How to prevent a letter insert in a int var ?

[YUS]

Hi,
I've got a problem.

I have a program that ask for a number to the user.
If the user put a letter instead of a number the program crashes (loop).

How can I prevent this ?
I've tried with isdigit but with no results.

int choise;

cout << "\nChoose one: ";
cin >> setw(sizeof(choice)) >> choise;
if(isdigit(choice)) cout <<"all ok";
if(isalpha(choice)) cout<<"error - number needed";

but the code doesn't work.
=(

Thanks to all.

 

[Alf P. Steinbach]

* YUS:

I've got a problem.

I have a program that ask for a number to the user.
If the user put a letter instead of a number the program crashes (loop).

How can I prevent this ?
I've tried with isdigit but with no results.

int choise;

cout << "\nChoose one: ";
cin >> setw(sizeof(choice)) >> choise;
if(isdigit(choice)) cout <<"all ok";
if(isalpha(choice)) cout<<"error - number needed";

but the code doesn't work.

Consider that attempted conversion to 'int' has already happened by the
time you call 'isdigit' and 'isalpha'.

Instead check the error state of the stream.

 

[YUS]

Consider that attempted conversion to 'int' has already happened by
the time you call 'isdigit' and 'isalpha'.

Instead check the error state of the stream.

Sorry but.. how ???

I really got no idea!

 

[Gavin Deane]

Sorry but.. how ???

I really got no idea!

http://www.parashift.com/c++-faq-lite/input-output.html#faq-15.2

Gavin Deane

 

[Howard]

Hi,
I've got a problem.

I have a program that ask for a number to the user.
If the user put a letter instead of a number the program crashes (loop).

How can I prevent this ?
I've tried with isdigit but with no results.

int choise;

cout << "\nChoose one: ";
cin >> setw(sizeof(choice)) >> choise;
if(isdigit(choice)) cout <<"all ok";
if(isalpha(choice)) cout<<"error - number needed";

but the code doesn't work.
=(

A common solution is to always enter data into a std::string, and then see
if the string can be converted into an integer (or double or whatever). If
it cannot be converted, then tell the user there's an error and loop back to
allow them to re-enter the data.

-Howard

 

[Mike Wahler]

Hi,
I've got a problem.

I have a program that ask for a number to the user.
If the user put a letter instead of a number the program crashes (loop).

How can I prevent this ?
I've tried with isdigit but with no results.

int choise;

cout << "\nChoose one: ";
cin >> setw(sizeof(choice)) >> choise;
if(isdigit(choice)) cout <<"all ok";
if(isalpha(choice)) cout<<"error - number needed";

but the code doesn't work.

Look again at the documentation for 'isdigit()'.
It doesn't do what you apparently believe.

=(

#include <istream>
#include <iostream>
#include <sstream>
#include <string>

int get_int()
{
int result(0);
std::string text;
std::istringstream iss;;

do
{
std::cout << "Enter an integer value: ";
std::cin >> text;
iss.clear();
iss.str(text);

if(!(iss >> result))
std::cerr << " * Not a valid integer value\n";

} while(!iss);

return result;
}

int main()
{
int i(0);

i = get_int();
std::cout << "You entered: " << i << '\n';
return 0;
}

-Mike

 

[YUS]

Thanks to all.

I repeat, thanks to all guys =)
Very useful.