Check if string is number in C++

In this tutorial, we will learn how to check whether the string is a number using C++. To better understand the problem, we are going to see some examples.

For Example:
Let us take three input strings 
string s1 = "CodeSpeedy"
string s2 = "c++"
string s3 = "54655449"

So Now the Output will be 

Not Number 
Not Number
Number

Now to solve this problem we are going to use the std::isdigit() function method. It is the fastest method to implement the solution.

Approach to the Problem

We just have to pass a string as a parameter that iterates and checks the string in each character-wise. As it finds out the first non-number character in the string, It returns False, and if not it returns True.

C++ program to check if a string is number or not

// Program to find the string is a number
#include <iostream>
#include <algorithm>

using namespace std;

// Function to check the string character by character
bool isNumber(string& str)
{
    for (char const &c : str) {
       
        // using the std::isdigit() function
        if (std::isdigit(c) == 0)
          return false;
    }
    return true;
}

//Driver Code
int main(){
    string str1 = "2Codespeedy";
    string str2 = "2222313232";
    string str3 = "a31C++";

    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 0;
}
OUTPUT 

Not Number
Number
Not Number

We can also use std::stoi() function and it will work on negative numbers too:

#include <iostream>
#include <string>

bool is_number(const std::string& s) {
    try {
        std::stoi(s);
    } catch (std::exception& e) {
        return false;
    }
    return true;
}

int main() {
    std::string s = "12345";
    if (is_number(s)) {
        std::cout << s << " is a number." << std::endl;
    } else {
        std::cout << s << " is not a number." << std::endl;
    }
    return 0;
}

Also read: How to check the current C++ version using Program

3 responses to “Check if string is number in C++”

  1. randomUser123 says:

    It won’t work for negative integers (“-69”) or if you specify positive integers with ‘+’ sign like “+69”.

  2. Mateusz says:

    You forget about empty condition.
    Empty string gives positive result in your case so it means it is number.

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