Convert a character digit to the corresponding integer in C

Question

Is there a way to convert a character to an integer in C    For example  from  5  to 5

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char numeralChar    4   int numeral    int   numeralChar -  0

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When I need to do something like this I prebake an array with the values I want   const static int lookup 256      -1       0 1 2 3 4 5 6 7 8 9            Then the conversion is easy  int digit to int  unsigned char c     return lookup  static cast lt int gt  c         This is basically the approach taken by many implementations of the ctype library  You can trivially adapt this to work with hex digits too

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Subtract char  0  or int 48 like this   char c    5   int i   c -  0     Explanation  Internally it works with ASCII value  From the ASCII table  decimal value of character 5 is 53 and 0 is 48  So 53 - 48   5  OR  char c    5   int i   c - 48     Because decimal value of char  0  is 48   That means if you deduct 48 from any numeral character  it will convert integer automatically

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use function  atoi for array to integer  atof for array to float type  or   char c    5   int b   c - 48  printf   d   b

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Subtract  0  like this   int i   c -  0     The C Standard guarantees each digit in the range  0    9  is one greater than its previous digit  in section 5 2 1 3 of the C99 draft   The same counts for C

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If it s just a single character 0-9 in ASCII  then subtracting the the value of the ASCII zero character from ASCII value should work fine   If you want to convert larger numbers then the following will do   char  string    24    int value   int assigned   sscanf string    d    amp value        don t forget to check the status  which should be 1 if it worked in the above case    Paul

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If  by some crazy coincidence  you want to convert a string of characters to an integer  you can do that too   char  num    1024   int val   atoi num      atoi   ASCII TO Int   val is now 1024  Apparently atoi   is fine  and what I said about it earlier only applies to me  on OS X  maybe  insert Lisp joke here     I have heard it is a macro that maps roughly to the next example  which uses strtol    a more general-purpose function  to do the conversion instead   char  num    1024   int val    int strtol num   char    NULL  10      strtol   STRing TO Long   strtol   works like this   long strtol const char  str  char   endptr  int base     It converts  str to a long  treating it as if it were a base base number  If   endptr isn t null  it holds the first non-digit character strtol   found  but who cares about that

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Just use the atol  function    include  lt stdio h gt   include  lt stdlib h gt   int main          const char  c    5       int d   atol c       printf   d n   d

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You would cast it to an int  or float or double or what ever else you want to do with it  and store it in anoter variable

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If your digit is  say   5   in ASCII it is represented as the binary number 0011 0101  53   Every digit has the highest four bits 0011 and the lowest 4 bits represent the digit in bcd  So you just do  char cdig    5   int dig   cdig  amp  0xf     dig contains the number 5   to get the lowest 4 bits  or  what its same  the digit  In asm  it uses and operation instead of sub as in the other answers

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Here are helper functions which allow to convert digit in char to int and vice versa   int toInt char c        return c -  0      char toChar int i        return i    0

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Check this   char s  A    int i    s lt   9    s- 0    s lt   F     s- A   10    s- a   10     for only 0 1 2      E F

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char chVal    5   char chIndex   if   chVal  gt    0    amp  amp   chVal  lt    9           chIndex   chVal -  0     else  if   chVal  gt    a    amp  amp   chVal  lt    z           chIndex   chVal -  a     else  if   chVal  gt    A    amp  amp   chVal  lt    Z           chIndex   chVal -  A     else       chIndex   -1     Error value

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As per other replies  this is fine   char c    5   int x   c -  0     Also  for error checking  you may wish to check isdigit c  is true first  Note that you cannot completely portably do the same for letters  for example   char c    b   int x   c -  a      x is now not necessarily 1   The standard guarantees that the char values for the digits  0  to  9  are contiguous  but makes no guarantees for other characters like letters of the alphabet

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To convert character digit to corresponding integer  Do as shown below   char c    8                       int i   c -  0     Logic behind the calculation above is to play with ASCII values  ASCII value of character 8 is 56  ASCII value of character 0 is 48  ASCII value of integer 8 is 8   If we subtract two characters  subtraction will happen between ASCII of characters   int i   56 - 48     i   8

[c] Convert a character digit to the corresponding integer in C

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