How to extract next integer in an arbitrary position in string?

Question

My code is below, I'm working on a simple text editor. The user needs to be able to input the following format:

I n
//where n is any integer representing the line number.

I used a switch statement below to see what the first character they typed is, but in case 'I' (insert) and case 'D'(delete) I need to be able to extract the integer they typed after that.

For example:

D 16 // deletes line 16
I 9 // Inserts string at line 9
L // lists all lines

I have tried a few different things but nothing works smoothly so I was wondering if there is a better way to do this.

void handle_choice(string &choice)
{   
    int line_number;

      // switch statement according to the first character in string choice.
    switch (choice[0]) 
    {

    case 'I':

       // code here to extract next integer in the string choice

      break;

    case 'D':

      break;

    case 'L':

      break;

    case 'Q':

      break;

    default:
      break;
    }

I tried a few different things like getline() and cin << but I can't get it to work properly in case the user doesn't input the line in that specific format and I was wondering if there is a way.

Thanks.

Answer 1

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

// This function takes the whole input string as input, and
// returns the first integer within that string as a string.

string first_integer(string input) {
   // The digits of the number will be added to the string
   // return_value. If no digits are found, return_value will
   // remain empty.
   string return_value;
   // This indicates that no digits have been found yet.
   // So long as no digits have been found, it's okay
   // if we run into non-digits.
   bool in_number = false;

   // This for statement iterates over the whole input string.
   // Within the for loop, *ix is the character from the string
   // currently being considered. 
   for(string::iterator ix = input.begin(); ix != input.end(); ix++) {
     // Check if the character is a digit.
     if(isdigit(*ix)) {
         // If it is, append it to the return_value. 
         return_value.push_back(*ix);
         in_number = true;
     } else if(in_number) {
         // If a digit has been found and then we find a non-digit
         // later, that's the end of the number.
         return return_value;
     }
   }
   // This is reached if there are no non-digit characters after
   // the number, or if there are no digits in the string. 
   return return_value;
}

Within your switch statement, you would use it like this:

case 'I':
     string number = first_integer(choice);
     // Convert the string to an int here.