C++ Excluding non-integer user inputs - c++

Question

I am making a number-guessing game where the user is asked to input a four-digit number. It is possible, however, that the user inputs less or more than four digits and/or a non-integer input (i.e. invalid input). My code stores the user input into an integer-type array. I just realized now that my code will still recognize "invalid inputs" as valid since the array where the input is being stored is declared as an integer-type. Below is a portion of my code:
#include <iostream>
using namespace std;
void guess(int num_guess[], int size);
int main(){
int list[4];
guess(list, 4);
for(int i = 0; i < 4; i++){
cout << list[i];
}
cout << endl;
}
void guess(int num_guess[], int size){
int number;
cin >> number;
for(int i = size-1; i >= 0; i--){
num_guess[i] = number%10;
number /= 10;
}
}
cout << list[i]; isn't really part of the original code, but this was how I found out that invalid inputs are still accepted. I encountered a similar problem before when I was making a rational roots calculator program in Python, but it was much easier then to detect and exclude unwanted inputs. My question is, how do I fix my code so that it can detect invalid inputs and output something like "Invalid input" and then proceed to ask the user for another input.

Answer 1

The following is a function to check if a string is a 4 digit positive integer. If the number could be negative, you just need to check if the s[0] == '-'.
bool check(string &s){
if(s.size() != 4) return false;
for(int i=0; i < 4; i++){
if(s[i] < '0' || s[i] > '9') return false;
}
return true;
}
The following is a function to convert a string to an int:
#include <stringstream>
int strToInt(string &s){
stringstream ss(s);
int ans;
ss >> ans;
return ans;
}

Answer 2

To exclude non integer inputs try the following:
void skip_to_int(){
// if is not an integer
if(cin.fail()){
// check character type
cin.clear();
char ch;
while(cin>>ch){
// throw away non digits
if(isdigit(ch)){
// put back if digit to be written
cin.unget();
return;}
}
}
else error ("no input");
}
And your input prompt function will look like this:
cout << "Please enter an integer" << endl;
int n=0;
if(cin>>n){
// integer OK, proceed
}
else{
cout << "That was not a numeric value, try again." << endl;
skip_to_int();}

Answer 3

Here's my solution. Beware, it uses C++11. Certainly not necessary if you use std::stringstream, but this should work pretty well.
I presume you don't want negative numbers. I also presume that any number of 0's in front doesn't make the number a 4-digit number. It will cut off padded 0's, so 01234 is a 4 digit number, but 0123 isn't.
void guess(int num_guess[], int size)
{
int number;
// if the length of the number isn't 4, try again until it is
do {
std::cin >> number;
if(std::to_string(number).length() != size)
std::cout << "You messed up the input. How hard could it be? Try again..." << std::endl;
} while(std::to_string(number).length() != size);
// by now, the size is definitely 4. insert it by digit into num_guess
for(int i = size-1; i >= 0; i++) {
num_guess[i] = number%10;
number /= 10;
}
}

Answer 4

#include <iostream>
#include <limits>
int main() {
int i = 0;
std::cout << "Please enter a number with four digits: ";
while( !(std::cin >> i) || !(i / 1000.0f >= 1.0f) )
{
std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
std::cout << "Invalid entry." << std::endl;
std::cout << "Please enter a number with four digits: ";
}
}
the std::cin.clear() clears all errors flags on current stream structure and std::cin.ignore() cleans up the input stream itself. Once we don't know the size of stream 'til this operation I have used the maximum possible value of a stream size to make sure any stream length could be cleaned.

Answer 5

add #include "math.h"
and change guess
void guess(int num_guess[], int size){
int number = 0;
bool firstTime = true;
do
{
if (!firstTime)
cout << " Error, try again " << endl;
firstTime = false;
cin >> number;
} while (number<pow(10, size-1) || number>=pow(10, size));
for(int i = size-1; i >= 0; i--){
num_guess[i] = number%10;
number /= 10;
}
}