Is a given string convertable to an integer [duplicate]

Question

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How do I tell if the c function atoi failed or if it was a string of zeros? (6 answers)

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Is there an effective way in C to check if a given string is convertable to an integer?
An output error should indicate if the string is convertable or not.
For example, "aa" is not convertable, "123" is convertable.

Answer 1

C++11 has the std::stoi, std::stol, and std::stoll functions, which throw an exception if the string cannot be converted.

Answer 2

With C, use the strtol(3) function with an end pointer:

 char* end=NULL;
 long l = strtol(cstr, &end, 0);
 if (end >= cstr && *end)
    badnumber = true;
 else
    badnumber = false;

With C++11, use the std::strtol function (it raises an exception on failure).

Answer 3

You could also loop through the string and appply isdigit function to each character

Answer 4

bool is_convertible_to_int(const std::string &s) {
  try {
    int t = std::stoi(s);
    return std::to_string(t).length() == s.length();
  }
  catch (std::invalid_argument) {
    return false;
  }
}