Hex to Dec in C

[gingit Dorot]

I am querying a device through an I2C buf for temp, I receive a hex value and I need to convert it to a decimal number to pass on to another device.
I need to convert for instance 0x21 to 33. I am using a char for both.

[David H]

Look up strtol

long int dec = strtol("0x21", NULL, 16);

[whodgson]

This may be one way:

int ch,hexnum;
cout<<"Converts a hex (base 16)format to a decimal format\n\n";
cout<<"How many hex characters ";
cin>>ch;
cout<<"Enter the number (w/- spaces) ";
 
for(int i=0;i<ch;i++)
{
cin>>hexnum; 
a[i]=hexnum;          
}
int x=0;
for(int i= 0;i<ch;i++)
{
x=16*x + a[i];
if (i>ch)break;}
cout<<"The dec value of hex ";
for(int i=0;i<ch;i++)
if(a[i]==15)cout<<'F';
else if(a[i]==14)cout<<'E';
else if(a[i]==13)cout<<'D';
else if(a[i]==12)cout<<'C';
else if(a[i]==11)cout<<'B';
else if(a[i]==10)cout<<'A';
else cout<<a[i];
cout<<" is "<<x;
cout<<endl;

[Banfa]

You seem to have done it the long way round whodgson for C++ try this

#include <iostream>
#include <sstream>
#include <string>
#include <iomanip>
#include <exception>

using namespace std;

int main()
{
	string input = "5b"; // 91

	istringstream iss(input);
	
	int output;
	
	iss >> hex >> output;
	
	cout << input << " <=> " << output << endl;
	
	return 0;
}

not to mention that the request was for C and this is clearly C++ :D

I personally would use David H solution.

[whodgson]

...yes Banfa ...agree entirely with you... one of my more pathetic solutions especially as it was not even C. The David H method is brief and simple but haven`t seen that NULL form before. However I assume alas it is also C++. Will study your stream method more closely in due course. Thanks.

[donbock]

The OP needs to state the problem more precisely.

When you say "I receive a hex value", do you mean:
> you receive a binary value;
> you receive a hex-ASCII value;
> something else?

When you say "I need to convert it to a decimal number", do you mean:
> you need to create a decimal-ASCII value;
> you need to create a binary value;
> something else?

It would help to know how large a number is represented by the "hex value" (1 byte, 2 bytes, etc). If it is more than one byte then that begs the question of endianness.

Note: when I refer to "hex-ASCII" or "decimal-ASCII", I mean a sequence of characters that encode the value in either hexadecimal or ascii digits. For example, the 1-byte value 0x1A can be encoded as either "1A" or "26". Both of those encodings are 2 bytes long.