Check if a string contains only digits in C++

This tutorial will discuss about unique ways to check if a string contains only digits in C++.

Technique 1: Using STL Algorithm std::all_of()

To check if string has only digits we can use the STL algorithm all_of(). t accepts three arguments,

• Iterator pointing to the first character of string.
• Iterator pointing to the end of string.
• A function ::isdigit(). It accepts a single character as an argument and returns true if the character is a digit.

The all_of() function will apply the ::isdigit() function on all the characters in string. If all the return values are true then they all_of() function will return true, otherwise it will return false.

If all_of() function return false, then it means there is atleast a character in the string that is not a digit. Whereas, if all_of() function returns true then it means all the characters in the string are digits.

Let’s see the complete example,

#include <iostream>
#include <string>
#include <algorithm>

int main()
{
    std::string str = "12345";

    // Check if string has only digits
    bool allDigits = std::all_of(
                          str.begin(),
                          str.end(),
                          ::isdigit);

    if (allDigits)
    {
        std::cout << "Yes, string contains all digits only." << std::endl;
    }
    else
    {
        std::cout << "No, string does not contains only digits." << std::endl;
    }

  return 0;
}

Output :

Yes, string contains all digits only.

Technique 2: Using Regex

We can use the std::regex_match() function, to check if a given string matches a regular pattern or not. To check if string contains only digits, we can use the regex pattern "\\d+". If the regex_match() function returns true, for this regex pattern and a string, then it means that all characters in that string are digits.

Let’s see the complete example,

#include <iostream>
#include <string>
#include <regex>

int main()
{
    std::string str = "12345";

    // Check if string has only digits
    bool allDigits = std::regex_match(str, std::regex("\\d+"));

    if (allDigits)
    {
        std::cout << "Yes, string contains all digits only." << std::endl;
    }
    else
    {
        std::cout << "No, string does not contains only digits." << std::endl;
    }

  return 0;
}

Output :

Yes, string contains all digits only.

Summary

Today we learned about several ways to check if a string contains only digits in C++. Thanks.