (Boolean Indexing)

Select all fruits that are 'Red'.

# Create a boolean condition: Color is 'Red'
red_fruits_condition = inventory_df['Color'] == 'Red'
print("\nBoolean Series for 'Color' == 'Red':")
print(red_fruits_condition)

# Apply the condition to the DataFrame
red_fruits_df = inventory_df[red_fruits_condition]
print("\nSelecting rows where Color is 'Red':")
print(red_fruits_df)

Boolean Series for 'Color' == 'Red':
F01     True
F02    False
F03    False
F04    False
F05     True
F06    False
Name: Color, dtype: bool

Selecting rows where Color is 'Red':
         Fruit Color  Price  Quantity
F01      Apple   Red    1.2       100
F05 Strawberry   Red    3.0       120

Select all fruits with a Price greater than $1.00$ and a Quantity less than 150. Use the & operator for combining conditions.

# Condition 1: Price > 1.0
price_condition = inventory_df['Price'] > 1.0
# Condition 2: Quantity < 150
quantity_condition = inventory_df['Quantity'] < 150

# Combine conditions using '&' (AND)
combined_condition = price_condition & quantity_condition

# Apply the combined condition
filtered_df = inventory_df[combined_condition]
print("\nSelecting rows where Price > 1.0 AND Quantity < 150:")
print(filtered_df)

Selecting rows where Price > 1.0 AND Quantity < 150:
         Fruit Color  Price  Quantity
F01      Apple   Red    1.2       100
F05 Strawberry   Red    3.0       120
F06  Blueberry  Blue    5.5        90

Select the Fruit name and Price for all fruits that are 'Red' OR have a Price less than $0.60$. Use the | operator.

# Condition 1: Color is 'Red'
color_condition = inventory_df['Color'] == 'Red'
# Condition 2: Price < 0.6
price_condition_low = inventory_df['Price'] < 0.6

# Combine conditions using '|' (OR)
combined_or_condition = color_condition | price_condition_low

# Apply the condition and select specific columns
filtered_subset = inventory_df.loc[combined_or_condition, ['Fruit', 'Price']]
print("\nSelecting 'Fruit' and 'Price' where Color is 'Red' OR Price < 0.6:")
print(filtered_subset)

Selecting 'Fruit' and 'Price' where Color is 'Red' OR Price < 0.6:
         Fruit  Price
F01      Apple    1.2
F02     Banana    0.5
F05 Strawberry    3.0

Notice how we combined boolean indexing with .loc to select specific columns for the filtered rows.

Task 5: Setting and Resetting the Index

Sometimes, it's useful to set one of the columns as the DataFrame index, especially if it contains unique identifiers. Let's set the Fruit column as the index.

# Set 'Fruit' column as the index
# inplace=True modifies the DataFrame directly
# inventory_df.set_index('Fruit', inplace=True)

# Alternatively, create a new DataFrame with the new index
inventory_df_fruit_index = inventory_df.set_index('Fruit')
print("\nDataFrame with 'Fruit' as index:")
print(inventory_df_fruit_index)

DataFrame with 'Fruit' as index:
             Color  Price  Quantity
Fruit                              
Apple          Red    1.2       100
Banana      Yellow    0.5       150
Orange      Orange    0.8        80
Grape       Purple    4.5       200
Strawberry     Red    3.0       120
Blueberry     Blue    5.5        90

Now, try selecting the 'Orange' row using .loc with the new index.

# Select 'Orange' using the new index
orange_data = inventory_df_fruit_index.loc['Orange']
print("\nSelecting 'Orange' data using the new fruit index:")
print(orange_data)

Selecting 'Orange' data using the new fruit index:
Color       Orange
Price          0.8
Quantity        80
Name: Orange, dtype: object

Finally, let's reset the index back to the default integer index, turning the 'Fruit' index back into a regular column.

# Reset the index
inventory_df_reset = inventory_df_fruit_index.reset_index()
print("\nDataFrame after resetting the index:")
print(inventory_df_reset)

DataFrame after resetting the index:
        Fruit   Color  Price  Quantity
0       Apple     Red    1.2       100
1      Banana  Yellow    0.5       150
2      Orange  Orange    0.8        80
3       Grape  Purple    4.5       200
4  Strawberry     Red    3.0       120
5   Blueberry    Blue    5.5        90

You can see the Fruit column is back, and a default integer index (0, 1, 2...) has been added.

This hands-on session provided practice in applying the core data selection techniques in Pandas. Being comfortable with [] for column selection, .loc for label-based access, .iloc for position-based access, and boolean indexing for conditional filtering is fundamental for almost any data analysis task. Experiment further with the inventory_df DataFrame or try loading your own datasets (as learned in the previous chapter) and practice selecting different subsets of data.