Convert a hexadecimal string to an integer efficiently in C

Question

In C  what is the most efficient way to convert a string of hex digits into a binary unsigned int or unsigned long   For example  if I have 0xFFFFFFFE  I want an int with the base10 value 4294967294

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Hexadecimal to decimal  Don t run it on online compilers  because it won t work    include lt stdio h gt  void main         unsigned int i      scanf   x   amp i       printf   d  i

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Why is a code solution that works   getting voted down  Sure  it s ugly         Perhaps because as well as being ugly it isn t educational and doesn t work   Also  I suspect that like me  most people don t have the power to edit at present  and judging by the rank needed - never will    The use of an array can be good for efficiency  but that s not mentioned in this code  It also takes no account of upper and lower case so it does not work for the example supplied in the question  FFFFFFFE

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If you don t have the stdlib then you have to do it manually   unsigned long hex2int char  a  unsigned int len        int i      unsigned long val   0       for i 0 i lt len i           if a i   lt   57          val     a i -48   1 lt  lt  4  len-1-i            else         val     a i -55   1 lt  lt  4  len-1-i          return val      Note  This code assumes uppercase A-F   It does not work if len is beyond your longest integer 32 or 64bits  and there is no error trapping for illegal hex characters

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Try this to Convert from Decimal to Hex       include lt stdio h gt       include lt conio h gt       int main void              int count 0 digit n i 0        int hex 5         clrscr          printf  enter a number             scanf   d   amp n          if n lt 10                    printf   d  n                  switch n                    case 10                printf  A                break            case 11                printf  B                break            case 12                printf  B                break            case 13                printf  C                break            case 14                printf  D                break            case 15                printf  E                break            case 16                printf  F                break            default                    while n gt 16                     digit n 16            hex i  digit            i              count              n n 16                   hex i  n          for i count i gt  0 i--                     switch hex i                            case 10                   printf  A                   break               case 11                   printf  B                   break               case 12                   printf  C                   break               case  13                   printf  D                   break               case 14                   printf  E                   break               case 15                   printf  F                   break               default                   printf   d  hex i                           getch         return 0

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In C you can convert a hexadecimal number to decimal in many ways  One way is to cast the hexadecimal number to an integer  I personally found this to be simple and small   Here is an sample code for converting a Hexadecimal number to a Decimal number with the help of casting    include  lt stdio h gt   int main        unsigned char Hexadecimal   0x6D      example hex number     int Decimal   0       decimal number initialized to 0           Decimal    int  Hexadecimal     conversion      printf  The decimal number is  d n   Decimal      output     return 0

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include  math h   include  stdio h                                                                      The bits arg represents the bit say 8 16 32                                                                                                                                                                                volatile long Hex To Int long Hex char bits        long Hex 2 Int      char byte      Hex 2 Int 0       for byte 0 byte lt bits byte                  if Hex amp  0x0001 lt  lt byte               Hex 2 Int  1  pow 2 byte            else             Hex 2 Int  0  pow 2 byte               return Hex 2 Int                                                                                                                                                                                                                                                        void main  void        int Dec         char Hex 0xFA      Dec  Hex To Int Hex 8      convert an 8-bis hexadecimal value to a number in base 10     printf  the number is  d  Dec

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Edit  Now compatible with MSVC  C   and non-GNU compilers  see end    The question was  most efficient way    The OP doesn t specify platform  he could be compiling for a RISC based ATMEL chip with 256 bytes of flash storage for his code   For the record  and for those  like me   who appreciate the difference between  the easiest way  and the  most efficient way   and who enjoy learning      static const long hextable           0     255    -1     bit aligned access into this table is considerably      0     0  1  2  3  4  5  6  7  8  9     faster for most modern processors       A     10  11  12  13  14  15           for the space conscious  reduce to      a     10  11  12  13  14  15           signed char               brief convert a hexidecimal string to a signed long    will not produce or process negative numbers except     to signal error          param hex without decoration  case insensitive           return -1 on error  or result  max  sizeof long  8 -1 bits      long hexdec unsigned const char  hex       long ret   0      while   hex  amp  amp  ret  gt   0          ret    ret  lt  lt  4    hextable  hex             return ret       It requires no external libraries  and it should be blindingly fast    It handles uppercase  lowercase  invalid characters  odd-sized hex input  eg  0xfff   and the maximum size is limited only by the compiler   For non-GCC or C   compilers or compilers that will not accept the fancy hextable declaration   Replace the first statement with this  longer  but more conforming  version   static const long hextable            -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 -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 -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 -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

