check if the given string is valid or not explanation with solution

Validate if a given string is numeric.

Some examples:
“0” => true
” 0.1 ” => true
“abc” => false
“1 a” => false
“2e10” => true

Below is the list of characters that can be in a valid decimal number:
• Numbers 0-9
• Exponent – “e”
• Positive/negative sign – “+”/”-”
• Decimal point – “.”

The problem can be solved in 3 methods:

1. Deterministic Finite Automaton
2. Regex
3. Simple Method

We are going to look at 3rd method. If anybody is able to solve by 1st and 2nd method, please leave the solution in the comment section of this post.

So to check if the given string is a number, we need to check the following cases:

1. Skip leading spaces.
2. Skip sign.
3. Integer, decimal point and fractional parts. Ensure it is {+, -, ., e, [0-9]}
4. Exponential bit. Check that no ‘.’ comes after ‘e’.
5. Skip following spaces.
6. Make sure that’s the end.

So we check it programmatically as below:

/*
* File     : check_if_the_number_is_valid.cpp
*/ 


#include<iostream>
#include<string>

using namespace std;


bool isNumber(string s)
{
   int n = s.size();
   if(n == 0) return false;

   int i = 0;
     //Skip leading spaces.
   while(s[i] == ' ') i++;

     //Significand
   if(s[i] == '+' || s[i] == '-') i++;

   int cnt = 0;
     //Integer part
   while(isdigit(s[i]))
   {
       i++;
       cnt++;
   }
     //Decimal point
   if(s[i] == '.') i++;
     //Fractional part
   while(isdigit(s[i]))
   {
       i++;
       cnt++;
   }
    
    if(cnt == 0) return false;  //No number in front or behind '.'
     
     //Exponential
     if(s[i] == 'e')
     {
       i++;
       if(s[i] == '+' || s[i] == '-') i++;
         if(!isdigit(s[i])) return false;    //No number follows
         while(isdigit(s[i])) i++;
     }
     
     //Skip following spaces;
     while(s[i] == ' ') i++;
    
    return s[i] == '\0';
}


int main()
{
    string str = "2e10";
//Other test case
    //string str = "abc";
    //string str = " 6e-1";
    //string str = " 99e2.5 ";

    if (isNumber(str))
        cout<<"The string is a valid number"<< endl;
    else
        cout<<"The string is not a valid number"<< endl;

}

Output:

The string is a valid number