Thread: Need help with the if statement

I made this calculator program here the beginning of it.

Code:

#include <iostream>
#include <string>

using namespace std;

int main() {

	float FirstNumber;
	char Sign;
	float SecondNumber;
	
	cout << ">";
	cin >> FirstNumber >> Sign >> SecondNumber;
	
	
	
	return(0);

}

I need to know if there a way to use the if statement so if FirstNumber or SecondNumber = a char then it will break or spit out a error message.....

in my head it looks like this...... Ya it doesn't work because if it did I wouldn't be posting this...

Code:

if ( ( FirstNumber == (char) || SecondNumber == ( char ) ) {

something....

}

Thanks....

What you want to check is whether the input succeeded. std::cin will go into a fail state if it didn't, and you can test for it.

Code:

if (cin >> FirstNumber >> Sign >> SecondNumber) {
   //input should be fine
}
else {
    //input must have failed somewhere
}

Otherwise a float simply can't contain characters (e.g you can't input 'N' into a float).

Originally Posted by anon

What you want to check is whether the input succeeded. std::cin will go into a fail state if it didn't, and you can test for it.

Code:

if (cin >> FirstNumber >> Sign >> SecondNumber) {
   //input should be fine
}
else {
    //input must have failed somewhere
}

Otherwise a float simply can't contain characters (e.g you can't input 'N' into a float).

You could of course do

Code:

float f = 'A';

which is identical to

Code:

float f = 65;

Some compilers MAY warn for it, but I'm pretty sure you don't get an error for it, even in C++.

However, reading in a float will fail, not set it to strange values.

--
Mats

Code:

std::string Input;
float f;
std::getline(std::cin, Input);
try
{
    f = boost::lexical_cast<float>(Input);
}
catch(boost::bad_lexical_cast)
{
    // Error; input not a float.
}

Rince and repeat. My favorite way.
Requires boost, though.