How do you output variable's declared as a double to a text file in C++ - c++

Question

I am very new to C++ and I am wondering how you output/write variables declared as double to a txt file. I know about how to output strings using fstream but I cant figure out how to send anything else. I am starting to think that you can't send anything but strings to a text file is that correct? If so then how would you convert the information stored in the variable to a string variable?
Here is my code that I'm trying to implement this concept into, Its fairly simple:
int main()
{
double invoiceAmt = 3800.00;
double apr = 18.5; //percentage
//compute cash discount
double discountRate = 3.0; //percentage
double discountAmt;
discountAmt = invoiceAmt * discountRate/100;
//compute amount due in 10 days
double amtDueIn10;
amtDueIn10 = invoiceAmt - discountAmt;
//Compute Interest on the loan of amount (with discount)for 20 days
double LoanInt;
LoanInt = amtDueIn10 * (apr /360/100) * 20;
//Compute amount due in 20 days at 18.5%.
double amtDueIn20;
amtDueIn20 = invoiceAmt * (1 + (apr /360/100) * 20);
return 0;
}
So what I'm trying to do is use those variables and output them to the text file. Also please inform me on the includes that I need to use for this source code. Feel free to give suggestions on how to improve my code in other ways as well please.
Thanks in advance.

Answer 1

As your tagging suggests, you use file streams:
std::ofstream ofs("/path/to/file.txt");
ofs << amtDueIn20;
Depending on what you need the file for, you'll probably have to write more stuff (like whitespaces etc.) in order to get decent formatting.
Edit due to rmagoteaux22's ongoing problems:
This code
#include <iostream>
#include <fstream>
const double d = 3.1415926;
int main(){
std::ofstream ofs("test.txt");
if( !ofs.good() ) {
std::cerr << "Couldn't open text file!\n";
return 1;
}
ofs << d << '\n';
return 0;
}
compiles for me (VC9) and writes this to test.txt:
3.14159
Can you try this?

Answer 2

Simply use the stream write operator operator<< which has an overloaded definition for double (defined in basic_ostream)
#include <fstream>
...
std::fstream stmMyStream( "c:\\tmp\\teststm.txt", std::ios::in | std::ios::out | std::ios::trunc );
double dbMyDouble = 23.456;
stmMyStream << "The value is: " << dbMyDouble;

Answer 3

To answer your first question, in C you use printf (and for file output fprintf). IIRC, cout has a large number of modifiers also, but I won't mention them as you originally mentioned fstream (more 'C' centric than C++) --
oops, missed the ofstream indicator, ignore my 'C' comments and use C++
to improve your program, be sure to use parentheses a lot when doing computations as above to be 100% sure things are evaluated the way you want them to be (do not rely on order of precedence)

Answer 4

Generally speaking methods to write to a output are printf, wprintf etc.
In case of files, these methods are named as fprintf_s, fsprintf_s etc.
Note that the '_s' methods are the new secure variations of previous formatting methods. You should always use these new secure versions.
For examples refer to:
http://msdn.microsoft.com/en-us/library/ksf1fzyy%28VS.80%29.aspx
Note these methods use a format specifier to convert a given type to text. For example %d acts as a place holder for integer. Similarly %f for double.

Answer 5

Just use the << operator on an output stream:
#include <fstream>
int main() {
double myNumber = 42.5;
std::fstream outfile("test.txt", std::fstream::out);
outfile << "The answer is almost " << myNumber << std::endl;
outfile.close();
}

Answer 6

I was having the exact same problem, where ofstream was outputting strings, but stopped as soon as it reached a variable. With a bit more Googling I found this solution in a forum post:
Under Xcode 3.2 when creating a new project based on stdc++ project template the target build settings for Debug configuration adds preprocessor macros which are incompatible with gcc-4.2:
_GLIBCXX_DEBUG=1
_GLIBXX_DEBUG_PEDANTIC=1
Destroy them if you want Debug/gcc-4.2 to execute correctly.
http://forums.macrumors.com/showpost.php?p=8590820&postcount=8