Python Program to Subtract K from each digit

In this article, we will learn how to write a Python program that subtracts a given value K from each digit of numbers in a list. When the result becomes negative, we use 0 instead.

Given an input list containing numbers and a K value, we subtract K from each individual digit and return the modified numbers.

Example

Let's understand with an example ?

Input list = [1034, 356, 2145, 6712, 8671]
K value = 3

For the number 1034, we subtract 3 from each digit:

• 1 3 = 2 ? 0 (negative results become 0)

• 0 3 = 3 ? 0

• 3 3 = 0

• 4 3 = 1

Result: 0001 ? 1 (leading zeros removed)

Complete output: [1, 23, 12, 3400, 5340]

Method 1: Using For Loop

This approach converts each number to a string, processes each digit, and reconstructs the result ?

# input list
input_numbers = [1034, 356, 2145, 6712, 8671]
print("Input list:", input_numbers)

# value to subtract from each digit
k = 3
result_list = []

# process each number in the list
for number in input_numbers:
    # convert number to string to access individual digits
    str_number = str(number)
    
    # subtract k from each digit, ensuring result is not negative
    modified_digits = ''.join([str(max(0, int(digit) - k)) for digit in str_number])
    
    # convert back to integer to remove leading zeros
    result_list.append(int(modified_digits))

print("Result after subtracting K from each digit:", result_list)

Input list: [1034, 356, 2145, 6712, 8671]
Result after subtracting K from each digit: [1, 23, 12, 3400, 5340]

Method 2: Using List Comprehension

A more concise approach using list comprehension for the same logic ?

# input list
input_numbers = [1034, 356, 2145, 6712, 8671]
print("Input list:", input_numbers)

k = 3

# process all numbers using list comprehension
result_list = [int(''.join([str(max(0, int(digit) - k)) for digit in str(num)])) 
               for num in input_numbers]

print("Result after subtracting K from each digit:", result_list)

Input list: [1034, 356, 2145, 6712, 8671]
Result after subtracting K from each digit: [1, 23, 12, 3400, 5340]

Method 3: Using NumPy Array

Using NumPy arrays for digit-wise operations ?

import numpy as np

# input list
input_numbers = [1034, 356, 2145, 6712, 8671]
print("Input list:", input_numbers)

k = 3
result_list = []

for number in input_numbers:
    # convert number to list of individual digits
    digits = [int(digit) for digit in str(number)]
    
    # create numpy array and subtract k
    digit_array = np.array(digits) - k
    
    # ensure no negative values
    digit_array = np.maximum(digit_array, 0)
    
    # reconstruct the number
    result_number = int(''.join(map(str, digit_array)))
    result_list.append(result_number)

print("Result after subtracting K from each digit:", result_list)

Input list: [1034, 356, 2145, 6712, 8671]
Result after subtracting K from each digit: [1, 23, 12, 3400, 5340]

Comparison

Conclusion

All three methods effectively subtract K from each digit while handling negative results. List comprehension offers the most concise solution, while the for loop provides better readability for beginners.