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In using "cin" for getting input I have about a 99% success rate. In the remaining
1% of cases the program does not pause for input. What causes this and how does one fix it?
In ancient times when we programmed in C rather than C++ the same problem occurred
occasionally if "getc" or "getchar" was used for input. You could solve the problem by
putting the line "flushall()" before the input statement but that doesn't seem to
work with cin.
I ought to give an example but the only ones I have are in longish programs that I
wouldn't ask anyone to read.
1% of cases the program does not pause for input. What causes this and how does one fix it?
In ancient times when we programmed in C rather than C++ the same problem occurred
occasionally if "getc" or "getchar" was used for input. You could solve the problem by
putting the line "flushall()" before the input statement but that doesn't seem to
work with cin.
I ought to give an example but the only ones I have are in longish programs that I
wouldn't ask anyone to read.
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same problem as in old days. when you enter a number and press <Enter> cin does not remove the "\n" from the keyboard -- you have to do that
int x; cin >> x; cin.ignore(); // remove '\n' key
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Originally Posted by murschech 
In using "cin" for getting input I have about a 99% success rate. In the remaining 1% of cases the program does not pause for input. What causes this and how does one fix it?
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Originally Posted by murschech
In ancient times when we programmed in C rather than C++ the same problem occurred occasionally if "getc" or "getchar" was used for input. You could solve the problem by putting the line "flushall()" before the input statement but that doesn't seem to work with cin.
#include <iostream>
#include <limits>
int main()
{
using namespace std;
// Create the problem by leaving a '\n' in the stream
char letter = 0;
cout << "Please type one letter: ";
cin.get( letter );
cout << "The next character is: " << (int)cin.get() << '\n';
// Create the same problem and fix it with cin.ignore()
cout << "Please type one letter: ";
cin.get( letter );
// Ignore the maximum value of the streamsize type or until '\n' is ignored
cin.ignore( numeric_limits<streamsize>::max(), '\n' );
// Now the cin.get() call will block because the stream is empty
cout << "The next character is: " << (int)cin.get() << '\n';
return 0;
}#include <iostream>
#include <limits>
#include <sstream>
#include <stdexcept>
#include <string>
namespace Raye {
using namespace std;
template <class T, class U>
T CType( U src )
{
stringstream ss;
T dst;
if ( !( ss << src ) )
throw runtime_error( "CType: stringize error" );
if ( !( ss >> dst ) )
throw runtime_error( "CType: invalid conversion" );
return dst;
}
template <class T>
T GetType( istream& is )
{
string s;
if ( !getline( is, s ) )
throw runtime_error( "GetInt: input failure" );
return CType<T>( s );
}
}
int main()
{
using namespace std;
int age;
string name;
// Create the problem as an example
cout << "Please type your age: ";
cin >> age;
cout << "Please type your name: ";
getline( cin, name );
cout << name << " is " << age << " years old\n";
// Clean up the stream for an accurate test
cin.ignore( numeric_limits<streamsize>::max(), '\n' );
// Eliminate the problem with GetType
cout << "Please type your age: ";
age = Raye::GetType<int>( cin );
cout << "Please type your name: ";
getline( cin, name );
cout << name << " is " << age << " years old\n";
return 0;
}
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Originally Posted by Ravalon
flushall() is not a standard function so you cannot expect it to work. You use cin.ignore() in C++.
#include <iostream> #include <limits> int main() { using namespace std; char letter = 0; cout << "Please type one letter: "; cin.get( letter ); cout << "The next character is: " << (int)cin.get() << '\n'; cout << "Please type one letter: "; cin.get( letter ); cin.ignore( numeric_limits<streamsize>::max(), '\n' ); cout << "The next character is: " << (int)cin.get() << '\n'; return 0; }
Though it would be really better if cin.clear( ) was placed before the call to cin.ignore( ) to deal with corrupted input stream along with the stray characters or junk in it. (eg. user enters a float in place of character).
Last edited by ~s.o.s~ : Jan 2nd, 2007 at 2:47 pm.
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Happiness corrupts people.
Failing to value the lives of others cheapens your own.
Happiness corrupts people.
Failing to value the lives of others cheapens your own.
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Keep in mind, cin is the C++ version of C's scanf() and must be used appropriately. IMAO that means -- don't! 
Last edited by Ancient Dragon : Jan 2nd, 2007 at 3:10 pm.
Got a cough? Go home tonight and eat a whole box of Ex-Lax. Tomorrow, you'll be afraid to cough.
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Yes true, but if already used, why not make it a bit more er... robust...
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Happiness corrupts people.
Failing to value the lives of others cheapens your own.
Happiness corrupts people.
Failing to value the lives of others cheapens your own.
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Originally Posted by WaltP
Keep in mind, cin is the C++ version of C's scanf() and must be used appropriately. IMAO that means -- don't!
Don't you mean >> operator is like c's scanf() ? cin by itself is similar to c's stdin file pointer.
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Originally Posted by ~s.o.s~
Though it would be really better if cin.clear( ) was placed before the call to cin.ignore( ) to deal with corrupted input stream along with the stray characters or junk in it. (eg. user enters a float in place of character).
Well, in real code I would check the error state of cin and deal with legitimate problems long before trying to do cin.ignore(). But real code is harder to read and has lots of extras that hide the point I'm trying to make.
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Originally Posted by WaltP
Keep in mind, cin is the C++ version of C's scanf() and must be used appropriately. IMAO that means -- don't!
Not to be nitpicky, but cin is the C++ version of C's stdin. The overloaded >> operator is the C++ version of scanf.
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Originally Posted by Ravalon
Not to be nitpicky, but cin is the C++ version of C's stdin. The overloaded >> operator is the C++ version of scanf.
Yes, you guys are right. So I change my statement:
Keep in mind,
cin << is the C++ version of C's scanf() and must be used appropriately. IMAO that means -- don't! Got a cough? Go home tonight and eat a whole box of Ex-Lax. Tomorrow, you'll be afraid to cough.
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Originally Posted by WaltP
So don't be nitpicky :p
Isn't nitpicking supposed to be one of life's true joys or something?•
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Originally Posted by WaltP
Keep in mind,cin <<is the C++ version of C's scanf() and must be used appropriately. IMAO that means -- don't!
cin >>, you mean. It's hard to be humble when you're as gifted as I am at pretending to be an expert.
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