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As if often happens  your question suffers from a serious terminological error ambiguity  In common speech it usually doesn t matter  but in the context of this specific problem it is critically important    You see  there s no such thing as  hex value  and  decimal value   or  hex number  and  decimal number     Hex  and  decimal  are properties of representations of values  Meanwhile  values  or numbers  by themselves have no representation  so they can t be  hex  or  decimal   For example  0xF and 15 in C syntax are two different representations of the same number   I would guess that your question  the way it is stated  suggests that you need to convert ASCII hex representation of a value  i e  a string  into a ASCII decimal representation of a value  another string   One way to do that is to use an integer representation as an intermediate one  first  convert ASCII hex representation to an integer of sufficient size  using functions from strto    group  like strtol   then convert the integer into the ASCII decimal representation  using sprintf    If that s not what you need to do  then you have to clarify your question  since it is impossible to figure it out from the way your question is formulated

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For AVR Microcontrollers I wrote the following function  including relevant comments to make it easy to understand          hex2int    take a hex string and convert it to a 32bit number  max 8 hex digits      uint32 t hex2int char  hex        uint32 t val   0      while   hex               get current character then increment         char byte    hex                transform hex character to the 4bit equivalent number  using the ascii table indexes         if  byte  gt    0   amp  amp  byte  lt    9   byte   byte -  0           else if  byte  gt    a   amp  amp  byte  lt   f   byte   byte -  a    10          else if  byte  gt    A   amp  amp  byte  lt   F   byte   byte -  A    10                 shift 4 to make space for new digit  and add the 4 bits of the new digit          val    val  lt  lt  4     byte  amp  0xF             return val      Example   char  z   82ABC1EF   uint32 t x   hex2int z   printf  Number is   X  n   x     Will output

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Try this    include  lt stdio h gt  int main         char s      fffffffe       int x      sscanf s    x    amp x       printf   u n   x

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For larger Hex strings like in the example I needed to use strtoul

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You want strtol or strtoul   See also the Unix man page

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Eric     Why is a code solution that works getting voted down  Sure  it s ugly and might not be the fastest way to do it  but it s more instructive that saying  strtol  or  sscanf   If you try it yourself you will learn something about how things happen under the hood    I don t really think your solution should have been voted down  but my guess as to why it s happening is because it s less practical   The idea with voting is that the  best  answer will float to the top  and while your answer might be more instructive about what happens under the hood  or a way it might happen   it s definitely not the best way to parse hex numbers in a production system   Again  I don t think there s anything wrong with your answer from an educational standpoint  and I certainly wouldn t  and didn t  vote it down   Don t get discouraged and stop posting just because some people didn t like one of your answers   It happens   I doubt my answer makes you feel any better about yours being voted down  but I know it s especially not fun when you ask why something s being voted down and no one answers

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Eric     I was actually hoping to see a C wizard post something really cool  sort of like what I did but less verbose  while still doing it  manually     Well  I m no C guru  but here s what I came up with   unsigned int parseHex const char   str        unsigned int val   0      char c       while c    str                  val  lt  lt   4           if  c  gt    0   amp  amp  c  lt    9                         val    c  amp  0x0F              continue                     c  amp   0xDF          if  c  gt    A   amp  amp  c  lt    F                         val     c  amp  0x07    9              continue                     errno   EINVAL          return 0             return val      I originally had more bitmasking going on instead of comparisons  but I seriously doubt bitmasking is any faster than comparison on modern hardware

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This currently only works with lower case but its super easy to make it work with both   cout  lt  lt    nEnter a hexadecimal number     cin  gt  gt  hexNumber  orighex   hexNumber   strlength   hexNumber length     for  i 0 i lt strlength i          hexa   hexNumber substr i 1       if   hexa gt   0    amp  amp   hexa lt   9                    cout  lt  lt   This is a numerical value  n             else                 cout  lt  lt   This is a alpabetical value  n           if  hexa   a   hexa  10            else if  hexa   b   hexa  11            else if  hexa   c   hexa  12            else if  hexa   d   hexa  13            else if  hexa   e   hexa  14            else if  hexa   f   hexa  15            else cout  lt  lt   INVALID ENTRY  ANSWER WONT BE CORRECT n                convert from string to integer      hx   atoi hexa c str         finalhex   finalhex    hx pow 16 0 strlength-i-1      cout  lt  lt   The hexadecimal number     lt  lt  orighex  lt  lt    is    lt  lt  finalhex  lt  lt    in decimal  n

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As written before  the efficiency basically depends on what one is optimizing for   When optiming for lines of code  or just working in environment without fully-equipped standard library one quick and dirty option could be      makes a number from two ascii hexa characters int ahex2int char a  char b        a    a  lt    9     a -  0     a  amp  0x7    9      b    b  lt    9     b -  0     b  amp  0x7    9       return  a  lt  lt  4    b          more in similar thread here  https   stackoverflow com a 58253380 5951263

[c] Convert a hexadecimal string to an integer efficiently in C?

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