Data Integrity

[Random Noob]

I'm trying to figure out how to write a few functions to check a user's input before continuing the program. Specifically, I'm trying to use the isdigit fuction to check if an integer really had an integer inputted to it, and not a letter or character. Most of the examples I've seen have it checking a string, so should I make a string value equal to the integer as a way to check? I'm kind of at a loss here. What do you, the experts have to say?

[Galik]

Use the input streams flags:

float f;
if(std::cin >> f)
{
    // read was a success so f is valid
}
// otherwise clear the error flag and try again
std::cin.clear();

[Random Noob]

I'm sorry, but I can't seem to get that code working. What I'm trying to do is protect when a user enters, say "Y" instead of say, "19.99" in that code.

[Galik]

Did you include iostream?

#include <iostream> 

[Random Noob]

iostream in included. I'm just not understanding why that works. How does that test an integer to see if an integer value was entered?

[Fresh Grass]

Try with:

float input;
cin >> input;
if(!cin) cout << "Invalid input!";

EDIT: Also, I wouldn't use isdigit, because it has some problems with various symbols (˘°˛`˙´˝¨¸ etc). In my opinion you should use your own function:

bool isDigit(char sign) {
return sign >= '0' && sign <= '9'; // || sign == '.' || sign =='-'
}

[Random Noob]

The top one worked, so I think I'll be using that from now on to test numbers. I'm not sure about testing characters or strings yet, but there's a few more things I'll try before asking for help. Thank you.

[Galik]

Random Noob wrote:

I'm just not understanding why that works. How does that test an integer to see if an integer value was entered?

The >> operator is actually a function call to read in characters and convert them to whatever type you are asking for. In this case a float. If the input does not conform to a number, then a flag is set. So you are basically testing that flag when you use >>

[Random Noob]

@Galik

I don't know what was happening before, but suddenly now your code is working. I probably dropped a semi-colon somewhere. Now I just have to make this work in a while loop while asking for new input. Thanks for helping me understand.

[Random Noob]

Alright, the while loop seems to be working backwards. I wrote it like:

int main()
{

int test;
cout<< "Enter a number";
cin>> test;

while (std::cin >> test)
{
cout<< "Enter a number";
cin>> test;
}
std::cin.clear();

return (0);
}

The while loop still executes when a valid entry is entered, and you have to make 2 entries. The loop stops when an invalid entry in entered. I tried using while (!std::cin >> test) , and it's skipped when there is a valid entry, but an invalid entry causes a loop of text. Any ideas about that?

[Random Noob]

Okay, new question. Is it practical to have all entries be stings, test them with isdigit or isalpah and then convert them to the data types that I need them to be for calculations?

[Galik]

You need to loop while the input failed. Try this:

int main()
{

    int test;
    cout<< "Enter a number: ";

    while (!(std::cin >> test)) // notice the ! meaning "not"
    {
        cout<< "Bad input, try again: ";

        cin.clear(); // clear error

        // clear the input of the failed attempt
        string line;
        getline(cin, line);
    }

    return (0);
}

EDIT: fixed bug

[Random Noob]

Nope, sorry. Still got the wall of text. I've mostly given up on doing it that way. I can still do it with a goto statement, but I was hoping not to.

[Galik]

Sorry, my bad, the while() should be like this:

while(!(std::cin >> test))

[Random Noob]

See, that was my mistake too. I didn't know to put the std::cin in paranthesis. Now, it's close. For some reason I have to put in a valid entry (number) twice. I'm not entirely sure why. Everything else works. Sorry to rack your brain with this.

[Galik]

That code works fine for me. Have you put it in exactly the same?