Python | Add comma between numbers

Sometimes, while working with currencies, we need to put commas between numbers to represent a currency, hence given a string, we can have a problem to insert commas into them. Let’s discuss certain ways in which this task can be performed. 

Method #1 : Using str.format() The string format function can be used by supplying the valid formatter to perform the formatting. The string formatter is called with the value as argument to perform this particular task. 

Python3

# Python3 code to demonstrate working of
# Adding comma between numbers
# Using str.format()

# initializing number
test_num = 1234567

# printing original number
print("The original number is : " + str(test_num))

# Using str.format()
# Adding comma between numbers
res = ('{:,}'.format(test_num))

# printing result
print("The number after inserting commas : " + str(res))

Output

The original number is : 1234567
The number after inserting commas : 1,234,567

Time complexity: O(1) as the formatting operation is a constant time operation.
Auxiliary space: O(1) as no additional data structure is used and the result is stored in a variable.

Method #2 : Using format() This task can also be performed without taking the help of format library of strings but the normal format library that can use the “d” notation of numbers and simply insert the commas where required. 

Python3

# Python3 code to demonstrate working of
# Adding comma between numbers
# Using format()

# initializing number
test_num = 1234567

# printing original number
print("The original number is : " + str(test_num))

# Using format()
# Adding comma between numbers
res = (format (test_num, ',d'))

# printing result
print("The number after inserting commas : " + str(res))

Output

The original number is : 1234567
The number after inserting commas : 1,234,567

Time complexity: O(1) The time complexity of the program is constant time because it does not depend on the size of the input. 

Auxiliary space: O(1) The program uses a constant amount of auxiliary space.

Method #3: Using re.sub()

The re.sub() method can be used along with regular expressions to match the required pattern of digits and insert commas in between them.

Python3

# Python3 code to demonstrate working of
# Adding comma between numbers
# Using re.sub()
import re

# initializing number
test_num = 1234567

# printing original number
print("The original number is : " + str(test_num))

# Using re.sub()
# Adding comma between numbers
res = re.sub(r'(\d{3})(?=\d)', r'\1,', str(test_num)[::-1])[::-1]

# printing result
print("The number after inserting commas : " + str(res))
#This code is contributed by Edula Vinay Kumar Reddy

Output

The original number is : 1234567
The number after inserting commas : 1,234,567

This method will match every 3 digits in the number, and insert a comma before it using a positive lookahead. The string is first reversed using slicing so that the commas are inserted from right to left, and then reversed back to get the final output.
Time Complexity and Auxiliary space of this approach is O(n)

Method#4: Using f-string

Python3

test_num = 1234567

# printing original number
print("The original number is : " + str(test_num))

res = f"{test_num:,}"

# printing result
print("The number after inserting commas : " + str(res))


#This code is contributed by Vinay Pinjala.

Output

The original number is : 1234567
The number after inserting commas : 1,234,567

Time Complexity: O(n)

Auxiliary space : O(1)

Method#5: Using insert_commas():

Python3

def insert_commas(num):
    num_str = str(num)
    result = ""
    for i, digit in enumerate(num_str[::-1]):
        if i > 0 and i % 3 == 0:
            result = "," + result
        result = digit + result
    return result
test_num = 1234567
print("The original number is:", test_num)
res = insert_commas(test_num)
print("The number after inserting commas:", res)
#This code is contributed by Jyothi pinjala

Output

The original number is: 1234567
The number after inserting commas: 1,234,567

Time Complexity : O(n)

Auxiliary space : O(n)

Method 6 : use recursion. 

 step-by-step approach:

1. Define a recursive function insert_commas_recursive() that takes two arguments: the number to be formatted as a string, and a counter that keeps track of the number of digits processed so far.
2. If the length of the input string is less than or equal to 3, return the input string as-is.
3. Otherwise, call insert_commas_recursive() recursively with the input string minus the last three digits, and increment the counter by 3.
4. Concatenate the result of the recursive call with a comma and the last three digits of the input string.
5. Return the final formatted string.
6. In the main code block, convert the input number to a string, and call the insert_commas_recursive() function with the string and a counter of 0.
7. Print the original number and the formatted number.

Python3

def insert_commas_recursive(num_str, counter):
    if len(num_str) <= 3:
        return num_str
    else:
        result = insert_commas_recursive(num_str[:-3], counter+3)
        return result + ',' + num_str[-3:]

test_num = 1234567
num_str = str(test_num)
res = insert_commas_recursive(num_str, 0)
print("The original number is:", test_num)
print("The number after inserting commas:", res)

Output

The original number is: 1234567
The number after inserting commas: 1,234,567

The time complexity : O(n) where n is the number of digits in the number.

 auxiliary space:  O(n).