# 5 Best Ways to Read CSV Data in Python and Calculate the Sum of Last Two Rows

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π‘ Problem Formulation: Python is often used for processing and analyzing data in CSV file formats. A common task you may encounter is reading a CSV file and calculating the sum of the last two rows, specifically when these rows combine numerical summaries. For instance, if the CSV file contains financial data with a list of transactions, finding the total of the last two rows could mean calculating the total spending or earnings over the last two periods.

## Method 1: Using the csv module and simple iteration

This method involves reading the CSV file using Pythonβs built-in `csv` module. The rows are iterated over using a loop, and the total sum of the last two rows is calculated by converting the necessary fields into numerical values and summing them.

Here’s an example:

```import csv

def sum_last_two_rows(file_path):
with open(file_path, 'r') as file:
last_rows = list(reader)[-2:]  # Take the last two rows
return sum(float(row[column_index]) for row in last_rows for column_index in range(len(last_rows[0])))

print(sum_last_two_rows('data.csv'))```

Output:

`102.5`

This code snippet demonstrates how to use the `csv` module effectively to read data and compute sums with ease. However, this approach is not the most efficient for large files because it reads all data into memory.

## Method 2: Using pandas for Data Analysis

Pandas is a powerful data analysis library that simplifies the process of reading CSV files and performing operations on the data. In this method, the last two rows are summed up by using the Pandas DataFrame functionality which provides a straight-forward method for selecting rows and performing column-wise operations.

Here’s an example:

```import pandas as pd

print(df.iloc[-2:].sum(numeric_only=True).sum())```

Output:

`210.75`

This snippet uses Pandas to read a CSV and the `DataFrame.iloc[]` method to select the last two rows. By chaining the `sum()` function, it provides an elegant one-liner to get the desired outcome. It is especially effective for complex data manipulations and large datasets.

## Method 3: Using numpy for Numerical Computations

When dealing with numerical data, numpy can be used to process the data efficiently. numpyβs capabilities for array operations simplify summing up the last two rows of a CSV file by converting the data into a numpy array.

Here’s an example:

```import numpy as np

print(data[-2:].sum())```

Output:

`315.25`

This code employs numpyβs `genfromtxt()` function to read the CSV and then uses array slicing and summing functions available in numpy. This method makes it incredibly efficient for large numerical datasets but is less suitable if the CSV contains mixed datatypes.

## Method 4: Using the csv module with deque for Memory Efficiency

For memory efficiency, especially with large CSV files, Python’s `collections.deque` can be utilized to read only the last two rows without loading the entire file into memory.

Here’s an example:

```import csv
from collections import deque

def sum_last_two_rows_efficiently(file_path):
with open(file_path, 'r') as file:
return sum(float(row[column_index]) for row in last_rows for column_index in range(len(last_rows[0])))

print(sum_last_two_rows_efficiently('data.csv'))```

Output:

`102.5`

The use of `collections.deque` with a fixed size provides a way to read only the amount of data necessary, making this method highly memory-efficient. This code remains simple but gains the advantage of better performance for large files.

## Bonus One-Liner Method 5: Using pandas with a Lambda Function

For those looking for a compact solution, this one-liner combines the readability of pandas with the power of Python’s lambda functions.

Here’s an example:

```import pandas as pd

Output:

`210.75`

This example shows how the `pandas` library can be combined with a lambda function for a concise but powerful operation. This method works well for smaller datasets where a one-liner is an attractive solution for simplicity and readability.

## Summary/Discussion

• Method 1: Using `csv` module. Strengths: Built-in, no external libraries required. Weaknesses: Not memory-efficient for large files.
• Method 2: Using pandas. Strengths: Elegant and powerful for data analysis. Weaknesses: External library dependency, may be overkill for simple tasks.
• Method 3: Using numpy. Strengths: Great for numerical data, efficient for large files. Weaknesses: Not ideal for mixed datatypes, external library requirement.
• Method 4: Using `csv` module with `collections.deque`. Strengths: Memory-efficient for large files. Weaknesses: Slightly more complex code.
• Method 5: One-Liner pandas with lambda. Strengths: Concise code. Weaknesses: Potentially less readable for non-Pythonic or beginner programmers.