C ++子串错误

Question

So, I made a program that will let a user enter a command, like print message and it will show him the message he entered. For example, if he types in: print Hello , the console's output will be Hello .

Now, here's my code:

#include <iostream>
using namespace std;

int main()
{
string command;
start:
cout << ">>> ";
cin >> command;
if (command.substr(0,5) == "print")
{
    if (command.substr(6,command.end) != "")
    {
        cout << command.substr(6,command.end);
        goto start;
    }
    else
    {
        cout << "Usage: print text";
        goto start;
    }
}
}

The thing is I get an error:

no matching function for call to 'std::basic_string::substr(int, )'|

and I'm not sure if I specified the substring length correctly. I wanted the first if to check if the first five words were print .

Answer 1

Try replacing command.end with command.length() .

Sidenote: It is good programming style to use the break, continue, and return statements instead of the goto statement whenever possible.

Answer 2

Your error is that you are providing command.end as an argument to the substr function which is not a valid parameter. It looks like you want to print the rest of the contents of command , in which case you can just call command.substr(6) .

Answer 3

First of all, you forgot to write #include <string> . Also it would be better not to use goto operator and a lot of magic numbers (5, 6 in your code). For reading string (from standard input) that can contain spaces you should use getline . In substr you omit the second argument if we want to get all characters until the end of the string .

I have refactored your solution and have the following code:

#include <iostream>
#include <string>

using namespace std;

int main()
{
  string command;
  string prompt("print");
  cout << ">>> ";
  while(getline(cin, command) &&
        command.size() >= prompt.size() && 
        command.substr(0, prompt.size()) == prompt)
  {                 
     if (command.size() > prompt.size())
     {
       cout << command.substr(prompt.size() + 1) << endl;
     }
     else
     {
       cout << "Usage: print text" << endl;
     }
    cout << ">>> ";
  }
  return 0;
}