5 Best Ways to Write a Python Code to Find the Second Lowest Value in Each Column in a Given DataFrame

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πŸ’‘ Problem Formulation: When analyzing data within a Pandas DataFrame, a common task might involve identifying not just the minimum value in a given column, but the second lowest value as well. This could provide insights into data trends and outliers. For instance, given a DataFrame of exam scores across different subjects, finding the second lowest score in each subject could help educators identify students who are just above the threshold for extra attention.

Method 1: Using the nsmallest() Method

The nsmallest() method in Pandas allows you to retrieve the n smallest values from a DataFrame. By using this method along with the iloc property, you can easily obtain the second lowest value in each column. This approach is straightforward and leverages built-in Pandas functions designed specifically for such operations.

Here’s an example:

import pandas as pd

# Create a sample DataFrame.
df = pd.DataFrame({
    'Math': [88, 76, 97, 85],
    'Science': [92, 88, 94, 89],
    'English': [79, 81, 78, 91]
})

# Find the second lowest value in each column.
second_lowest = df.apply(lambda col: col.nsmallest(2).iloc[-1])

print(second_lowest)
    

Output:

Math       85
Science    89
English    79
dtype: int64

In this snippet, we define a DataFrame called df with three exam subjects. We then apply a lambda function to each column, using the nsmallest(2) to obtain the two smallest values and selecting the last one (the second smallest) using iloc[-1]. The output shows the second lowest scores for Math, Science, and English.

Method 2: Sorting and Selecting

An alternative method involves sorting each column and then selecting the value at the second index position. This is applicable since sorting the column in ascending order will place the second smallest value right after the smallest. Although this method is a bit more manual, it is still quite intuitive and allows for easy customization.

Here’s an example:

# Assume the same DataFrame 'df' from the previous example.

# Find the second lowest value by sorting and selecting.
second_lowest = df.apply(lambda col: sorted(col)[1])

print(second_lowest)
    

Output:

Math       85
Science    89
English    79
dtype: int64

Much like in Method 1, we use the DataFrame df. Here, however, we apply a lambda function that sorts the values in each column and selects the second element (index 1). The results are identical to the previous method, providing a simple alternative to find the second lowest values.

Method 3: Using the drop_duplicates() and min() Methods

This method combines the drop_duplicates() and min() functions to exclude the lowest value and find the new minimum in the list. It is particularly useful when dealing with duplicate lowest values, ensuring that the second unique value is retrieved. However, it may require additional steps if data cleaning is necessary.

Here’s an example:

# Assume the same DataFrame 'df' from the previous examples.

# Find the second lowest value by dropping duplicates.
second_lowest = df.apply(lambda col: col.drop_duplicates().nsmallest(2).min())

print(second_lowest)
    

Output:

Math       85
Science    89
English    79
dtype: int64

In this code example, we’re filtering out duplicate values within each column using drop_duplicates(). Then, we find the smallest two values that remain (with nsmallest(2)) and take the minimum of those (in most cases, it should be the second smallest overall).

Method 4: Using a For Loop and Conditional Statements

For those who prefer a more traditional approach or need more control over the selection process, using a for loop and conditional statements can be a good solution. This method can be modified to handle a wide array of specific conditions and is very clear in terms of logic.

Here’s an example:

# Assume the same DataFrame 'df' from the previous examples.

# Find the second lowest value using a for loop.
second_lowest = {}
for column in df:
    unique_sorted = sorted(df[column].unique())
    second_lowest[column] = unique_sorted[1] if len(unique_sorted) > 1 else None

print(second_lowest)
    

Output:

{'Math': 85, 'Science': 89, 'English': 79}

This iteration-based approach creates a sorted list of unique values for each column in a traditional for loop. We then extract the second element, ensuring there are enough unique values to do so, otherwise, we return None. This method gives a clear, step-by-step process for finding the second lowest values.

Bonus One-Liner Method 5: Using List Comprehension

Python’s list comprehension provides a concise way to achieve this task in a single line within a dictionary comprehension. This method is for those who appreciate Python’s ability to condense operations while maintaining readability.

Here’s an example:

# Assume the same DataFrame 'df' from the previous examples.

# One-liner to find second lowest values.
second_lowest = {col: sorted(df[col].unique())[1] if len(df[col].unique()) > 1 else None for col in df}

print(second_lowest)
    

Output:

{'Math': 85, 'Science': 89, 'English': 79}

Here we use a dictionary comprehension to iterate over columns. We sort the unique values for each column and then select the second element, with a conditional to handle columns with fewer than two unique values. It’s a compact and elegant solution.

Summary/Discussion

  • Method 1: nsmallest() Method. Direct and concise, utilizes Pandas built-in functions for optimal performance. May not be as efficient for large DataFrames due to the application of a function over each column.
  • Method 2: Sorting and Selecting. Intuitive and easy to understand. However, can be inefficient if the DataFrame is very large, as each column is fully sorted even though only the second smallest value is needed.
  • Method 3: drop_duplicates() and min() Methods. Excellent for unique values and deals well with duplicates. It can be a bit overkill for simple scenarios where there are no duplicate smallest values.
  • Method 4: For Loop with Conditional Statements. Offers detailed control and is very adaptable, but it’s more verbose and potentially slower than vectorized Pandas operations.
  • Bonus Method 5: Using List Comprehension. Compact and Pythonic, but readability might suffer for those not familiar with comprehensions. Also, less efficient for larger DataFrames compared to built-in Pandas methods.