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 <iostream>   using namespace std; int main(int argc,int argvx[]) {     int i=1;     int answer = 23;     int temp;      // decode arguments     if(argc < 2) {         printf("You must provide at least one argument\n");         exit(0);     }      // Convert it to an int here  } 

Answer 1

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 <cstdlib>  int x = atoi(argv[1]); 

The C way with input checking:

#include <cstdlib>  errno = 0; char *endptr; long int x = strtol(argv[1], &endptr, 10); if (endptr == argv[1]) {   std::cerr << "Invalid number: " << argv[1] << '\n'; } else if (*endptr) {   std::cerr << "Trailing characters after number: " << argv[1] << '\n'; } else if (errno == ERANGE) {   std::cerr << "Number out of range: " << argv[1] << '\n'; } 

The C++ iostreams way with input checking:

#include <sstream>  std::istringstream ss(argv[1]); int x; if (!(ss >> x)) {   std::cerr << "Invalid number: " << argv[1] << '\n'; } else if (!ss.eof()) {   std::cerr << "Trailing characters after number: " << argv[1] << '\n'; } 

Alternative C++ way since C++11:

#include <stdexcept> #include <string>  std::string arg = argv[1]; try {   std::size_t pos;   int x = std::stoi(arg, &pos);   if (pos < arg.size()) {     std::cerr << "Trailing characters after number: " << arg << '\n';   } } catch (std::invalid_argument const &ex) {   std::cerr << "Invalid number: " << arg << '\n'; } catch (std::out_of_range const &ex) {   std::cerr << "Number out of range: " << arg << '\n'; } 

All four variants assume that argc >= 2. All accept leading whitespace; check isspace(argv[1][0]) if you don't want that. All except atoi reject trailing whitespace.

Answer 2

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 <sstream>  int main(int argc, char *argv[]) {     if (argc >= 2)     {         std::istringstream iss( argv[1] );         int val;          if (iss >> val)         {             // Conversion successful         }     }      return 0; }