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Remove leading zeros from a Number given as a string

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  • Last Updated : 05 Jul, 2022

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:
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:

Java




// Java Program to remove all the leading
//zeros from a given numeric string
import java.io.*;
 
class GFG
{
     
    // Function to remove the leading zeros
    static String removeLeadingZeros(String num)
    {
        //traverse the entire string
        for(int i=0;i<num.length();i++){
             
            //check for the first non-zero character
            if(num.charAt(i)!='0'){
                //return the remaining string
                String res = num.substring(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
    public static void main(String[] args)
    {
        String num = "1023";
        System.out.println(removeLeadingZeros(num));
             
        num = "00123";
        System.out.println(removeLeadingZeros(num));
    }
}
 
// This code is contributed by shruti456rawal


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:

C++




// C++ Program to implement
// the above approach
#include <iostream>
#include <regex>
using namespace std;
 
// Function to remove all leading
// zeros from a given string
void removeLeadingZeros(string str)
{
    // Regex to remove leading
    // zeros from a string
    const regex pattern("^0+(?!$)");
 
    // Replaces the matched
    // value with given string
    str = regex_replace(str, pattern, "");
    cout << str;
}
 
// Driver Code
int main()
{
    string str = "0001234";
    removeLeadingZeros(str);
    return 0;
}


Java




// Java Program to implement
// the above approach
import java.util.regex.*;
class GFG
{
 
    // Function to remove all leading
    // zeros from a given string
    public static void removeLeadingZeros(String str)
    {
 
        // Regex to remove leading
        // zeros from a string
        String regex = "^0+(?!$)";
 
        // Replaces the matched
        // value with given string
        str = str.replaceAll(regex, "");
 
        System.out.println(str);
    }
 
    // Driver Code
    public static void main(String args[])
    {
        String str = "0001234";
 
        removeLeadingZeros(str);
    }
}


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)
 
# This code is contributed by avanitrachhadiya2155


C#




// C# program to implement
// the above approach
using System;
using System.Text.RegularExpressions;
 
class GFG{
 
// Function to remove all leading
// zeros from a given string
public static void removeLeadingZeros(string str)
{
     
    // Regex to remove leading
    // zeros from a string
    string regex = "^0+(?!$)";
 
    // Replaces the matched
    // value with given string
    str = Regex.Replace(str, regex, "");
 
    Console.WriteLine(str);
}
 
// Driver Code
public static void Main(string[] args)
{
    string str = "0001234";
 
    removeLeadingZeros(str);
}
}
 
// This code is contributed by ukasp


Javascript




<script>
// Javascript Program to implement
// the above approach
 
// Function to remove all leading
    // zeros from a given string
function removeLeadingZeros(str)
{
    // Regex to remove leading
        // zeros from a string
        const regex = new RegExp("^0+(?!$)",'g');
  
        // Replaces the matched
        // value with given string
        str = str.replaceAll(regex, "");
  
        document.write(str);
}
 
// Driver Code
let str = "0001234";
  
removeLeadingZeros(str);
 
// This code is contributed by unknown2108
</script>


Output

1234

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

Java-specific approach: Refer to this article for the Java-specific approach using StringBuffer.


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