Lambda expression in Python Program to rearrange positive and negative numbers

In this tutorial, we will write an anonymous function using lambda to rearrange positive and negative numbers in a list. The goal is to place all negative numbers first, followed by all positive numbers.

Algorithm

Let's see how to solve the problem step by step ?

1. Initialize a list with negative and positive numbers.
2. Write a lambda expression that takes a list as an argument.
   2.1. Iterate over the list and get negative numbers
   2.2. Same for positive numbers
   2.3. Combine both using concatenation operator.
3. Return the resultant list.

Note − Use list comprehension to get negative and positive numbers.

Using Lambda Expression

Here's how to implement the solution using lambda with list comprehension ?

# initializing a list
arr = [3, 4, -2, -10, 23, 20, -44, 1, -23]

# lambda expression to rearrange numbers
rearrange_numbers = lambda arr: [x for x in arr if x < 0] + [x for x in arr if x >= 0]

# rearranging the array
new_arr = rearrange_numbers(arr)

# printing the resultant array
print("Original array:", arr)
print("Rearranged array:", new_arr)

Original array: [3, 4, -2, -10, 23, 20, -44, 1, -23]
Rearranged array: [-2, -10, -44, -23, 3, 4, 23, 20, 1]

How It Works

The lambda function performs two operations ?

• [x for x in arr if x < 0] − Creates a list of all negative numbers
• [x for x in arr if x >= 0] − Creates a list of all positive numbers (including zero)
• The + operator concatenates both lists

Alternative Approach with Regular Function

For comparison, here's the same logic using a regular function ?

def rearrange_numbers(arr):
    negatives = [x for x in arr if x < 0]
    positives = [x for x in arr if x >= 0]
    return negatives + positives

# test with the same array
arr = [3, 4, -2, -10, 23, 20, -44, 1, -23]
result = rearrange_numbers(arr)
print("Result:", result)

Result: [-2, -10, -44, -23, 3, 4, 23, 20, 1]

Conclusion

Lambda functions provide a concise way to write short operations. In this case, the lambda expression efficiently rearranges numbers by separating negatives and positives using list comprehension and concatenation.