check if input is valid

[jsmith613]

I have been trying to make a very simply programme that checks if the inputted information is an integer or not (i.e: that it contains no other characters).

I have tried using the isdigit function (but this only works for single characters)
i have tried cin.clear, cin.ignore (1000) but this doesn't work either..

can someone please suggest an effective way to check if x in the following programme has been entered correctly

#include <iostream>
using namespace std;

int main()
{
cout << "Please enter an integer (positive or negative)" << endl;
int x;
cin >> x;
HERE I WOULD LIKE CODE TO CHECK IF THE USERS INPUT IS VALID
}

thanks

[coder777]

Use the stream function good():

http://www.cplusplus.com/reference/ios/ios/good/

cin >> x;
if(cin.good())
  alright();
else
  wrong();

[j3Rz8vqX]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

#include <iostream> using std::cout; using std::cin; int main() { int choice; bool error;//Our error flag do { error = false;//Flag false, safe cout<<"Enter a integer value: "; cin>>choice; if(cin.fail()) { error = true;//Flag true, error has occurred cout<<"Error, invalid entry.\n\n"; } cin.clear(); //Clears any error flags cin.ignore(1000,'\n'); //Ignores 1000 characters, or until '\n' }while(error); cout<<"\nThe integer value entered was: "<<choice<<'\n'; cout<<"Press <enter> to exit console: "; cin.get(); return 0; }

Enter a integer value: d Error, invalid entry. Enter a integer value: s Error, invalid entry. Enter a integer value: a Error, invalid entry. Enter a integer value: 50 The integer value entered was: 50 Press <enter> to exit console:

http://www.cplusplus.com/reference/ios/ios/clear/

[jsmith613]

thanks. but the code seems to say "88j9" is "88"...."88j9" is clearly wrong though!!
how do I get around this?
thanks

[Chervil]

"88j9" is clearly wrong

Get the input as a string and either test each individual character is a digit, then convert the string to an int.

Or maybe something like this

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

using std::cout;
using std::cin;
using std::endl;

int main()
{
    int choice;

    while (true)
    {
        cout << "Please enter an integer: ";
        std::string input, test;        
        cin >> input;
        std::istringstream ss(input);
        bool goodint = (ss >> choice);
        bool goodstr = (ss >> test);
        if (goodint && !goodstr)
            break;
        cout << "Invalid input: " << input << endl;
    }
        
    cout << "The integer was: " << choice << endl;

    return 0;
}

Demo: http://cpp.sh/8kb

[jsmith613]

amazing
thanks