Char to int conversion in C

Question

If I want to convert a single numeric char to it s numeric value  for example  if   char c    5     and I want c to hold 5 instead of  5   is it 100  portable doing it like this   c   c -  0     I heard that all character sets store the numbers in consecutive order so I assume so  but I d like to know if there is an organized library function to do this conversion  and how it is done conventionally  I m a real beginner

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Since you re only converting one character  the function atoi   is overkill   atoi   is useful if you are converting string representations of numbers   The other posts have given examples of this   If I read your post correctly  you are only converting one numeric character   So  you are only going to convert a character that is the range 0 to 9   In the case of only converting one numeric character  your suggestion to subtract  0  will give you the result you want   The reason why this works is because ASCII values are consecutive  like you said   So  subtracting the ASCII value of 0  ASCII value 48 - see ASCII Table for values  from a numeric character will give the value of the number   So  your example of c   c -  0  where c    5   what is really happening is 53  the ASCII value of 5  - 48  the ASCII value of 0    5   When I first posted this answer  I didn t take into consideration your comment about being 100  portable between different character sets   I did some further looking around around and it seems like your answer is still mostly correct   The problem is that you are using a char which is an 8-bit data type   Which wouldn t work with all character types   Read this article by Joel Spolsky on Unicode for a lot more information on Unicode   In this article  he says that he uses wchar t for characters   This has worked well for him and he publishes his web site in 29 languages  So  you would need to change your char to a wchar t   Other than that  he says that the character under value 127 and below are basically the same   This would include characters that represent numbers   This means the basic math you proposed should work for what you were trying to achieve

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Yes  this is a safe conversion  C requires it to work  This guarantee is in section 5 2 1 paragraph 2 of the latest ISO C standard  a recent draft of which is N1570      Both the basic source and basic execution character sets shall have the following   members            the 10 decimal digits          0 1 2 3 4 5 6 7 8 9           In both the source and execution basic character sets  the   value of each character after 0 in the above list of decimal digits shall be one greater than   the value of the previous    Both ASCII and EBCDIC  and character sets derived from them  satisfy this requirement  which is why the C standard was able to impose it  Note that letters are not contiguous iN EBCDIC  and C doesn t require them to be   There is no library function to do it for a single char  you would need to build a string first   int digit to int char d     char str 2     str 0    d   str 1      0    return  int  strtol str  NULL  10       You could also use the atoi   function to do the conversion  once you have a string  but strtol   is better and safer   As commenters have pointed out though  it is extreme overkill to call a function to do this conversion  your initial approach to subtract  0  is the proper way of doing this  I just wanted to show how the recommended standard approach of converting a number as a string to a  true  number would be used  here

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int i   c -  0     You should be aware that this doesn t perform any validation against the character - for example  if the character was  a  then you would get 91 - 48   49   Especially if you are dealing with user or network input  you should probably perform validation to avoid bad behavior in your program   Just check the range   if   0   lt   c  amp  amp   c  lt    9         i   c -  0     else          handle error        Note that if you want your conversion to handle hex digits you can check the range and perform the appropriate calculation   if   0   lt   c  amp  amp  c  lt    9         i   c -  0     else if   a   lt   c  amp  amp  c  lt    f         i   10   c -  a     else if   A   lt   c  amp  amp  c  lt    F         i   10   c -  A     else          handle error        That will convert a single hex character  upper or lowercase independent  into an integer

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As others have suggested  but wrapped in a function   int char to digit char c        return c -  0       Now just use the function  If  down the line  you decide to use a different method  you just need to change the implementation  performance  charset differences  whatever   you wont need to change the callers   This version assumes that c contains a char which represents a digit  You can check that before calling the function  using ctype h s isdigit function

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Normally  if there s no guarantee that your input is in the  0    9  range  you d have to perform a check like this   if  c  gt    0   amp  amp  c  lt    9         int v   c -  0          safely use v     An alternative is to use a lookup table  You get simple range checking and conversion with less  and possibly faster  code      one-time setup of an array of 256 integers     all slots set to -1 except for ones corresponding    to the numeric characters static const int CHAR TO NUMBER           -1  -1  -1           0  1  2  3  4  5  6  7  8  9      0    9      -1  -1  -1             Now  all you need is   int v   CHAR TO NUMBER c    if  v    -1           safely use v     P S  I know that this is an overkill  I just wanted to present it as an alternative solution that may not be immediately evident

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Since the ASCII codes for  0   1   2      are placed from 48 to 57 they are essentially continuous  Now the arithmetic operations require conversion of char datatype to int datatype Hence what you are basically doing is   53-48 and hence it stores the value 5 with which you can do any integer operations Note that while converting back from int to char the compiler gives no error but just performs a modulo 256 operation to put the value in its acceptable range

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Yes  This is safe as long as you are using standard ascii characters  like you are in this example

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You can simply use theatol  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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Try this    char c    5  -  0

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You can use atoi  which is part of the standard library

[c] Char to int conversion in C

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