How to Convert an Integer to a List of Digits in Python?

In Python, you can convert an integer to a list of digits in the following ways:

Converting Numeric String to List of Characters

If you are certain that the integer is always going to be positive, then you can simply do the following:

  1. Convert integer to numeric string;
  2. Convert numeric string to a list of characters;
  3. Use list comprehension to convert each list item to an integer.

For example, this can be implemented like so: 

def num_to_list(num):
    # 1: convert integer to string
    # 2: convert numeric string to list of chars
    # 3: convert each numeric string character to integer
    return [int(char) for char in str(num)]

print(num_to_list(12345)) # [1, 2, 3, 4, 5]
print(num_to_list(0)) # [0]

However, if the integer can also be negative, then the following are some ways in which you can handle the negative sign (-) when converting an integer to a list of digits:

Make the Most Significant Digit Negative:

Making the most significant digit negative means that in the resulting list, if the original number is negative, the first digit will have the negative sign. This means that: 

# 12345 becomes [1, 2, 3, 4, 5]
# -12345 becomes [-1, 2, 3, 4, 5]

For example, this can be implemented like so: 

def num_to_list(num):
    # 1: convert integer in absolute form to string
    # 2: convert numeric string to list of chars
    # 3: convert each numeric string character to integer
    digits = [int(char) for char in str(abs(num))]
    # 4: make first digit negative if original number is negative
    digits[0] = -digits[0] if num < 0 else digits[0]

    return digits

print(num_to_list(12345)) # [1, 2, 3, 4, 5]
print(num_to_list(-12345)) # [-1, 2, 3, 4, 5]

print(num_to_list(0)) # [0]
print(num_to_list(-0)) # [0]

Add the Minus Sign as a String:

Adding the minus sign (-) as a string means that in the resulting list, if the original number is negative, the first element of the list will have the minus sign as a string: 

# 12345 becomes [1, 2, 3, 4, 5]
# -12345 becomes ['-', 1, 2, 3, 4, 5]

For example, this can be implemented like so: 

def num_to_list(num):
    # 1: convert integer in absolute form to string
    # 2: convert numeric string to list of chars
    # 3: convert each numeric string character to integer
    digits = [int(char) for char in str(abs(num))]

    # 4: add minus sign if original number is negative
    if num < 0:
        digits.insert(0, '-')

    return digits

print(num_to_list(12345)) # [1, 2, 3, 4, 5]
print(num_to_list(-12345)) # ['-', 1, 2, 3, 4, 5]

print(num_to_list(0)) # [0]
print(num_to_list(-0)) # [0]

Add a Leading Digit to Represent the Sign

Adding a leading digit to represent the sign means that in the resulting list, the first element can be used to represent the sign (i.e. 0 for positive integers and 1 for negative integers): 

# 12345 becomes [0, 1, 2, 3, 4, 5]
# -12345 becomes [1, 1, 2, 3, 4, 5]

For example, this can be implemented like so: 

def num_to_list(num):
    # 1: convert integer in absolute form to string
    # 2: convert numeric string to list of chars
    # 3: convert each numeric string character to integer
    digits = [int(char) for char in str(abs(num))]

    # 4: add 0 or 1 as first element in list
    digits.insert(0, 1 if num < 0 else 0)

    return digits

print(num_to_list(12345)) # [0, 1, 2, 3, 4, 5]
print(num_to_list(-12345)) # [1, 1, 2, 3, 4, 5]

print(num_to_list(0)) # [0, 0]
print(num_to_list(-0)) # [0, 0]

Reducing to Single Digit

If you are certain that the integer is always going to be positive, then you can simply do the following:

  1. Create a loop;
  2. Add the last digit (i.e. remainder of x % 10) as the first element to a new list;
  3. Remove the last digit from the integer in each iteration till there are no digits left.

For example, this can be implemented like so: 

def num_to_list(num):
    digits = []

    # 1: create a loop
    while True:
        # 2: add last digit as first element in new list
        digits.insert(0, num % 10)
        # 3: remove last digit from integer
        num //= 10

        # ...till there are no digits left
        if num == 0:
          break

    return digits

print(num_to_list(12345)) # [1, 2, 3, 4, 5]
print(num_to_list(0)) # [0]

However, if the integer can also be negative, then you can handle the negative sign (-) in the following ways:

Make the Most Significant Digit Negative:

Making the most significant digit negative means that in the resulting list, if the original number is negative, the first digit will have the negative sign. This means that: 

# 12345 becomes [1, 2, 3, 4, 5]
# -12345 becomes [-1, 2, 3, 4, 5]

For example, this can be implemented like so: 

def num_to_list(num):
    # 1: convert integer to absolute form
    abs_num = abs(num)
    digits = []

    # 2: create a loop
    while True:
        # 3: add last digit as first element in new list
        digits.insert(0, abs_num % 10)
        # 4: remove last digit from integer
        abs_num //= 10

        # ...till there are no digits left
        if abs_num == 0:
          break

    # 5: make first digit negative if original number is negative
    if num < 0:
        digits[0] = -digits[0]

    return digits

print(num_to_list(12345)) # [1, 2, 3, 4, 5]
print(num_to_list(-12345)) # [-1, 2, 3, 4, 5]

print(num_to_list(0)) # [0]
print(num_to_list(-0)) # [0]

Add the Minus Sign as a String:

Adding the minus sign (-) as a string means that in the resulting list, if the original number is negative, the first element of the list will have the minus sign as a string: 

