C++ iostream: Using cin >> var and getline(cin, var) input errors

2
votes

I'm creating a simple console application in C++ that gets string and char inputs from the user. To make things simple, I would like to use the string and char data types to pass input from cin to.

To get string inputs, I'm using the getline method:

string var;
cin.ignore(); //I used ignore() because it prevents skipping a line after using cin >> var
getline(cin, var);

To get char inputs, I'm using the cin >> var method:

char var;
cin >> var;

This works fine for the most part. However, when I enter a string using getline, it ignores the first character of my string.

Is it possible to use getline and cin >> without having to use ignore, or a method I can call to ensure that my first character isn't skipped?

This is a full sample of code where I use both getline and cin >>:

string firstName;
string lastName;
char gender = 'A';

cout << "First Name: ";
cin.ignore();
getline(cin, firstName);


cout << "Last Name: ";
cin.ignore();
getline(cin, lastName);

while(genderChar != 'M' && genderChar != 'F')
{
    cout << "Gender (M/F): ";
    cin >> genderChar;
    genderChar = toupper(genderChar);
}
5
c++inputcingetline
I don't understand your comments for cin.ignore(). Based on its api definition, it will discard the first character of string you input, just as the scenario you encounter now. – Jin Chen
@JinChen sorry, fixed. I meant to say that if I DON'T use ignore(), it skips a line of input. – miguelarcilla
still weird, cin.ignore() should not behavior like that. Can you provide more code of your problem? – Jin Chen
@JinChen updated again with some sample code. The comments are where my problems are – miguelarcilla

5 Answers

6
votes

cin>>var;

only grabs the var from the buffer, it leaves the \n in the buffer, which is then immediately grabbed up by the getline

So, following is just fine, (if I understood correctly your problem)

cin>>var;
cin.ignore();     //Skip trailing '\n'
getline(cin, var);

As per your edited post

You don't have to use cin.ignore(); for geline

This extracts characters from buffer and stores them into firstName or (lastName) until the delimitation character here -newline ('\n').

1
votes

ignore() does not skip a line, it skips a character. Could you send example code and elaborate on the need for cin.ignore()?

1
votes

std::cin.ignore() will ignore the first character of your input.

For your case, use std::cin.ignore() after std::cin and then getline() to ignore newline character as:

cin>>ch;
cin.ignore();  //to skip the newline character in the buffer
getline(cin,var);
0
votes

You are using std::isstream::ignore() before std::getline(). std::cin.ignore() will extract the first character from the input sequence and discard that.

http://www.cplusplus.com/reference/istream/istream/ignore/

0
votes

So basically, cin>>var leaves the '\n' character out of its buffer. So now when you call getline it reads the '\n' character and stops. Therefore we use cin.ignore() to ignore the first character getline reads i.e '\n' when we use it after cin.

But getline doesn't leave '\n' character instead it stores everything in its buffer till it find '\n' character, then stores '\n' character as well and then stops.

So in your code when you are using cin.ignore() after a getline and again uses getline to take input, it ignores the first character of the string instead of '\n'. That is why the first character is missing.

Hope this answers your question.