How to convert a command-line argument to int

Question

I need to get an argument and convert it to an int  Here is my code so far    include  lt iostream gt    using namespace std  int main int argc int argvx          int i 1      int answer   23      int temp          decode arguments     if argc  lt  2            printf  You must provide at least one argument n            exit 0                 Convert it to an int here

User · Answer

As WhirlWind has pointed out  the recommendations to use atoi aren t really very good  atoi has no way to indicate an error  so you get the same return from atoi  0    as you do from atoi  abc     The first is clearly meaningful  but the second is a clear error   He also recommended strtol  which is perfectly fine  if a little bit clumsy  Another possibility would be to use sscanf  something like    if  1  sscanf argv 1     d    amp temp          successful conversion else        couldn t convert input   note that strtol does give slightly more detailed results though -- in particular  if you got an argument like 123abc  the sscanf call would simply say it had converted a number  123   whereas strtol would not only tel you it had converted the number  but also a pointer to the a  i e   the beginning of the part it could not convert to a number    Since you re using C    you could also consider using boost  lexical cast  This is almost as simple to use as atoi  but also provides  roughly  the same level of detail in reporting errors as strtol  The biggest expense is that it can throw exceptions  so to use it your code has to be exception-safe  If you re writing C    you should do that anyway  but it kind of forces the issue

User · Answer

Note that your main arguments are not correct  The standard form should be   int main int argc  char  argv      or equivalently   int main int argc  char   argv    There are many ways to achieve the conversion  This is one approach    include  lt sstream gt   int main int argc  char  argv          if  argc  gt   2                std  istringstream iss  argv 1             int val           if  iss  gt  gt  val                           Conversion successful                      return 0

User · Answer

Since this answer was somehow accepted and thus will appear at the top  although it s not the best  I ve improved it based on the other answers and the comments   The C way  simplest  but will treat any invalid number as 0    include  lt cstdlib gt   int x   atoi argv 1      The C way with input checking    include  lt cstdlib gt   errno   0  char  endptr  long int x   strtol argv 1    amp endptr  10   if  endptr    argv 1       std  cerr  lt  lt   Invalid number     lt  lt  argv 1   lt  lt    n     else if   endptr      std  cerr  lt  lt   Trailing characters after number     lt  lt  argv 1   lt  lt    n     else if  errno    ERANGE      std  cerr  lt  lt   Number out of range     lt  lt  argv 1   lt  lt    n       The C   iostreams way with input checking    include  lt sstream gt   std  istringstream ss argv 1    int x  if    ss  gt  gt  x       std  cerr  lt  lt   Invalid number     lt  lt  argv 1   lt  lt    n     else if   ss eof        std  cerr  lt  lt   Trailing characters after number     lt  lt  argv 1   lt  lt    n       Alternative C   way since C  11    include  lt stdexcept gt   include  lt string gt   std  string arg   argv 1   try     std  size t pos    int x   std  stoi arg   amp pos     if  pos  lt  arg size          std  cerr  lt  lt   Trailing characters after number     lt  lt  arg  lt  lt    n         catch  std  invalid argument const  amp ex      std  cerr  lt  lt   Invalid number     lt  lt  arg  lt  lt    n     catch  std  out of range const  amp ex      std  cerr  lt  lt   Number out of range     lt  lt  arg  lt  lt    n       All four variants assume that argc  gt   2  All accept leading whitespace  check isspace argv 1  0   if you don t want that  All except atoi reject trailing whitespace

User · Answer

Like that we can do      int main int argc  char  argv           int a  b  c          Converting string type to integer type        using function  atoi  argument          a   atoi argv 1             b   atoi argv 2        c   atoi argv 3

User · Answer

Take a look at strtol    if you re using the C standard library

User · Answer

The approach with istringstream can be improved in order to check that no other characters have been inserted after the expected argument    include  lt sstream gt   int main int argc  char  argv          if  argc  gt   2                std  istringstream iss  argv 1             int val           if   iss  gt  gt  val   amp  amp  iss eof       Check eofbit                          Conversion successful                      return 0

User · Answer

std  stoi from string could also be used        include  lt string gt       using namespace std       int main  int argc  char   argv                 if  argc  gt   2                          int val   stoi argv 1                                                return 0

[c++] How to convert a command-line argument to int?

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