Add leading zeros to a C++ string

This post will discuss how to add leading zeros to a string in C++.

1. Using `std::string` constructor

A simple solution is to create a string consisting of all zeros of the required length using the std::string constructor. Then append the original string to it. Following is a C++ implementation of the same:

#include <iostream>

#include <string>

#include <algorithm>

int main()

{

std::string str = "11011";

size_t n = 8;

int precision = n - std::min(n, str.size());

std::string s = std::string(precision, '0').append(str);

std::cout << s << std::endl; // 00011011

return 0;

}

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2. Using `string::insert`

The above solution creates a new string. To insert the leading zeros in the original string, consider using the string::insert function. Its usage is demonstrated below:

#include <iostream>

#include <string>

#include <algorithm>

int main()

{

std::string str = "11011";

size_t n = 8;

int precision = n - std::min(n, str.size());

str.insert(0, precision, '0');

std::cout << str << std::endl; // 00011011

return 0;

}

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3. Using `std::ostringstream`

Another option is to use the std::setw manipulator with std::ostringstream to pad a string with leading zeros. This would need <sstream> and <iomanip> headers.

#include <iostream>

#include <string>

#include <algorithm>

#include <sstream>

#include <iomanip>

int main()

{

std::string str = "11011";

size_t n = 8;

std::ostringstream ss;

ss << std::setw(n) << std::setfill('0') << str;

std::string s = ss.str();

std::cout << s << std::endl; // 00011011

return 0;

}

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4. Using `std::format`

Starting with C++20, we can use the formatting library to add leading zeros to the string. It provides the std::format function in the header <format>. With C++17 and before, we can use the {fmt} library to achieve the same.

#include <iostream>

#include <string>

#include <algorithm>

#include <format>

int main()

{

std::string str = "11011";

std::cout << std::format("{:08}", str); // 00011011

return 0;

}

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That’s all about adding leading zeros to a string in C++.

Add leading zeros to a C++ string

1. Using std::string constructor

2. Using string::insert

3. Using std::ostringstream

4. Using std::format

Share this:

1. Using `std::string` constructor

2. Using `string::insert`

3. Using `std::ostringstream`

4. Using `std::format`