Python Program for Remove leading zeros from a Number given as a string

Given numeric string str, the task is to remove all the leading zeros from a given string. If the string contains only zeros, then print a single “0”.

Examples:

Input: str = “0001234” 
Output: 1234 
Explanation:
Removal of leading substring “000” modifies the string to “1234”. 
Hence, the final answer is “1234”.

Input: str = “00000000” 
Output: 0 
Explanation:
There are no numbers except 0

Naive Approach:
The simplest approach to solve the problem is to traverse the string up to the first non-zero character present in the string and store the remaining string starting from that index as the answer. If the entire string is traversed, it means all the characters in the string are ‘0’. For this case, store “0” as the answer. Print the final answer. 

Below is the implementation of the above idea:

Python3

# Python Program to remove all the leading
# zeros from a given numeric string

# Function to remove the leading zeros
def removeLeadingZeros(num):

    # traverse the entire string
    for i in range(len(num)):

        # check for the first non-zero character
        if num[i] != '0':
            # return the remaining string
            res = num[i::];
            return res;

    # If the entire string is traversed
    # that means it didn't have a single
    # non-zero character, hence return "0"
    return "0";

# Driver Code
num = "1023";
print(removeLeadingZeros(num));

num = "00123";
print(removeLeadingZeros(num));

Output

1023
123

Time Complexity: O(N) 
Auxiliary Space: O(N)

Space-Efficient Approach:
Follow the steps below to solve the problem in constant space using Regular Expression: 

• Create a Regular Expression as given below to remove the leading zeros

regex = “^0+(?!$)”
where: 
^0+ match one or more zeros from the beginning of the string. 
(?!$) is a negative look-ahead expression, where “$” means the end of the string. 

• Use the inbuilt replaceAll() method of the String class which accepts two parameters, a Regular Expression, and a Replacement String.
• To remove the leading zeros, pass a Regex as the first parameter and empty string as the second parameter.
• This method replaces the matched value with the given string.

Below is the implementation of the above approach:

Python3

# Python3 Program to implement
# the above approach
import re

# Function to remove all leading
# zeros from a given string
def removeLeadingZeros(str):

    # Regex to remove leading
    # zeros from a string
    regex = "^0+(?!$)"

    # Replaces the matched
    # value with given string
    str = re.sub(regex, "", str)

    print(str)

# Driver Code
str = "0001234"
removeLeadingZeros(str)

Output

1234

Time Complexity: O(N), where N is the length of the string. 
Auxiliary Space: O(1)

Built in methods:

use the built-in string method lstrip to remove leading zeros. This method removes all leading characters that match a specified set of characters from a string. To remove leading zeros, we can pass ‘0’ as the set of characters to remove.

Here is an example of how to use lstrip to remove leading zeros from a string:

Python3

def remove_leading_zeros(num):
    return num.lstrip('0')

print(remove_leading_zeros('0001234')) # Output: '1234'
print(remove_leading_zeros('00000000')) # Output: ''

Output

1234

 Time complexity:  O(N) 
 Auxiliary space: O(1), where N is the length of the string.

Please refer complete article on Remove leading zeros from a Number given as a string for more details!