My code is written twice for uncomprehensible reas

Question

This code is written in C++ and for reasons that I don't quite understand it is written twice. I would expect that after inputting a random char it would display the char once and the String lower to it once as well. But I don't get this as output. What am I missing?

Solution: Adding a cin.ignore() statement disregards the return that is read in as well. Making my code go through the loop once.

#include <iostream>

using std::cin;
using std::cout;
using std::endl;

int main()
{
    char letter;

    letter = cin.get();
    while (letter!= 'X')
    {
        cout << letter << endl;
        cout << "this will be written twice for ununderstandable reasons";                
        letter = cin.get();
    }
}

Example: If I were to write in cmd scrn c, I'd get a c back + twice the phrase this will be written twice for ununderstandable reasons. So what I thought to be the output

c
this will be written twice for ununderstandable reasons

is actually

c
this will be written twice for ununderstandable reasons
this will be written twice for ununderstandable reasons

Answer 1

The text 'this will be written twice..' will not necessarily print twice.

Type 'qwerty' + ENTER and your stream will have "qwerty\n" within and you'll see this output:

this will be written twice for ununderstandable reasons
this will be written twice for ununderstandable reasons
this will be written twice for ununderstandable reasons
this will be written twice for ununderstandable reasons
this will be written twice for ununderstandable reasons
this will be written twice for ununderstandable reasons
this will be written twice for ununderstandable reasons

Exactly that many as string "qwerty\n" has characters. The problem is that

cin.get()

Puts all chars that you type into a stream/buffer (not your letter char) but handles one char every cin.get() invocation.

When you type 'abcXd' + enter - the program will print above line 3 times and stop on X.

Answer 2

as everyone already mentioned, cin will append the newline marker \n every time you hit enter. another solution is to place cin.ignore(); after every cin.get();.

#include <iostream>

using std::cin;
using std::cout;
using std::endl;

int main()
{
    char letter;

    letter = cin.get();
    cin.ignore();
    while (letter!= 'X')
    {
          cout<<letter<<endl;
          cout<<"this will be written twice for ununderstandable reasons";
          letter= cin.get();
          cin.ignore();
          }
}

Answer 3

You forgot about the newline. cin reads every character, which includes the newline you type after typing your character. If you don't want this behaviour, you have to specifically check for newline.

while (letter!= 'X')
{
      if (letter == '\n')
      {
          letter = cin.get();
          continue;
      }
      cout<<letter<<endl;
      cout<<"this will be written twice for ununderstandable reasons";
      letter= cin.get();
}

Answer 4

It happens because cin.get() reads new-line character too. Try to press Enter without any symbols or type some string, like abc. You need to handle it, e.g.:

while (letter = cin.get()) {
    if (!isalpha(letter)) { continue; }
    // handling user inputted alpha
}

Answer 5

You are reading every character with the unformatted get() function, including the newline character each time you hit return.

Depending on what you're trying to do, you could use formatted input (cin >> c) to skip all whitespace; or you could test each character and ignore things like newline that don't interest you; or you could use getline(cin, some_string) to read a whole line, and then process that.

Answer 6

When you type in a character the new-line character (from pressing enter) is also in your input buffer.

From the C-Reference:

The delimiting character is not extracted from the input sequence if found, and remains there as the next character to be extracted from the stream (see getline for an alternative that does discard the delimiting character).

Just use a cin.sync() after every cin.get() to clear the buffer and you should be good to go.