Convert char array to a int number in C

Question

I want to convert a char array   like   char myarray 4      -   1   2   3      where the - means it is negative   So it should be the integer  -1234 using standard libaries in C  I could not find any elegant way to do that   I can append the   0  for sure

User · Answer

It s not what the question asks but I used  Rich Drummond  s answer for a char array read in from stdin which is null terminated  char  buff  size t buff size   100  int choice  do      buff    char   malloc buff size  sizeof char        getline  amp buff   amp buff size  stdin       choice   atoi buff       free buff                         while  choice lt 1  amp  amp  choice gt 9

User · Answer

So  the idea is to convert character numbers  in single quotes  e g   8   to integer expression  For instance char c    8   int i   c -  0    would yield integer 8  And sum up all the converted numbers by the principle that 908 9 100 0 10 8  which is done in a loop   char t 5      -    9    0    8     0      Should be terminated properly   int s   1  int i   -1  int res   0   if  c 0      -       s   -1    i   0     while  c   i       0       iterate until the array end   res   res 10    c i  -  0      generating the integer according to read parsed numbers     res   res s    answer  -908

User · Answer

Why not just use atoi   For example   char myarray 4      -   1   2   3     int i   atoi myarray    printf   d n   i     Gives me  as expected   -123   Update  why not - the character array is not null terminated  Doh

User · Answer

It isn t that hard to deal with the character array itself without converting the array to a string  Especially in the case where the length of the character array is know or can be easily found  With the character array  the length must be determined in the same scope as the array definition  e g    size t len sizeof myarray sizeof  myarray    For strings you  of course  have strlen available    With the length known  regardless of whether it is a character array or a string  you can convert the character values to a number with a short function similar to the following      convert character array to integer    int char2int  char  array  size t n            int number   0      int mult   1       n    int n  lt  0   -n   n           quick absolute value check             for each character in array        while  n--                   if not digit or  -   check if number  gt  0  break or continue            if   array n   lt   0     array n   gt   9    amp  amp  array n      -                 if  number                  break              else                 continue                     if  array n      -             if  -  if number  negate  break                if  number                    number   -number                  break                                  else                           convert digit to numeric value                  number     array n  -  0     mult              mult    10                       return number      Above is simply the standard char to int conversion approach with a few additional conditionals included  To handle stray characters  in addition to the digits and  -   the only trick is making smart choices about when to start collecting digits and when to stop    If you start collecting digits for conversion when you encounter the first digit  then the conversion ends when you encounter the first  -  or non-digit  This makes the conversion much more convenient when interested in indexes such as  e g  file 0127 txt    A short example of its use    include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   int char2int  char  array  size t n    int main  void         char myarray 4      -   1   2   3         char  string    some-goofy-string-with-123-inside       char  fname    file-0123 txt        size t mlen   sizeof myarray sizeof  myarray      size t slen   strlen  string       size t flen   strlen  fname        printf    n myarray 4      -   1   2   3    n n        printf      char2int  myarray  mlen     d n n   char2int  myarray  mlen         printf    string     some-goofy-string-with-123-inside    n n        printf      char2int  string  slen      d n n   char2int  string  slen         printf    fname     file-0123 txt    n n        printf      char2int  fname  flen       d n n   char2int  fname  flen         return 0      Note  when faced with  -  delimited file indexes  or the like   it is up to you to negate the result   e g  file-0123 txt compared to file 0123 txt where the first would return -123 while the second 123    Example Output      bin atoic array   myarray 4      -   1   2   3        char2int  myarray  mlen    -123   string    some-goofy-string-with-123-inside       char2int  string  slen     -123   fname    file-0123 txt       char2int  fname  flen      -123   Note  there are always corner cases  etc  that can cause problems  This isn t intended to be 100  bulletproof in all character sets  etc   but instead work an overwhelming majority of the time and provide additional conversion flexibility without the initial parsing or conversion to string required by atoi or strtol  etc

User · Answer

I personally don t like atoi function  I would suggest sscanf   char myarray 5      -    1    2    3     0    int i  sscanf myarray    d    amp i     It s very standard  it s in the stdio h library     And in my opinion  it allows you much more freedom than atoi  arbitrary formatting of your number-string  and probably also allows for non-number characters at the end   EDIT I just found this wonderful question here on the site that explains and compares 3 different ways to do it - atoi  sscanf and strtol  Also  there is a nice more-detailed insight into sscanf  actually  the whole family of  scanf functions    EDIT2 Looks like it s not just me personally disliking the atoi function  Here s a link to an answer explaining that the atoi function is deprecated and should not be used in newer code

[c] Convert char array to a int number in C

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