I have this code:
double i;
while(cin >> i)
{
if ( i < 0.0)
cout << "ENTER A POSITIVE NUMBER: ";
else
break;
}
I want code like this ( I don't want to use break):
while((cin >> i) < 0.0)
{
cout << "ENTER A POSITIVE NUMBER: ";
}
I get an error on this line: while((cin >> i) < 0.0) saying invalid operands to binary expression.
What am I missing?
Use it like this.
while ((cin >> i) && (i < 0.0))
The overloaded function for cin returns an object by reference of istream class. So you cannot compare it to a double value.
cin >> i
|-------| //this is an object of istream class
&& is a sequence point (old C++03 parlance) and thus it is guaranteed that (i<0.0) will only be evaluated after cin >> i.
The expression (cin >> i) does not return a double.
You can write the same statement, without a break, as:
double i;
while ((cin >> i) && (i < 0.0)) {
cout << "ENTER A POSITIVE NUMBER: ";
}
-1 as the code is correct. You might want to revisit the explanation, as cin >> i does not yield a boolean value, but a reference to the stream (which can be then converted to a bool)you want to check the value of i, not the return of cin
while((cin >> i) && ( i < 0.0))
{
cout << "ENTER A POSITIVE NUMBER: ";
}
The return value of cin >> i is the stream, not the read value. This is to allow chaining of operands
cin >> i >> j;
You could try this:
while( (cin >> i, i) < 0. )
{
cout << "ENTER A POSITIVE NUMBER: ";
}
The comma operator should return the value of i but I haven't tested it.
EDIT: Don't use this approach, as David Rodríguez has noted this discards the result of the read. Use while( (cin >>i) && (i<0.) ) instead.
i is negative and the user closes the input stream (or types a letter which fails to parse as a double and sets the fail bit in the stream), then the subsequent reads will fail, and the value of i will not be updated being still positive in an infinite loop.Do:
while( cin >> i && i < 0.0 )
{
cout << "ENTER A POSITIVE NUMBER: ";
}
The error is caused due to the fact that the expression (cin >> i) is not valid for comparison with a double.
operator,; are you guaranteed that cin >> i is run before the i < 0.0 when the thing on the left of the comma is an object?Alex. The comma operator discards the first expression. If the input is -1 a you run into an infinite loop. In the first iteration i is set to -1, after that the read fails, i is unmodified and you enter an infinite loop.