Re: writing huge C-array to a file

FROM : Bill Bumgarner
DATE : Fri Jul 21 18:01:20 2006

On Jul 21, 2006, at 8:04 AM, PGM wrote:
> In my app, I have a document containing a potentially huge two-
> dimensional C-array filled with doubles that I need to write to a 
> file (with a user-specified number of decimals). My original plan 
> was to use NSDocument's "dataRepresentationOfType" and fill an 
> NSMutableString with the text and use that. However, this takes 
> forever because the NSMutableString has to allocate additional 
> memory again and again ("initWithCapacity" did not help much).
>
> What I did then is to use "writeToFile:ofType:" and then use plain 
> C to write the data to a file:
>
>         FILE *writing;
>      double **matrix = [myData distanceMatrix];
>      const char *out_name;
>      out_name = [fileName cString];
>      writing = fopen(out_name, "w");
>     fprintf(writing, everything); //looping over the matrix
>
> This works, but I am afraid that it will break when the fileName 
> contains non-ASCII characters. Is there another fast way to write a 
> large C-array to a file that does work with non-ASCII filenames?


(Others have pointed out not to use cString, etc..)

You are writing binary data to a file.    It isn't a string and, 
thus, NSMutableString is completely inappropriate for the task at hand.

You can use NSFileHandle:
+ (id)fileHandleForWritingAtPath:(NSString *)path

http://developer.apple.com/documentation/Cocoa/Reference/Foundation/
Classes/NSFileHandle_Class/Reference/Reference.html#//apple_ref/occ/
clm/NSFileHandle/fileHandleForWritingAtPath:

Followed by a series of calls to writeData:

- (void)writeData:(NSData *)data

http://developer.apple.com/documentation/Cocoa/Reference/Foundation/
Classes/NSFileHandle_Class/Reference/Reference.html#//apple_ref/occ/
instm/NSFileHandle/writeData:

Of course, you will need to get your double* arrays into an instance 
of NSData for writing.  You will want to avoid copying the data in 
doing so:

+ (id)dataWithBytesNoCopy:(void *)bytes length:(unsigned)length

http://developer.apple.com/documentation/Cocoa/Reference/Foundation/
Classes/NSData_Class/Reference/Reference.html#//apple_ref/occ/clm/
NSData/dataWithBytesNoCopy:length: