How to forbid the inclusion of non-numbers? - c++

Question

I have this program:
#include <iostream>
#include <string>
using namespace std;
int main()
{
int i=0;
float reader, tot = 0;
while(i!=10)
{
cout << "Insert a number: " << endl;
cin >> reader;
if(cin)
{
tot += reader;
i++;
}
else
{
cout << "Error. Please retry." << endl;
cin.clear();
}
}
cout << "Media: " << tot/i << endl;
return 0;
}
In the IF() i want the user to insert ONLY FLOAT VALUES in "reader" variable.
I want that if the user inserts a number, the program continues... else the program should re-ask to the user to insert a correct value.
How to do this checking the INPUT? I tried with a TRY CATCH but it didn't work.
Thanks in advance!

Answer 1

"How to do this checking the INPUT?"
It's already ensured by
cin >> reader;
that only valid float values can be entered by the user. The way to check for validity is
if( cin >> reader ) {
tot += reader;
i++;
}
else {
cout << "Error. Please retry." << endl;
cin.clear();
std::string dummy;
cin >> dummy; // <<< Read invalid stuff up to next delimiter
}
Here's the fully working sample.
"I tried with a TRY CATCH but it didn't work."
To get exceptions from std::istream operations, set the std::istream::exceptions mask.

Answer 2

Just check the result of operator>>:
if (cin >> reader) {
// user gave you a float
tot += reader;
i++;
}
else {
cout << "Error. Please retry." << endl;
// clear the error flag on cin, and skip
// to the next newline.
cin.clear();
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}

Answer 3

Your original program was fine except you need to skip over that bad input when you get an error. Simply clearing the error is not enough:
#include <iostream>
#include <string>
#include <limits> // include this!!!
using namespace std;
int main()
{
int i=0;
float reader, tot = 0.0f; // tot needs to be float
while(i!=10)
{
cout << "Insert a number: " << endl;
if( cin >> reader )
{
tot += reader;
i++;
}
else
{
cout << "Error. Please retry." << endl;
cin.clear();
// Then you ned to skip past the bad input so you
// don't keep tripping the same error
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
}
cout << "Media: " << tot/i << endl;
return 0;
}
The function cin.ignore() ignores as many input characters as possibly up until the end-of-line character '\n'. The std::numeric_limits<std::streamsize>::max() function tells us the maximum number possible characters that can be stored in an input buffer.
If it is confusing an alternate way to skip the bad input would be to simply read up to the next line into a std::string.
else
{
cout << "Error. Please retry." << endl;
cin.clear();
// Then you need to skip past the bad input so you
// don't keep tripping the same error
std::string skip;
std::getline(cin, skip); // read the bad input into a string
}