带有 getline 的循环不会因用户输入而结束

Question

I thought getline stops at a newline character, but the while loop does not end? it returns the correct data but it just sits in the terminal window. For example:

 Enter an expression: #5+4#5+4 (blinking cursor)

(can enter data forever and press enter forever and it wont exit)

my code, (main.cpp):

    int main()
    {
    string exp;
        cout << "Enter an Infix Expression:";

        while (getline(cin, exp, '#'))
        {
            string token = exp;
            string post;
            cout << token << endl;
            IntoPost *infix = new IntoPost(token.length());
            post = infix->inToPost(token);
            cout << post << endl;
        }
        cin.get();
    }

Answer 1

The Solution Using EOF

Your current program is looping endlessly because getline returns std::basic_istream , so while(getline()) will never equate to 'false'.

As @0x499602D2 has stated, your program is working as intended, but the extraction from getline can only end in two ways, as indicated by the reference here :

Extracts characters from is and stores them into str until the delimitation character delim is found (or the newline character, '\n', for when no delimiter is specified).

The extraction also stops if the end of file is reached in is or if some other error occurs during the input operation.

The first condition is difficult to pull off, as inputs on console are triggered by the \n character.

As for the second condition, as per @DavidC.Rankin:

You can also generate a manual EOF on Linux with [Ctrl+d] or windows with [Ctrl+z] (generally twice is required)

This means the solution is to use [Ctrl+d] or [Ctrl+z] to trigger the second condition to end your while loop at any time.

---

Alternative Using a Break Statement

One alternative way you can try to end the loop instead is breaking on input of an 'exit' string:

(1)

#include <algorithm>
//...
    while (getline(cin, exp, '#'))
    {
        // removes meaningless endline chars from input
        exp.erase(std::remove(exp.begin(), exp.end(), '\n'), exp.end()); 
        if (exp == "exit"){
            break;
        }
        //... Your While Block Code Here!
    }

To break out of your while loop, you can simply use:

exit#

# Note, the endls from your couts in the loop will bleed into your inputs on your next while (getline(cin, exp, '#')) , giving us unwanted newlines. To prevent this, we can get rid of the endlines from the inputs by using std::erase(). If you wish to keep those endlines in your input, simply set string token = exp; in front of the erase() line.

Answer 2

That's right, getline blocks the execution of the loop until a line separator is received and returns while that all is well, in the next step everything is repeated. If you want the loop not to be infinite - then put the Boolean variable key in the loop condition, and from the input check if the last character is an exit symbol and if so switch the variable key

Answer 3

Well, in a loop you must include some sort of operator. Right now your code reads:

while (getline(cin, exp, '#'))
{
   //code
}` 

And the problem with this is that it's basically saying,

while(exp == true){
//Execute code forever
}

In order for your loop to stop, you must pass some sort of argument. Example:

while(getline(cin, exp, '#') == "What ever you want"){
//Exicute this code while exp is what I want
}

I hope you can implement this into your project, although I don't know specifically what you want your loop to do, so just remember to pass an argument that allows your while loop to stop at some point.