How to convert a string to integer in C

Question

I am trying to find out if there is an alternative way of converting string to integer in C   I regularly pattern the following in my code   char s      45    int num   atoi s     So  is there a better way or another way

User · Answer

You can always roll your own     include  lt stdio h gt   include  lt string h gt   include  lt math h gt   int my atoi const char  snum        int idx  strIdx   0  accum   0  numIsNeg   0      const unsigned int NUMLEN    int strlen snum           Check if negative number and flag it         if snum 0     0x2d          numIsNeg   1       for idx   NUMLEN - 1  idx  gt   0  idx--                   Only process numbers from 0 through 9             if snum strIdx   gt   0x30  amp  amp  snum strIdx   lt   0x39              accum     snum strIdx  - 0x30    pow 10  idx            strIdx                  Check flag to see if originally passed -ve number and convert result if so         if  numIsNeg          return accum      else         return accum   -1     int main            Tests           printf  Returned number is   d n   my atoi  34574         printf  Returned number is   d n   my atoi  -23          return 0      This will do what you want without clutter

User · Answer

Don t use functions from ato    group  These are broken and virtually useless  A moderately better solution would be to use sscanf  although it is not perfect either   To convert string to integer  functions from strto    group should be used  In your specific case it would be strtol function

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Ok  I had the same problem I came up with this solution It worked for me the best I did try atoi   but didn t work well for me So here is my solution   void splitInput int arr    int sizeArr  char num          for int i   0  i  lt  sizeArr  i               We are subtracting 48 because the numbers in ASCII starts at 48          arr i     int num i  - 48

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I think this way we could go   int my atoi const char  snum     int nInt 0    int index 0    while snum index          if  nInt          nInt     int  snum index   - 48      else               nInt    nInt    10      int  snum index  - 48             index        return nInt      int main         printf  Returned number is   d n   my atoi  676987         return 0

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This function will help you  int strtoint n char  str  int n        int sign   1      int place   1      int ret   0       int i      for  i   n-1  i  gt   0  i--  place    10                int c   str i           switch  c                        case  -                   if  i    0  sign   -1                  else return -1                  break              default                  if  c  gt    0   amp  amp  c  lt    9     ret     c -  0     place                  else return -1                       return sign   ret     int strtoint char  str        char  temp   str      int n   0      while   temp      0                 n            temp              return strtoint n str  n       Ref  http   amscata blogspot com 2013 09 strnumstr-version-2 html

