c++ - 如何提取字符串中任意位置的下一个整数？

Question

我的代码在下面，我正在开发一个简单的文本编辑器。用户需要能够输入以下格式:

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

我在下面使用了一个 switch 语句来查看他们输入的第一个字符是什么，但是在 case 'I'(插入)和 case 'D'(删除)中我需要能够提取他们之后输入的整数。

例如:

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

我已经尝试了一些不同的方法，但没有一个是顺利的，所以我想知道是否有更好的方法来做到这一点。

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;
    }

我尝试了一些不同的东西，比如 getline() 和 cin << 但是如果用户没有以特定格式输入行，我就无法让它正常工作，我想知道是否有办法。

谢谢。

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;
}

在您的 switch 语句中，您可以像这样使用它:

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