cin.ignore();

This is a discussion on cin.ignore(); within the C++ Programming forums, part of the General Programming Boards category; I am a begginner in C++ and in programming generally. I am reading http://www.cprogramming.com/tutorial/lesson1.html this tutorial and I have one ...

  1. #1
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    Post cin.ignore();

    I am a begginner in C++ and in programming generally. I am reading http://www.cprogramming.com/tutorial/lesson1.html this tutorial and I have one question.

    Code:
    #include <iostream>
    
    using namespace std;
    
    int main()
    {
      int thisisanumber;
    
      cout<<"Please enter a number: ";
      cin>> thisisanumber;
      cin.ignore();
      cout<<"You entered: "<< thisisanumber <<"\n";
      cin.get();
    }
    I could not understand the purpose of the cin.ignore() function. I recompile the module without the cin.ignore() function and the result was the same.

    While running the program, I typed a character instead of a number and the result was the number 2. eg. I typed "s" and the output was "2". I typed 43243432767436482 and the output was 2 again. So what the cin.ignore() really does ?

  2. #2
    Deathray Engineer MacGyver's Avatar
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    I believe cin.ignore() just reads a character from stdin and discards it. In this case, it's probably throwing away the '\n' that you are entering when you press Enter, assuming you enter a legit int.

    If you are not entering a legit int, then you're messing things up. Don't expect the output to make sense.

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    When mixing cin >> and getline, the extra newline causes problems. One solution is to always call cin.ignore() after calling cin >> because it doesn't cause any problems when you don't use getline or get afterwards.

    In this case, get() is called later, so it does matter. The purpose of the cin.get() at the end of the program is to force the user to respond before the program finishes (so that the output can be read before the console window closes on some platforms). If you don't call cin.ignore(), then the newline in the input stream after the cin>> thisisanumber call will be read in by cin.get(). Then the window won't stay open and the user won't be able to read the output.

    If you're console window stays open automatically, then you don't have to worry about it in this instance, but others do so that is added.


    Also, if you initialize the variable to some number, then input s or 43243432767436482, then chances are the output would be whatever number you initialized it to. In this case the random value in memory it gets when uninitialized is probably 2.

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    Daved, what is "getline" ?

    Instead of using cin.get() , may I use system("PAUSE"); ?

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    getline is a function that reads an entire line of text into a string (among other things). It is similar to get, which reads a single character instead of a string of characters.

    Instead of cin.get(), you could use system("PAUSE"), but there's no real reason to. The code in the original example will work everywhere, and there's nothing wrong with leaving the cin.ignore() in the code.

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    Well after experimenting with these I found out and solve my query.
    When I use cin.ignore() in the code the MS-DOS Window does not dissappear immediately, even with cin.get() at the bottom of the code. So, cin.ignore() in combination with cin.get() is a "must" to make MS-DOS problems run away. Although, I discover that using system("PAUSE") instead of both cin.get() and cin.ignore, it does the same thing.

    So, as you said previously, there are no errors. Have a nice day :P
    Thanks a lot.

  7. #7
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    Note that the cin.ignore() is only a "must" with the cin.get() if you use cin >> previously in the program.

  8. #8
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    I got that master. Good point worth mentioning.

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