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Determine if a String Is a Number in C++

Created: September-24, 2020 | Updated: March-30, 2021

  1. Use std::isdigit Method to Determine if a String Is a Number
  2. Use std::isdigit With std::ranges::all_of to Determine if a String Is a Number
  3. Use find_first_not_of Method to Determine if a String Is a Number

This article explains how to find out if a given C++ string is a number. Before we dig in, it should be noted that the following methods are only compatible with single-byte character strings and decimal integral numbers.

Use std::isdigit Method to Determine if a String Is a Number

The first version is probably the most obvious way to implement the solution. Namely, pass a string as a parameter to a function isNumber, which iterates over every single char in the string and checks with isdigit method. When it finds the first non-number the function returns false, if none is found returns true. 

              #include <iostream>  using std::cout; using std::cin; using std::endl using std::string;  bool isNumber(const string& str) {     for (char const &c : str) {         if (std::isdigit(c) == 0) return false;     }     return true; }  int main(){     string str1 = "231524randstr23";     string str2 = "23152423";     string str3 = "a3152423";      isNumber(str1) ? cout << "Number\n" : cout << "Not number\n";     isNumber(str2) ? cout << "Number\n" : cout << "Not number\n";     isNumber(str3) ? cout << "Number\n" : cout << "Not number\n";      return EXIT_SUCCESS; }

Output:

              Not number Number Not number

Notice that we output verdict on each string via ? : ternary conditional operator, which is a concise variant of if-else.

Use std::isdigit With std::ranges::all_of to Determine if a String Is a Number

The previous method was quite basic for the mighty C++, so let's implement a more eloquent solution using C++20 method std::ranges::all_of and some lambda expressions. In our case ranges::all_of checks if the specified lambda returns true for every element of the given range s.begin(), s.end() and returns true if condition is satisfied.

              #include <iostream> #include <algorithm>  using std::cout; using std::cin; using std::endl using std::string;  bool isNumber(const string& s) {     return std::ranges::all_of(s.begin(), s.end(),                   [](char c){ return isdigit(c) != 0; }); }  int main(){     string str1 = "231524randstr23";     string str2 = "23152423";     string str3 = "a3152423";      isNumber(str1) ? cout << "Number\n" : cout << "Not number\n";     isNumber(str2) ? cout << "Number\n" : cout << "Not number\n";     isNumber(str3) ? cout << "Number\n" : cout << "Not number\n";      return EXIT_SUCCESS; }

Use find_first_not_of Method to Determine if a String Is a Number

This version utilizes a built-in string search algorithm. The algorithm searches for the first character equal to none of the characters in string passed as an argument (in our case - "0123456789"). If the character is not found, string::npos is returned, thus we return the result of comparison from isNumber. 

              #include <iostream>  using std::cout; using std::cin; using std::endl using std::string;  bool isNumber(const string& str) {     return str.find_first_not_of("0123456789") == string::npos; }  int main(){     string str1 = "231524randstr23";     string str2 = "23152423";     string str3 = "a3152423";      isNumber(str1) ? cout << "Number\n" : cout << "Not number\n";     isNumber(str2) ? cout << "Number\n" : cout << "Not number\n";     isNumber(str3) ? cout << "Number\n" : cout << "Not number\n";      return EXIT_SUCCESS; }

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