Selecting Rows Using Integer Position (.iloc)

Data selection in Pandas often requires accessing information based on its exact position rather than its labels. Sometimes you need to retrieve data based purely on its integer position, regardless of the index labels or column names assigned to a DataFrame. This approach is particularly useful when the order of rows or columns is known but their specific labels are not, or when working with DataFrames that utilize default integer indices. For this specific need, Pandas offers the .iloc accessor.

The .iloc accessor behaves much like standard Python list and NumPy array indexing. It uses 0-based integer positions to select rows and columns. Remember that Python slicing conventions apply: the start bound is inclusive, and the end bound is exclusive.

Basic Syntax

The general syntax is DataFrame.iloc[row_indexer, column_indexer]. Both row_indexer and column_indexer accept integers, lists of integers, slices with integers, or boolean arrays (though boolean indexing is often clearer using standard [] or .loc). If you only provide one indexer, it's assumed to be for the rows.

Let's use a sample DataFrame to illustrate:

import pandas as pd
import numpy as np

data = {'col_a': [10, 20, 30, 40, 50],
        'col_b': [0.1, 0.2, 0.3, 0.4, 0.5],
        'col_c': ['x', 'y', 'z', 'x', 'y']}
# Note the custom string index
df_example = pd.DataFrame(data, index=['row1', 'row2', 'row3', 'row4', 'row5'])

print("Sample DataFrame:")
print(df_example)

Sample DataFrame:
      col_a  col_b col_c
row1     10    0.1     x
row2     20    0.2     y
row3     30    0.3     z
row4     40    0.4     x
row5     50    0.5     y

Selecting Single Rows

To select a single row by its integer position, pass the integer to .iloc:

# Select the first row (position 0)
first_row = df_example.iloc[0]
print("\nFirst row (position 0):")
print(first_row)

# Select the third row (position 2)
third_row = df_example.iloc[2]
print("\nThird row (position 2):")
print(third_row)

First row (position 0):
col_a     10
col_b    0.1
col_c      x
Name: row1, dtype: object

Third row (position 2):
col_a     30
col_b    0.3
col_c      z
Name: row3, dtype: object

Notice that even though our index consists of strings ('row1', 'row2', etc.), .iloc accesses the rows based on their 0-based integer position. The result is a Pandas Series containing the data from that row, with the original column names as its index.

Selecting Multiple Rows using Lists or Slices

You can select multiple specific rows by providing a list of integers, or a range of rows using slice notation.

# Select the first and third rows (positions 0 and 2)
rows_0_2 = df_example.iloc[[0, 2]]
print("\nRows at positions 0 and 2:")
print(rows_0_2)

# Select rows from position 1 up to (but not including) position 4
rows_1_to_3 = df_example.iloc[1:4] # Selects rows at index 1, 2, 3
print("\nRows from position 1 up to 4:")
print(rows_1_to_3)

# Select from the beginning up to position 3 (exclusive)
first_three_rows = df_example.iloc[:3]
print("\nFirst three rows:")
print(first_three_rows)

# Select from position 3 to the end
last_rows = df_example.iloc[3:]
print("\nRows from position 3 to end:")
print(last_rows)

Rows at positions 0 and 2:
      col_a  col_b col_c
row1     10    0.1     x
row3     30    0.3     z

Rows from position 1 up to 4:
      col_a  col_b col_c
row2     20    0.2     y
row3     30    0.3     z
row4     40    0.4     x

First three rows:
      col_a  col_b col_c
row1     10    0.1     x
row2     20    0.2     y
row3     30    0.3     z

Rows from position 3 to end:
      col_a  col_b col_c
row4     40    0.4     x
row5     50    0.5     y

As expected, selecting multiple rows returns a new DataFrame containing the specified rows. The slicing behavior start:end includes start but excludes end, consistent with Python standards.

Selecting Rows and Columns Simultaneously

.iloc really shines when you need to select specific elements or subsections based on both row and column positions. You provide the row indexer first, followed by the column indexer, separated by a comma.

# Select the element at row position 1, column position 0
element_1_0 = df_example.iloc[1, 0]
print(f"\nElement at row position 1, column position 0: {element_1_0}")

# Select the first row (position 0) and the first two columns (positions 0, 1)
row0_cols01 = df_example.iloc[0, 0:2]
print("\nFirst row, first two columns:")
print(row0_cols01)

# Select the first three rows (positions 0, 1, 2) and the first and third columns (positions 0, 2)
subset = df_example.iloc[0:3, [0, 2]]
print("\nFirst three rows, columns 0 and 2:")
print(subset)

# Select all rows and the last column (position -1 works!)
last_col = df_example.iloc[:, -1]
print("\nAll rows, last column:")
print(last_col)

Element at row position 1, column position 0: 20

First row, first two columns:
col_a     10
col_b    0.1
Name: row1, dtype: object

First three rows, columns 0 and 2:
      col_a col_c
row1     10     x
row2     20     y
row3     30     z

All rows, last column:
row1    x
row2    y
row3    z
row4    x
row5    y
Name: col_c, dtype: object

Using : selects all rows or all columns, similar to NumPy slicing. Negative indices count from the end (e.g., -1 is the last column).

Distinction: .iloc vs .loc

It's fundamental to remember the difference:

• .loc: Selects based on labels (index labels, column names). Slicing with labels is inclusive of both start and end labels.
• .iloc: Selects based on integer position (0-based). Slicing with integers is exclusive of the end position.

Trying to use labels with .iloc or integer positions with .loc (unless the labels happen to be integers) will result in an error. Understanding this distinction is essential for correctly retrieving the data you need.

Mastering .iloc provides a precise way to access data based on its position within the DataFrame structure, complementing the label-based selection offered by .loc.