How can I check if the user has entered valid value? - c++

Question

I'm writing a project and I have to check if the user has entered a valid value for specific things. For example, first case is to check whether he inputted an integer, which is no longer than 6 length and >0. I am working with struct, so my code is this:
#include <iostream>
using namespace std;
int dolzina_int(int vlez);
int main()
{
struct pole{
int sifra; // sifra na artiklot
string opis; // opis na artiklot
float cena; // edinecna cena
int vlez_kol; // vlezna kolicina
int izlez_kol; // izlezna kolicina
float dan_stapka; // danocna stapka
float iznos; // iznos
int datum; // datum na vlez i izlez (GGMMDD)
}artikli[100]; // maksimalen broj na artikli e 100
for (int i = 0; i < 5; i++){
cin >> artikli[i].sifra;
while(!(cin >> artikli[i].sifra) ||
(artikli[i].sifra < 0 || (dolzina_int(artikli[i].sifra) > 6)))
{
cout << "Error" << endl;
cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
}
return 0;
}
// funkcija za dolzina na integer
int dolzina_int(int vlez){
int dolzina = 1;
while(vlez > 0){
dolzina++;
vlez /= 10;
}
return dolzina;
}
So, my code seems to work, the first part, it does check if it's >6 or <0, but the second else if, it doesn't work properly to check if it's an integer or not. So my question is the following, how can i make sure that it's an integer value that the user enters and how can I make sure the length of integer is not >6 or the value is <0 and meanwhile, if it is, to make him re-enter it, and also check the newly entered value, as my code doesn't do that currently.
for (int i = 0; i < 5; i++){
artikli[i].sifra = 1;
do{
cout << "Enter a code: ";
while(!(cin >> artikli[i].sifra)){
cin.clear();
cin.ignore();
cout << "Invalid entry. Enter a new value: ";
}
}while(artikli[i].sifra > 0 && dolzina_int(artikli[i].sifra) > 6);
}

Answer 1

You should note that the extraction will only succeed if the contents read from the file can be used as a correct value of the type of the object (in this case the integer sifra). If it doesn't work, ios_base::failbit will be set in the stream state.
So what needs to be done is that when an incorrect value is given, we discard the wrong text, clear the stream state, and re-try the extaction. For example:
#include <iostream>
#include <string>
#include <limits>
int main()
{
for (int i = 0; i < 5; ++i)
{
while (!(std::cin >> artikli[i].sifra) ||
((artikli[i].sifra < 0) || (artikli[i].sifra > 6)))
{
std::cout << "Entered an incorrect value, try again.\n";
std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
// ...
}
}

Answer 2

First you should check this out:
http://www.cplusplus.com/reference/cctype/
Second, I think your over complicating it...
1) Read in int
2) check value of int
3) repeat
So I would do this
for (i = 0 to i < 5 add one to i)
while (entered value is not digit)
get digit until value is valid
Something like this:
for(i = 0; i < 5; i++) {
cout << "Enter the code" << endl;
cin >> artikli[i].sifra;
while(!is_digit) {
cout << "Invalid, please repeat" << endl;
cin >> artikli[i].sifra;
}
}

Answer 3

Assuming you want to restrict your input to integers and have them in a specific [min, max] range, you would do it like this:
int input = -1;
do
{
std::cout << "Enter a value in the range [min, max]: ";
while (!(std::cin >> input)) // while the input is invalid
{
std::cin.clear(); // clear the fail bit
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // ignore the invalid entry
std::cout << "Invalid Entry! Please Enter a valid value: ";
}
} while (min > input || max < input);
Where min and max are whatever values you are looking to keep the value between.