Writing a double type value to a text file

Question

The below code is writing unreadable characters to the text file:

int main () 
{
    ofstream myfile ("example.txt");

    if (myfile.is_open())
    {
        double value       = 11.23444556;
        char   *conversion = (char *)&value;
        strcat (conversion, "\0");

        myfile.write (conversion, strlen (conversion));
        myfile.close();
    }   

    return 0;
}

I want to see the actual number written in the file :( Hints please.

EDIT Seeing the answers below, I modified the code as:

int main () 
{
    ofstream myfile ("example.txt");

    if (myfile.is_open())
    {
        double value       = 11.23444556;

        myfile << value;
        myfile.close();
    }   

    return 0;
}

This produces the putput: 11.2344 while the actual number is 11.23444556. I want the complete number.

Editing the post to notify everyone: The unreadable characters are due to ofstream's write function:

This is an unformatted output function

This quote is from: http://www.cplusplus.com/reference/iostream/ostream/write/

@Anisha - The default is to display 6 digits, if you want a higher precision, you can do `myfile << setprecision(10) << value;` to get 10 digits in the number. — Bo Persson, Jun 03 '11 at 11:15
@Bo Persson: to get 10 digits in the number, `std::fixed` is also necessary, or else the trailing zeros willn't get printed. — Nawaz, Jun 03 '11 at 11:21
Casting a double like that, other than wrong in this case, violates strict aliasing rule, so one way or the other your code wouldn't have worked. — King_DuckZ, Jun 03 '11 at 12:35
You need to make a union with double and the type you want to cast to, or use memcopy. The latter on recent compilers should be optimized away and the resulting code would be exactly what you expect, but formally correct. Check this out: http://stackoverflow.com/questions/4163126/c-dereferencing-type-punned-pointer-will-break-strict-aliasing-rules-warning — King_DuckZ, Jun 03 '11 at 12:52

score 29 · Accepted Answer · edited Jan 04 '19 at 23:01

29

Why don't you simply do this (updated answer after the edit in the question):

 #include <iomanip>

 myfile << std::fixed << std::setprecision(8) << value;
 myfile.close();

Now, you can see the actual number written in the file.

See the documentation of std::setprecision. Note: you have to include the <iomanip> header file.

edited Jan 04 '19 at 23:01

JROS

17
8

answered Jun 03 '11 at 11:01

Nawaz

353,942
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1

@Anisha: See it now. I updated it. Use `std::fixed` and `std::setprecision(8)` – Nawaz Jun 03 '11 at 11:13
1

Thanks, now it worked brilliantly! Can you tell WHAT was wrong with the original code? – Aquarius_Girl Jun 03 '11 at 11:17

score 3 · Answer 2 · answered Jun 03 '11 at 11:03

It's easier here to use the stream operators:

#include <iostream>
#include <fstream>
#include <string.h>
using namespace std;

int main () 
{
    ofstream myfile ("example.txt");

    if (myfile.is_open())
    {
        double value       = 11.23444556;

        myfile << value;
        myfile.close();
    }   

    return 0;
}

gives you what you want.

score 2 · Answer 3 · answered Jun 03 '11 at 11:12

Others suggested better ways but if you really want to do it the pointer way there should be casts to convert char* to double * and vice versa

#include <iostream>
#include <fstream>
#include <string>
#include <cstring>
using namespace std;

int main () 
{
  ofstream myfile ("example.txt");
  if (myfile.is_open())
  {
    double value       = 11.23444556;
    char     *conversion = reinterpret_cast<char *>(&value);
    strcat (conversion, "\0");
    //myfile.write (*conversion, strlen (conversion));
    myfile << *(reinterpret_cast<double *>(conversion));
    myfile.close();
  }
  return 0;
}

score 0 · Answer 4 · answered Jun 03 '11 at 11:04

0

You can write the value to file like this:

myfile << value;

Using the adress of value as parameter strcat corrupts your stack, since strcat expects a 0-terminated c-string as first argument (and actually allocated space).

answered Jun 03 '11 at 11:04

maxschlepzig

35,645
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Writing a double type value to a text file

4 Answers4