How to read in a double from a file in c++

问题

How do you read in a double from a file in C++?

For ints I know you can use the getline() and then atoi, but I am not finding an array to double function. What is available for reading in doubles, or converting a char array to a double?

回答1:

You can use stream extraction:

std::ifstream ifs(...);
double d;
ifs >> d;

This work provided that other then whitespace, the next data in the stream should be a double in textual representation.

After the extraction, you can check the state of the stream to see if there were errors:

ifs >> d;
if (!ifs)
{
    // the double extraction failed
}

回答2:

Do not consider using atof(), or any of the ato.. functions, as they do not allow you to diagnose errors. Take a look at strtod and strtol. Or use the stream extraction operators.

回答3:

I'm wondering, does one need to be careful about locale settings (e.g. a locale could use comma instead of dot to separate the decimal part) or do stringstreams always default to some standard "C locale" notation?

回答4:

You can leverage istringstream For example, here are toDouble and toInt:

double toDouble(string s) {
  double r = 0;
  istringstream ss(s);
  ss >> r;
  return r;
}

int toInt(string s) {
  int r=0;
  istringstream ss(s);
  ss >> r;
  return r;
}

来源：https://stackoverflow.com/questions/2615078/how-to-read-in-a-double-from-a-file-in-c