Check If a Given String is a Valid Number in C++

Concept

It should be validated if a given string is numeric.

Input − str = "12.5"

Output − true

Input − str = "def"

Output − false

Input − str = "2e5"

Output − true

Input − 10e4.4

Output − false

Method

We have to handle the following cases in the code.

• We have to ignore the leading and trailing white spaces.

• We have to ignore the ‘+’, ‘-‘ and’.’ at the start.

• We have to ensure that the characters in the string belong to {+, -, ., e, [0-9]}

• We have to ensure that no ‘.’ comes after ‘e’.

• A digit should follow a dot character ‘.’.

• We have to ensure that the character ‘e’ should be followed either by ‘+’, ‘-‘, or a digit.

Example

 Live Demo

// C++ program to check if input number
// is a valid number
#include <bits/stdc++.h>
#include <iostream>
using namespace std;
int valid_number1(string str1){
   int i = 0, j = str1.length() - 1;
   while (i < str1.length() && str1[i] == ' ')
      i++;
   while (j >= 0 && str1[j] == ' ')
      j--;
   if (i > j)
      return 0;
   if (i == j && !(str1[i] >= '0' && str1[i] <= '9'))
      return 0;
   if (str1[i] != '.' && str1[i] != '+' && str1[i] != '-' && !(str1[i] >= '0' && str1[i] <= '9'))
      return 0;
   bool flagDotOrE = false;
   for (i; i <= j; i++) {
      // If any of the char does not belong to
      // {digit, +, -, ., e}
      if (str1[i] != 'e' && str1[i] != '.'
      && str1[i] != '+' && str1[i] != '-'
      && !(str1[i] >= '0' && str1[i] <= '9'))
      return 0;
      if (str1[i] == '.') {
         if (flagDotOrE == true)
            return 0;
         if (i + 1 > str1.length())
            return 0;
         if (!(str1[i + 1] >= '0' && str1[i + 1] <= '9'))
            return 0;
      }
      else if (str1[i] == 'e') {
         flagDotOrE = true;
         if (!(str1[i - 1] >= '0' && str1[i - 1] <= '9'))
            return 0;
         if (i + 1 > str1.length())
            return 0;
         if (str1[i + 1] != '+' && str1[i + 1] != '-'
            && (str1[i + 1] >= '0' && str1[i] <= '9'))
         return 0;
      }
   }
   return 1;
}
// Driver code
int main(){
   char str1[] = "0.1e10";
   if (valid_number1(str1))
      cout << "true";
   else
      cout << "false";
   return 0;
}

Output

true