C++ switch statement odd output - c++

Question

I'm working on a final assignment for an Intro to C++ course. What I've coded so far works, but it's producing some interesting output that I'm looking for clarification on. Here's my code:
(Caveat: Yes, I know using void main() sucks, but we're using Visual Studio in class, and this is the instructors preference.)
#include <string>
#include <iostream>
using namespace std;
void conversion(int);
void main()
{
int decimal_number, answer;
cout << "Please enter a whole decimal number (e.g. 20): ";
cin >> decimal_number;
if (decimal_number == 0)
{
answer = 0;
cout << "The hexadecimal value of your number is: " << answer;
getchar();
getchar();
}
else if (decimal_number < 0)
{
cout << "INVALID ENTRY" ;
getchar();
getchar();
}
else if (decimal_number > 0)
{
conversion(decimal_number);
}
getchar();
getchar();
}
void conversion (int decimal_number)
{
int count = 0, remainder, reverse_order;
char hexadecimal_number[10] = { NULL };
while (decimal_number != 0)
{
remainder = decimal_number % 16;
switch (remainder)
{
case 0:
hexadecimal_number[count] = '0';
count++;
break;
case 1:
hexadecimal_number[count] = '1';
count++;
break;
case 2:
hexadecimal_number[count] = '2';
count++;
break;
case 3:
hexadecimal_number[count] = '3';
count++;
break;
case 4:
hexadecimal_number[count] = '4';
count++;
break;
case 5:
hexadecimal_number[count] = '5';
count++;
break;
case 6:
hexadecimal_number[count] = '6';
count++;
break;
case 7:
hexadecimal_number[count] = '7';
count++;
break;
case 8:
hexadecimal_number[count] = '8';
count++;
break;
case 9:
hexadecimal_number[count] = '9';
count++;
break;
case 10:
hexadecimal_number[count] = 'A';
count++;
break;
case 11:
hexadecimal_number[count] = 'B';
count++;
break;
case 12:
hexadecimal_number[count] = 'C';
count++;
break;
case 13:
hexadecimal_number[count] = 'D';
count++;
break;
case 14:
hexadecimal_number[count] = 'E';
count++;
break;
case 15:
hexadecimal_number[count] = 'F';
count++;
break;
default:
cout << decimal_number << "+++ " << hexadecimal_number;
cout << "INVALID ENTRY";
getchar();
getchar();
}
decimal_number = decimal_number / 16;
}
cout << "The hexadecimal value of your number is: ";
for (reverse_order = count -1; reverse_order >= 0; reverse_order--)
{
cout << hexadecimal_number[reverse_order];
}
getchar();
getchar();
}
So, like I said: my code works. I can take any number input as a decimal, and convert it to its hexadecimal equivalent. However, I've found that I've had to include an IF statement within the main function of the code, because if the user inputs anything other than a decimal number into the decimal_number variable, the program will store a string of decimal numbers, into decimal_number, and I have no idea where those numbers come from. They don't appear to be the ASCII equivalents of anything.
... If none of this makes any sense, I'm sorry. just input cout << decimal_number after the line cin >> decimal_number, then run the code and see what weird number comes out. I hope that makes things clearer.
Anyway, my instructors stumped, and I'm stumped. I've got the above workaround in place that the instructor will accept, but for my own sanity, I just want to figure out what's going on. Any help or pointers is appreciated. Cheers!

Answer 1

You can test whether the result of cin >> decimal_number succeeded, like
if(!(cin>>decimal_number))
throw std::runtime_error("Oops, not a decimal number!");
This is a bit too extreme, you can also validate the input:
while(!(cin>>decimal_number))
{
std::cout << "Not decimal, input again ";
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
}
If you're not performing this kind of validation, then you leave the stream in an invalid state whenever reading a non-decimal, and the variable you think you're reading into will end up un-initialized.

Answer 2

However, I've found that I've had to include an IF statement within the main function of the code, because if the user inputs anything other than a decimal number into the decimal_number variable, the program will store a string of decimal numbers, into decimal_number, and I have no idea where those numbers come from.
Well, you did not initialise decimal_number to anything, and you do not have any error checking around the cin >> decimal_number call. So I'm not sure what else you expected but an unspecified value for decimal_number!
Your instructor should know this. It's worrying that, on top of teaching you to write code that is ill-formed per the International Standard (void main!!), they failed to discover this problem or note that you have no error checking.

Answer 3

What's the point of the assignment?
The conversion can be simplified to:
cout << hex << decimal_value << endl;
Or if you need it in a string:
std::string convert_decimal_to_hex_string(int decimal_value)
{
std::ostringstream output;
output << hex << value;
return output.str();
}
I believe the class should show you how to use std::string and existing language features (such as the hex manipulator). Using char for a string is dangerous.
Also, since you don't know the size or limit of the decimal values, you will need to dynamically allocate (i.e. during run-time) the array holding the characters. Think about allocating 2 characters and entering the value 1024; buffer overflow.
Change your program to use std::string. Refrain from character (C-Style) arrays.