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Robust C89 strtol-based solution  With    no undefined behavior  as could be had with the atoi family  a stricter definition of integer than strtol  e g  no leading whitespace nor trailing trash chars  classification of the error case  e g  to give useful error messages to users  a  testsuite       include  lt assert h gt   include  lt ctype h gt   include  lt errno h gt   include  lt limits h gt   include  lt stdio h gt   include  lt stdlib h gt   typedef enum       STR2INT SUCCESS      STR2INT OVERFLOW      STR2INT UNDERFLOW      STR2INT INCONVERTIBLE   str2int errno      Convert string s to int out         param out  out The converted int  Cannot be NULL         param in  s Input string to be converted            The format is the same as strtol         except that the following are inconvertible            - empty string        - leading whitespace        - any trailing characters that are not part of the number           Cannot be NULL         param in  base Base to interpret string in  Same range as strtol  2 to 36          return Indicates if the operation succeeded  or why it failed      str2int errno str2int int  out  char  s  int base        char  end      if  s 0       0     isspace s 0            return STR2INT INCONVERTIBLE      errno   0      long l   strtol s   amp end  base          Both checks are needed because INT MAX    LONG MAX is possible         if  l  gt  INT MAX     errno    ERANGE  amp  amp  l    LONG MAX           return STR2INT OVERFLOW      if  l  lt  INT MIN     errno    ERANGE  amp  amp  l    LONG MIN           return STR2INT UNDERFLOW      if   end      0           return STR2INT INCONVERTIBLE       out   l      return STR2INT SUCCESS     int main void        int i         Lazy to calculate this size properly         char s 256           Simple case         assert str2int  amp i   11   10     STR2INT SUCCESS       assert i    11           Negative number          assert str2int  amp i   -11   10     STR2INT SUCCESS       assert i    -11           Different base         assert str2int  amp i   11   16     STR2INT SUCCESS       assert i    17           0        assert str2int  amp i   0   10     STR2INT SUCCESS       assert i    0           INT MAX         sprintf s    d   INT MAX       assert str2int  amp i  s  10     STR2INT SUCCESS       assert i    INT MAX           INT MIN         sprintf s    d   INT MIN       assert str2int  amp i  s  10     STR2INT SUCCESS       assert i    INT MIN           Leading and trailing space         assert str2int  amp i    1   10     STR2INT INCONVERTIBLE       assert str2int  amp i   1    10     STR2INT INCONVERTIBLE           Trash characters         assert str2int  amp i   a10   10     STR2INT INCONVERTIBLE       assert str2int  amp i   10a   10     STR2INT INCONVERTIBLE           int overflow                 if  needed to avoid undefined behaviour        on  INT MAX   1  if INT MAX    LONG MAX              if  INT MAX  lt  LONG MAX            sprintf s    ld    long int INT MAX   1L           assert str2int  amp i  s  10     STR2INT OVERFLOW                 int underflow        if  LONG MIN  lt  INT MIN            sprintf s    ld    long int INT MIN - 1L           assert str2int  amp i  s  10     STR2INT UNDERFLOW                 long overflow        sprintf s    ld0   LONG MAX       assert str2int  amp i  s  10     STR2INT OVERFLOW           long underflow        sprintf s    ld0   LONG MIN       assert str2int  amp i  s  10     STR2INT UNDERFLOW        return EXIT SUCCESS      GitHub upstream   Based on  https   stackoverflow com a 6154614 895245

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Just wanted to share a solution for unsigned long aswell   unsigned long ToUInt char  str        unsigned long mult   1      unsigned long re   0      int len   strlen str       for int i   len -1   i  gt   0   i--                re   re     int str i  -48  mult          mult   mult 10            return re

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int atoi const char  str       int num   0      int i   0      bool isNegetive   false      if str i      -            isNegetive   true          i              while  str i   amp  amp   str i   gt    0   amp  amp  str i   lt    9             num   num   10    str i  -  0            i              if isNegetive  num   -1   num      return num

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You can code atoi   for fun  int my getnbr char  str      int result    int puiss     result   0    puiss   1    while    -       str         str                     if   str     -           puiss   puiss   -1        str          while    str  gt    0    amp  amp    str  lt    9              result    result   10       str  -  0          str          return  result   puiss      You can also make it recursive  which can fold in 3 lines

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There is strtol which is better IMO  Also I have taken a liking in strtonum  so use it if you have it  but remember it s not portable   long long      strtonum const char  nptr  long long minval  long long maxval       const char   errstr    You might also be interested in strtoumax and strtoimax which are standard functions in C99  For example you could say  uintmax t num   strtoumax s  NULL  10   if  num    UINTMAX MAX  amp  amp  errno    ERANGE         Could not convert      Anyway  stay away from atoi   The call atoi str  shall be equivalent to   int  strtol str   char    NULL  10   except that the handling of errors may differ  If the value cannot be represented  the behavior is undefined

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In C    you can use a such function   template  lt typename T gt  T to const std  string  amp  s        std  istringstream stm s       T result      stm  gt  gt  result       if stm tellg      s size            throw error       return result      This can help you to convert any string to any type such as float  int  double

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Yes  you can store the integer directly   int num   45    If you must parse a string  atoi or strol is going to win the  shortest amount of code  contest

[c] How to convert a string to integer in C?

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