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:

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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).

Quote:

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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:

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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.