Decimal to Hexadecimal code

[NaNaNaB]

Hi! I have to write a code that converts a hexadecimal value to a decimal value..
So far I have

#include <iostream>
#include <cmath>
#include <string>

using namespace std;

int num;
char g;
int rem;


int main(){

cout << " input num: "; cin >> num;

while ( num > 0) {

rem = num%16;

num = num/16;


if (rem <= 9){

}

if ( rem > 9) {

g = rem + 55;


}
}
return 0;
}

I think i need to put the g and rem value into a string... which I'm not sure how to do since g will be a char value and rem will be a int value... and after I believe i need to then flip the numbers in the string.. oh it has to be in the format of 0000.. which I'm lost with.. :/

[iQChange]

#include <iostream>
#include <string>
#include <algorithm>
std::string dectobin(int num)
{
    std::string return_;
    bool is_negative = false;
    if(num < 0)
    {
        is_negative = true;
        num = -num;
    }
    while(num) //the algorithm
    {
        int rem = num % 16;
        num /= 16;
        return_.push_back(rem > 9? (rem - 10) + 'A' : rem + '0');
    }
    std::reverse(return_.begin(), return_.end());
    return is_negative? std::string("-")+ return_ : return_;
}
int main(int argc, char* argv [])
{
    std::cout << dectobin(15);
}

[Disch]

You have a critical misunderstanding here that is causing this problem to not make any sense.

A number is a number. A number cannot be hexadecimal, octal, decimal, binary, or whatever. It just is what it is.

What can change, is the textual representation of that number. IE: how we display that number though text.

Example:

seventeen
17
0x11
%00010001

These are all different ways to print the same number through text.


In C++, if you have an integer... then there is no text involved. So there is nothing to convert. If your int contains 15, then it contains 15. There's nothing more to it.


What matters here is how the number 15 gets printed to the user. Do you print it as "15"? Or as "0x0F"? Or as "fifteen"? Or what?

The other side of this coin is that the user is giving their input in the form of text as well. They can't input a number... they can only input text. So there's also a translation from text to number that cin is doing when it gets the input from the user.


So there's really 2 conversions here:

1) Converting the user's input (text) to a number (int)
2) Converting the resulting number (int) back to text

The number itself never changes. The only thing you're doing is converting to/from text in different ways.



The first conversion is happening here:

int num;
cin >> num;  // <- this is already wrong 

This is wrong because, by default, cin is going to assume the user is inputting a number in decimal and will convert accordingly.

Fortunately, cin knows all about hexadecimal text... so to tell it the user is inputting something in hex, you just have to give it a qualifier:

#include <iomanip> // add this to your includes
//..

int num;
cin >> hex >> num;  // get the number, but convert the text as if it were in hex 

So really... this problem is very simple:

#include <iostream>
#include <iomanip>

int main()
{
    cout << "Please insert a number in hexadecimal:  ";

    int num;
    cin >> hex >> num;

    cout << "Your number in decimal is:  " << num << endl;

   return 0;
}

That's it.




Now if this is a school assignment... then you probably can't use the hex qualifier like that, since that'd be too easy.

So instead... you'll have to convert the text to a number yourself.

This means, you cannot use cin to do the conversion. Which means you must read a string from cin to get the input:

string input;
cin >> input;

// ... then examine characters from the string one at a time
//  so you can convert the number they typed in 

[iQChange]

@Disch Look at the title and then look at the text. Informations are not equal! Dec to Hex or Hex to Dec?

[Disch]

Whoops!

You're right. I had it backwards.

Well what I said still mostly applies, I just have it backwards. =P

The problem is still pretty simple:

#include <iostream>
#include <iomanip>

int main()
{
    cout << "Please insert a number in decimal:  ";

    int num;
    cin >> num; // <- this is no longer wrong, because converting from dec text is what
        // you want

    cout << "Your number in hexadecimal is:  " << hex << num << endl; // <- print as hex

   return 0;
}

[iQChange]

I knew it :P But I think he needs it for some assignment.