Double to Const Char*

Question

How can I convert a double into a const char, and then convert it back into a double?

I'm wanting to convert the double to a string, to write it to a file via fputs, and then when I read the file, it will need to be converted back into a double.

I'm using Visual C++ 2010 Express Edition.

If you are using C++ do you really want to use char and fputs? — David Heffernan, Jun 19 '11 at 19:24
Better to use binary files, instead of text files and store the double "as-is". Then read it "as-is". Ensure that "structure" of file doesn't change. — Ajay, Jun 19 '11 at 19:39
I'd like to support what Ajay said: it is much more effective to use binary files and store the doubles as doubles rather than converting them to a decimal representation (if only because this is likely to introce rounding errors!), only to be able to write it to a text file in string form. — leftaroundabout, Jun 19 '11 at 22:41
Another thing: don't say "convert `double` to const `char*`", you never want to do this. What you actually mean is "convert `double` to array of char and give me a pointer to this array". Or simply "convert `double` to `string`". — leftaroundabout, Jun 19 '11 at 22:44

score 8 · Answer 1 · edited Aug 28 '13 at 18:45

8

Since you added C++ to your tags, I suggest you use std::stringstream:

#include <sstream>

stringstream ss;
ss << myDouble;
const char* str = ss.str().c_str();
ss >> myOtherDouble;

edited Aug 28 '13 at 18:45

answered Jun 19 '11 at 19:24

Paul Manta

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You should add that after any call to a non-const member function the pointer should not be treated as valid. – lccarrasco Jun 19 '11 at 19:34

score 8 · Accepted Answer · edited Aug 28 '13 at 18:45

8

If you just want to write the double values to file, you can simply write it, without converting them into const char*. Converting them into const char* is overkill.

Just use std::ofstream as:

 std::ofstream file("output.txt")'

 double d = 1.989089;

 file << d ; // d goes to the file!

 file.close(); //done!

edited Aug 28 '13 at 18:45

answered Jun 19 '11 at 19:27

Nawaz

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Thanks! I actually didn't know this was possible, LOL. I've only been working with C++ for a few days. – Justin White Jun 19 '11 at 19:35

score 3 · Answer 3 · answered Jun 19 '11 at 19:29

You can use these functions to convert to and from:

template <class T>
bool convertFromStr(string &str, T *var) {
  istringstream ss(str);
  return (ss >> *var);
}

template <class T>
string convertToStr(T *var) {
  ostringstream ss;
  ss << *var;
  return ss.str();
}

Example:

double d = 1.234567;
string str = convertToStr<double>(&d);
cout << str << endl;
double d2;
convertFromStr<double>(str, &d2);
cout << d2 << endl;

score 1 · Answer 4 · answered Jun 23 '15 at 14:18

1

Use this funcition :

const char* ConvertDoubleToString(double value){
    std::stringstream ss ;
    ss << value;
    const char* str = ss.str().c_str();
    return str;
}

answered Jun 23 '15 at 14:18

Milton Quirino

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Double to Const Char*

4 Answers4

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