How to convert a command-line argument to int?

nosedive25 picture nosedive25 · May 9, 2010 · Viewed 140.5k times · Source

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

Thomas picture Thomas · May 9, 2010

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.