为什么while循环不需要用户在c ++中输入

Question

I'm learning c++ and reading c++ primer plus, but I don't understand why this code need two "cin >> ch". I know the first cin will read character that was user input.but then I delete first "cin >> ch" and run code ,the program have no error.So the fist cin is necessary? why the second cin needn't user to input?

#include <iostream>
int main()
{
    using namespace std;
    char ch;
    int count = 0;
    cout << "Enter characters; enter # to quit:\n";
    cin >> ch; //get a character
    while (ch != '#')
    {
        cout << ch;
        ++count;
        cin >> ch; // get the next character
    }
    cout << endl << count << " characters read\n";
    return 0;
}

Answer 1

You can evaluate your input right inside condition of while loop.

#include <iostream>

int main()
{
    char ch;
    int count = 0;
    std::cout << "Enter characters; enter # to quit:\n";

    while (std::cin >> ch && ch != '#')
    {
        std::cout << "entered: " << ch << std::endl;
        ++count;
    }

    std::cout << std::endl << count << " characters read" << std::endl;

    return 0;
}

When while condition is entered it will wait for you to enter anything first. Once input is received it will check if the input is not #. If input is not # the loop is entered, input printed out, counter increased, and back to waiting for another input. If # is entered, condition becomes false, and loop is aborted.

Answer 2

If you remove the first cin then count will never be incremented. The user can enter # character before entering the loop and the program can never enter it therefore.

Answer 3

The first cin>>ch is obviously used to take input from user but you have again accepting data in while loop using the same variable name "ch" , So when you run the program it will not give u error but accept only first value that you have accept before the while loop not in while loop. In while loop you can assign new value to variable "ch" but not accept the new value again.