validating input question

[layzer]

howdy all, i need to validate user input. A string of numbers that allows "- +" signs, but not any other signs like "$,%". i have it all working except for allowing one period. EI. 67.89 should be valid. I cant tell if i just messed up the punctuation statement or what. And i also do not know how to make it check for only one punctuation mark and return invalid for a second or third punctuation EI 76.54.32 would be invalid.

Any help would be appreciated!

punctuation validation is at line 53.

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

bool isvalidInt(string);  // function prototype (declaration)

int main()
{
  string value;
  double number;

  cout << "Enter an integer: ";
  getline(cin, value);

  if (!isvalidInt(value))
   cout << "The number you entered is not a valid integer.\n\n";
  else
  {
	number = atoi(value.c_str());
    cout << "The integer you entered is " 
         << number
         << endl;
    return 0;
  }
  
}

   bool isvalidInt(string str)
   {
     int start = 0;
     int i;
     bool valid = true;  // assume a valid 
     bool sign = false;  // assume no sign 
	
   
     // check for an empty string
     if (str.length() == 0)  valid = false;
	
     // check for a leading sign
     if ( valid  &&  ( str.at(0) == '-'|| str.at(0) == '+' ) ) 
     { 
	      sign = true;
	      start = 1;  // start checking for digits after the sign
     }

     // check that there is at least one character after the sign
     if (sign && str.length() == 1) valid = false;
	
     // now check the string, which we know has at least one non-sign char
     i = start;
     //check for punctuation mark only 1
     if (ispunct(str)) 
        
          i++;
     // we have checked i number of chars so far
     while(valid && i != str.length())
     {
	    if(!isdigit(str.at(i)))  valid = false;  //found a non-digit character
	    i++;  // move to next character
     }
     cout<<"chars checked: "<<i<<"\n\n";
	return valid;
  }  

Edit:

I tried moving it into the while loop, and that got rid of errors and it now runs but it does not seem to be checking for the punctuation at all.

 while(valid && i != str.length())
     {
	    if(!isdigit(str.at(i)))  valid = false;  //found a non-digit character
	    i++;  // move to next character
            
            //check for punctuation mark only 1
         
            if (ispunct(str.at(i))) valid = false;
        
          i++;
     }
     cout<<"chars checked: "<<i<<"\n\n";
	return valid;
  }