how do I validate user input as a double in C++?
How would I check if the input is really a double?
double x;
while (1) {
cout << '>';
if (cin >> x) {
// valid number
break;开发者_StackOverflow
} else {
// not a valid number
cout << "Invalid Input! Please input a numerical value." << endl;
}
}
//do other stuff...
The above code infinitely outputs the Invalid Input!
statement, so its not prompting for another input. I want to prompt for the input, check if it is legitimate... if its a double, go on... if it is NOT a double, prompt again.
Any ideas?
Try this:
while (1) {
if (cin >> x) {
// valid number
break;
} else {
// not a valid number
cout << "Invalid Input! Please input a numerical value." << endl;
cin.clear();
while (cin.get() != '\n') ; // empty loop
}
}
This basically clears the error state, then reads and discards everything that was entered on the previous line.
failbit
will be set after using an extraction operator if there was a parse error, there are a couple simple test functions good
and fail
you can check. They are exactly the opposite of each other because they handle eofbit
differently, but that's not an issue in this example.
Then, you have to clear failbit
before trying again.
As casablanca says, you also have to discard the non-numeric data still left in the input buffer.
So:
double x;
while (1) {
cout << '>';
cin >> x;
if (cin.good())
// valid number
break;
} else {
// not a valid number
cout << "Invalid Input! Please input a numerical value." << endl;
cin.clear();
cin.ignore(100000, '\n');
}
}
//do other stuff...
I would use:
double x;
while (!(std::cin >> x)) {
std::cin.clear();
std::cin.ignore(2147483647, '\n');
std::cout << "Error.\n";
}
or
double x;
while ((std::cout << "> ") && !(std::cin >> x)) {
std::cin.clear();
std::cin.ignore(2147483647, '\n');
std::cout << "Error.\n";
}
#include <iostream>
#include <string>
bool askForDouble(char const *question, double &ret)
{
using namespace std;
while(true)
{
cout << question << flush;
cin >> ret;
if(cin.good())
{
return true;
}
if(cin.eof())
{
return false;
}
// (cin.fail() || cin.bad()) is true here
cin.clear(); // clear state flags
string dummy;
cin >> dummy; // discard a word
}
}
int main()
{
double x;
if(askForDouble("Give me a floating point number! ",x))
{
std::cout << "The double of it is: " << (x*2) << std::endl;
} else
{
std::cerr << "END OF INPUT" << std::endl;
}
return 0;
}
bool is_double(double val)
{
bool answer;
double chk;
int double_equl = 0;
double strdouble = 0.0;
strdouble = val;
double_equl = (int)val;
chk = double_equl / strdouble;
if (chk == 1.00)
{
answer = false; // val is integer
return answer;
} else {
answer = true; // val is double
return answer;
}
}
One way is to check for floating number equality.
double x;
while (1) {
cout << '>';
cin >> x;
if (x != int(x)) {
// valid number
break;
} else {
// not a valid number
cout << "Invalid Input! Please input a numerical value." << endl;
}
}
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