How to use fprintf

//////////////////////////////////////////
//                                      //
// Reading and writing textual data     //
//                                      //
// You found this at bobobobo's weblog, //
// http://bobobobo.wordpress.com        //
//                                      //
// Creation date:  Feb 8/08             //
// Last modified:  Feb 8/08             //
//                                      //
//////////////////////////////////////////
#include <stdio.h>
#include <stdlib.h>

// How do you write to a file in C/C++?
// Its easy!

int main()
{
    ///////////////////
    // fprintf() - stands for File-PRINT-Formatted.
    //
    // Say you've got some TEXTUAL data
    // you want to print out to a simple
    // text file for your program.
    //
    // You know where to turn to.  fprintf() !
    //
    // There are a few steps we must follow here:
    // 1.  OPEN FILE TO WRITE TO
    // 2.  WRITE!
    // 3.  CLOSE THE FILE.

    // 1.  In order to write out to a file, you
    // first have to OPEN A FILE FOR WRITING.
    FILE * outfile = fopen( "shoppingList.txt", "w" );  // open "shoppingList.txt" for "w"RITING
    // 'outfile' is now a "file pointer" to
    // the file "shoppingList.txt" on disk.  outfile
    // can ONLY BE WRITTEN TO, because I opened
    // the file in a WRITE mode, indicated by
    // the "w" as the second argument to fopen()

    // Notice when I call fopen()
        // A.  If "shoppingList.txt" doesn't exist, then
        //     it will be created.
    
        // B.  If "shoppingList.txt" DOES exist, then
        //     shoppingList.txt's contents are CLEARED OUT

    // 2.  So, now that the file is open, let's write to it!
    fprintf( outfile, "Shopping list:\n" );
    fprintf( outfile, "   - Blueberries\n" );
    fprintf( outfile, "   - Raspberries\n" );
    fprintf( outfile, "   - Granola bars\n" );
    fprintf( outfile, "   - Beer\n" );

    // What if we want to write variable values?
    // Easy!  Just like printf() works, you use
    // the %d, %f, %s to tell fprintf() what types
    // of variables its printing.

    // fprint an int
    int qtips = 5000;
    fprintf( outfile, "   - %d Qtips\n", qtips );

    // fprint a float
    float purity = 99.4;
    fprintf( outfile, "   - Soap that is %.1f%% pure\n", purity );

    // 3.  DONE, so CLOSE THE FILE!
    fclose( outfile );

    ////////////////////
    // HOW DO YOU OPEN A FILE WITHOUT
    // OVERWRITING ITS CONTENTS?
    //
    // 1.  OPEN:  You open the file using "APPEND" mode.
    outfile = fopen( "shoppingList.txt", "a" ); // open "shoppingList.txt" for "a"PPENDING

    // 2.  PRINT:  anything we fprintf() here will automatically be
    // tacked onto the end of "shoppingList.txt"
    fprintf( outfile, "Don't forget the milk !" );

    // 3.  CLOSE!
    fclose( outfile );

    printf("\n\n***************************\n* Part 2 - Reading from a file\n");
    system("pause");
    ////////////////////
    // READING FROM A FILE.
    //
    // Now, we're going to write a bit of code
    // to read that file we just wrote to.

    // Here's how you do it:
    // 1.  OPEN FILE FOR READING
    // 2.  READ!
    // 3.  CLOSE FILE!

    // 1.  OPEN:
    FILE * readfile = fopen( "shoppingList.txt", "r" ); // open "shoppingList.txt" in "r"EAD mode
    char buf[300];      // create a buffer of 300 chars
                        // to temporarily dump data
                        // from the file into.

    while( !feof( readfile ) )
    {
        // 2.  READ!
        // let's read one line at a time
        fgets( buf, 300, readfile );

        // print out that line we just read in
        // to the console so we can see it
        printf( "%s", buf );
    }

    // 3. CLOSE!
    fclose( readfile );


    printf("\nPart 3 - printing the source of __this__ code file.\n");
    system("pause");
    //////////////////
    // A fairly common interview question comes next.
    //
    // "Write a C program that prints out
    // its own source code when it is
    // compiled and run."
    //
    // The trick here about this question is
    // you have to know about the __FILE__ macro.
    // 
    // __FILE__ will be automatically identified
    // with a string containing the full path
    // of the source file.
    printf("\nThe source file for this code is:  %s\n", __FILE__ );
    
    FILE * source = fopen( __FILE__, "r" ); // open __this__ file for reading
    while( !feof(source) )
    {
        // print every character in the file
        // to the console.
        printf( "%c", fgetc( source ) );
    }
}


//////////////////
// END NOTES:
//
// Note that there IS a C++ "way" to do file output
// that is "object oriented" provided through
// the 'ofstream' object.
//
// I do NOT recommend you use the ofstream
// class however.  WHY?  Because its REALLY slow.
// In simple tests I've conducted using a high performance
// counter, ofstream takes TWICE AS LONG to print
// the same stream of characters to disk
// as fprintf() does.

// See
// http://bobobobo.wordpress.com/2008/02/07/speed-tests-fprintf-vs-ofstream-and-fprintf-vs-fwrite/
// for a speed test.)

// OOP is good, but it sometimes has
// a pretty bad performance cost.

Download the Visual Studio 2005 project files hosted by esnips (thanks esnips!)

Refs
fprintf entry @ cplusplus.com
fprintf entry @ cppreference.com

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