isdigit() in C++

The isdigit() function in C++ checks whether a given character is a digit or not. Let's take a look at an example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    char c = '5';

    if (isdigit(c))
        cout << c << " is a Digit";
    else
        cout << c << " is not a Digit";

    return 0;
}

5 is a Digit

Explanation: The isdigit() function returns a non-zero value if the character is a digit. Since c is a digit, it returns a non-zero (true) value.

This article provides an overview of the syntax, working and use cases of the isdigit() function in C++.

Syntax of isdigit()

The isdigit() function is defined inside the <cctype> header file.

isdigit(c);

Parameter

• c: Character to check if it is a digit.

Return Value

• Returns non-zero (true) if the character is a digit ('0'–'9').
• Returns 0 (false) otherwise.

Working of isdigit() Function

As all the characters in C++ are represented by their ASCII values and all the characters representing numeric digits lies in the range [48, 57]. The isdigit() function works by checking if the ASCII value of the given character lies in the range from 48 to 57 (or '0' to '9').

• If the character falls within this range, it returns a non-zero value (true).
• Otherwise, it returns 0 (false).

Examples of isdigit()

Below are some common use cases of the isdigit() function in C++ that demonstrate its use in validating and processing numeric characters.

Sum of Digits in a String

#include <bits/stdc++.h>
using namespace std;

int main() {
    string s = "g1f2g3";
    int sum = 0;
  
    for (char c : s) {
      
      	// If the given character is a digit
      	// Convert it to int and add
        if (isdigit(c))
            sum += c - '0';
    }
    cout << sum;

    return 0;
}

6

Phone Number Validation

As a phone number should only contain numbers, a string representing a phone number should only contains characters that represent numbers.

#include <iostream>
#include <cctype>
using namespace std;

int main() {
  
  	// Here alphabet 'O' is used instead of '0'
    string ph_num = "9124782O21";
  
  	// Check if all characters are numbers
    for (char c : ph_num) {
        if (!isdigit(c)) {
            cout << "Invalid number!";
            return 0;
        }
    }
    cout << "Valid number";
  
    return 0;
}

Invalid number!