CODEWITHSUNDEEP
c++while-loop
Snerd
Snerd

Reputation: 1553

negative value input in while loop c++

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?

Upvotes: 4

Views: 4879

Answers (5)

NominSim
NominSim

Reputation: 8511

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.

Upvotes: 0

Alex
Alex

Reputation: 7838

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.

Upvotes: 2

Aamir
Aamir

Reputation: 5440

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: "; 
}

Upvotes: 4

RvdK
RvdK

Reputation: 19790

you want to check the value of i, not the return of cin

while((cin >> i) && ( i < 0.0))
{
    cout << "ENTER A POSITIVE NUMBER: ";
}

Upvotes: 4

Coding Mash
Coding Mash

Reputation: 3346

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

Upvotes: 10

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