C++ Exercises: Check if a given string is a decimal number or not

C++ Math: Exercise-21 with Solution

A decimal number is defined as a number whose whole number and fractional parts are separated by a decimal point in algebra. Write a C++ program to check if a given string is a decimal number or not.

List of characters of a valid decimal number:
Numbers: 0-9
Positive/negative sign - "+"/"-"
Decimal point - "."
Exponent - "e"

Sample Input: s = 9
Sample Output: Is 0 a decimal number? 1

Sample Input: s = abc 123
Sample Output: Is abc 123 a decimal number? 0

Sample Solution:

C++ Code :

#include <iostream>

using namespace std;

// Function to check if the given string is a decimal number or not
bool isNumber(string str_num) {
    int i = 0;
    int str_len = str_num.size();

    // Skipping leading spaces
    while (i < str_len && str_num[i] == ' ')
        i++;

    // Handling sign (+/-)
    if (i < str_len && (str_num[i] == '+' || str_num[i] == '-'))
        i++;

    int digits = 0, dots = 0;

    // Loop to check for digits and dots in the string
    while (i < str_len && ((str_num[i] >= '0' && str_num[i] <= '9') || (str_num[i] == '.'))) {
        if (str_num[i] >= '0' && str_num[i] <= '9')
            digits++;
        else if (str_num[i] == '.')
            dots++;
        i++;
    }

    // Conditions for an invalid number
    if (digits == 0 || dots > 1)
        return false;

    // Handling exponential notation (e)
    if (i < str_len && str_num[i] == 'e') {
        // Handling sign (+/-) after 'e'
        if (++i < str_len && (str_num[i] == '+' || str_num[i] == '-'))
            i++;

        // Checking for valid exponential notation
        if (i == str_len || (str_num[i] < '0' || str_num[i] > '9'))
            return false;

        // Skipping exponential part
        while (i < str_len && (str_num[i] >= '0' && str_num[i] <= '9'))
            i++;
    }

    // Skipping optional trailing spaces
    while (i < str_len && str_num[i] == ' ')
        i++;

    return (i == str_len); // If at the end of the string, then it's a valid number
}

// Main function to test isNumber() function
int main() {
    string s = "0";
    cout << "\n Is " << s << " a decimal number? " << isNumber(s) << endl;
    s = "abc 123";
    cout << "\n Is " << s << " a decimal number? " << isNumber(s) << endl;
    s = "abc";
    cout << "\n Is " << s << " a decimal number? " << isNumber(s);
    s = "0.12";
    cout << "\n Is " << s << " a decimal number? " << isNumber(s) << endl;
    s = "123.33";
    cout << "\n Is " << s << " a decimal number? " << isNumber(s) << endl;
    s = "76.4e93";
    cout << "\n Is " << s << " a decimal number? " << isNumber(s) << endl;
    s = "+123";
    cout << "\n Is " << s << " a decimal number? " << isNumber(s) << endl;
    s = "+-33";
    cout << "\n Is " << s << " a decimal number? " << isNumber(s) << endl;
    return 0;
}

Sample Output:

 Is 0 a  decimal number? 1

 Is abc 123 a  decimal number? 0

 Is abc a  decimal number? 0
 Is 0.12 a  decimal number? 1

 Is 123.33 a  decimal number? 1

 Is 76.4e93 a  decimal number? 1

 Is +123 a  decimal number? 1

 Is +-33 a  decimal number? 0

Flowchart:

C++ Code Editor:

Contribute your code and comments through Disqus.

Previous: Write a C++ program to calculate the product of two positive integers represented as strings.