Hex character to int in C++

Question

How can I change a hex character, not string, into a numerical value?

While typing this question, I found many answers on how to convert hex strings to values. However, none work for chars. I remember reading somewhere that this works for strings:

std::string mystr = "12345";
unsigned int myval;
std::stringstream(mystr) >> std::hex >> myval;

However, if I do mystr[x] in a loop, this code will not work. I have tried adding a new line with std::string temp = mystr[x] and changing std::stringstream(mystr) to std::stringstream(temp), but that's not working either.

So how should I do this? Currently, I'm searching through a string of the hex chars ("0123456789abcdef".find(mystr[x]);) and using the index for the value. However, since it searches, it's slow, even if it's only searching through 16 characters.

http://ideone.com/dIyD4

Its hard to understand what you have tried. Why not post all the variants you have tried and explain for each why they are not working. — Martin York, Jun 23 '11 at 16:36
possible duplicate of [C++ convert hex string to signed integer](http://stackoverflow.com/questions/1070497/c-convert-hex-string-to-signed-integer) — quetzalcoatl, Jul 02 '13 at 22:19

score 11 · Answer 1 · answered Jun 23 '11 at 16:37

11

int intval = (hexchar >= 'A') ? (hexchar - 'A' + 10) : (hexchar - '0');

answered Jun 23 '11 at 16:37

levis501

4,117
23
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Is there a version of your code that works with hex chars of different cases? (eg 'a', 'A') – quamrana Jun 23 '11 at 16:50
For that just convert hexchar to uppercase letter before using that expression. – Ajay Jun 23 '11 at 17:01
@quamrana: I've added lower case support in my answer. – Nick Westgate Feb 13 '17 at 23:50

Nick Westgate · Answer 2 · 2017-02-13T23:58:41.997

The ternary from levis501 can be expanded to:

int v = (c >= 'A') ? (c >= 'a') ? (c - 'a' + 10) : (c - 'A' + 10) : (c - '0');

But if you want error checking it gets a bit messy:

int v = (c < '0')  ? -1 :
        (c <= '9') ? (c - '0') :
        (c < 'A')  ? v = -1 :
        (c <= 'F') ? (c - 'A' + 10) :
        (c < 'a')  ? v = -1 :
        (c <= 'f') ? (c - 'a' + 10) : -1;

It doesn't look much better in if-else blocks:

int v = -1;
if ((c >= '0') && (c <= '9'))
    v = (c - '0');
else if ((c >= 'A') && (c <= 'F'))
    v = (c - 'A' + 10);
else if ((c >= 'a') && (c <= 'f'))
    v = (c - 'a' + 10);

Since I wanted a fast implementation and validation, I went for a lookup table:

int ASCIIHexToInt[] =
{
    // ASCII
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
     0,  1,  2,  3,  4,  5,  6,  7,  8,  9, -1, -1, -1, -1, -1, -1,
    -1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,

    // 0x80-FF (Omit this if you don't need to check for non-ASCII)
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
};

In this case it's just:

int v = ASCIIHexToInt[c];
if (v < 0)
    // Invalid input

Runnable examples are (hopefully) here and here.

An universal digit-to-value routine up to base 36 (/[0-9A-Za-z]+/) can be expressed as follows: `byte digit(signed char c) { if (c<='9') return c-'0'; c&=~0x20; // change small to capital letters if (c<'A') return 0xFF; return c-'A'+10; ` All return values equal or greater than desired base are then treated as invalid characters. — Henrik Haftmann, Dec 22 '22 at 08:21

score 3 · Accepted Answer · answered Jun 23 '11 at 16:43

3

You already have a solution that works with strings. Use it for chars too:

#include <string>
#include <sstream>
#include <iostream>
#include <iomanip>
using namespace std;

int main()
{
  char val = 'A';
  unsigned int myval;
  std::stringstream ss;
  ss << val;
  ss >> std::hex >> myval;
  cout << myval << endl;
}

Code

answered Jun 23 '11 at 16:43

Bill

14,257
4
43
55

A good way but cannot throw an error if the input character is outside of the valid range. In such cases, `stringstream` always returns `0`. – dizcza Jan 11 '23 at 15:51

score -1 · Answer 4 · answered Jun 23 '11 at 16:42

Something like the following?

//!         \return
//!             The numeric value of ch, if it is hex, otherwise -1
int
fromHexChar( char ch )
{
    static char const hexChars[] = "0123456789ABCDEF";
    char const* p = std::find( begin( hexChars ), end( hexChars ),
                               ::tolower( (unsigned char)ch ) );
    return p == end( hexChars )
        ? -1
        : p - begin( hexChars );
}

Hex character to int in C++

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