# 12345 becomes [1, 2, 3, 4, 5]
# -12345 becomes ['-', 1, 2, 3, 4, 5]

For example, this can be implemented like so: 

def num_to_list(num):
    # 1: convert integer to absolute form
    abs_num = abs(num)
    digits = []

    # 2: create a loop
    while True:
        # 3: add last digit as first element in new list
        digits.insert(0, abs_num % 10)
        # 4: remove last digit from integer
        abs_num //= 10

        # ...till there are no digits left
        if abs_num == 0:
          break

    # 5: add '-' to front of list
    if num < 0:
        digits.insert(0, '-')

    return digits

print(num_to_list(12345)) # [1, 2, 3, 4, 5]
print(num_to_list(-12345)) # ['-', 1, 2, 3, 4, 5]

print(num_to_list(0)) # [0, 0]
print(num_to_list(-0)) # [0, 0]

Add a Leading Digit to Represent the Sign:

Adding a leading digit to represent the sign means that in the resulting list, the first element can be used to represent the sign (i.e. 0 for positive integers and 1 for negative integers): 

# 12345 becomes [0, 1, 2, 3, 4, 5]
# -12345 becomes [1, 1, 2, 3, 4, 5]

For example, this can be implemented like so: 

def num_to_list(num):
    # 1: convert integer to absolute form
    abs_num = abs(num)
    digits = []

    # 2: create a loop
    while True:
        # 3: add last digit as first element in new list
        digits.insert(0, abs_num % 10)
        # 4: remove last digit from integer
        abs_num //= 10

        # ...till there are no digits left
        if abs_num == 0:
          break

    # 5: add 0 or 1 as first element in list
    digits.insert(0, 1 if (num < 0) else 0)

    return digits

print(num_to_list(12345)) # [0, 1, 2, 3, 4, 5]
print(num_to_list(-12345)) # [1, 1, 2, 3, 4, 5]

print(num_to_list(0)) # [0, 0]
print(num_to_list(-0)) # [0, 0]

Using Regular Expression

If you are certain that the integer is always going to be positive, then you can simply do the following:

  1. Use re.findall() with a regular expression to convert numeric string to a list of numeric string characters;
  2. Use list comprehension to convert each list item to an integer.

For example, this can be implemented like so: 

import re

def num_to_list(num):
    # 1: use regex to match numbers
    matches = re.findall(r'\d', str(num))
    # 2: convert each item to an integer
    return [int(match) for match in matches]

print(num_to_list(12345)) # [1, 2, 3, 4, 5]
print(num_to_list(0)) # [0]

However, if the integer can also be negative, then you can handle the negative sign (-) in the following ways:

Make the Most Significant Digit Negative:

Making the most significant digit negative means that in the resulting list, if the original number is negative, the first digit will have the negative sign: 

# 12345 becomes [1, 2, 3, 4, 5]
# -12345 becomes [-1, 2, 3, 4, 5]

For example, this can be implemented like so: 

import re

def num_to_list(num):
    # 1: use regex to match numbers
    matches = re.findall(r'-?\d', str(num))
    # 2: convert each item to an integer
    return [int(match) for match in matches]

print(num_to_list(12345)) # [1, 2, 3, 4, 5]
print(num_to_list(-12345)) # [-1, 2, 3, 4, 5]

print(num_to_list(0)) # [0]
print(num_to_list(-0)) # [0]

Add the Minus Sign as a String:

Adding the minus sign (-) as a string means that in the resulting list, if the original number is negative, the first element of the list will have the minus sign as a string: 

# 12345 becomes [1, 2, 3, 4, 5]
# -12345 becomes ['-', 1, 2, 3, 4, 5]

For example, this can be implemented like so: 

import re

def num_to_list(num):
    # 1: use regex to match numbers
    matches = re.findall(r'\d', str(num))
    # 2: convert each item to an integer
    digits = [int(match) for match in matches]
    # 3: add '-' to the front of list
    if num < 0:
        digits.insert(0, '-')

    return digits

print(num_to_list(12345)) # [1, 2, 3, 4, 5]
print(num_to_list(-12345)) # ['-', 1, 2, 3, 4, 5]

print(num_to_list(0)) # [0]
print(num_to_list(-0)) # [0]

Add a Leading Digit to Represent the Sign:

Adding a leading digit to represent the sign means that in the resulting list, the first element can be used to represent the sign (i.e. 0 for positive integers and 1 for negative integers): 

# 12345 becomes [0, 1, 2, 3, 4, 5]
# -12345 becomes [1, 1, 2, 3, 4, 5]

For example, this can be implemented like so: 

import re

def num_to_list(num):
    # 1: use regex to match numbers
    matches = re.findall(r'\d', str(num))
    # 2: convert each item to an integer
    digits = [int(match) for match in matches]
    # 3: add 0 or 1 as first element in list
    digits.insert(0, 1 if (num < 0) else 0)

    return digits

print(num_to_list(12345)) # [0, 1, 2, 3, 4, 5]
print(num_to_list(-12345)) # [1, 1, 2, 3, 4, 5]

print(num_to_list(0)) # [0, 0]
print(num_to_list(-0)) # [0, 0]

This post was published by Daniyal Hamid. Daniyal currently works as the Head of Engineering in Germany and has 20+ years of experience in software engineering, design and marketing. Please show your love and support by sharing this post.