Read and Write Text Files on IPhone OS

/Method writes a string to a text file
-(void) writeToTextFile{
      //get the documents directory:
      NSArray *paths = NSSearchPathForDirectoriesInDomains
           (NSDocumentDirectory, NSUserDomainMask, YES);
      NSString *documentsDirectory = [paths objectAtIndex:0];

      //make a file name to write the data to using the documents directory:
      NSString *fileName = [NSString stringWithFormat:@"%@/textfile.txt", 
                                   documentsDirectory];
      //create content - four lines of text
      NSString *content = @"One\nTwo\nThree\nFour\nFive";
      //save content to the documents directory
      [content writeToFile:fileName 
                   atomically:NO 
                       encoding:NSStringEncodingConversionAllowLossy 
                          error:nil];

}

//Method retrieves content from documents directory and
//displays it in an alert
-(void) displayContent{
      //get the documents directory:
      NSArray *paths = NSSearchPathForDirectoriesInDomains
                      (NSDocumentDirectory, NSUserDomainMask, YES);
      NSString *documentsDirectory = [paths objectAtIndex:0];

      //make a file name to write the data to using the documents directory:
      NSString *fileName = [NSString stringWithFormat:@"%@/textfile.txt", 
                                   documentsDirectory];
      NSString *content = [[NSString alloc] initWithContentsOfFile:fileName
                                                      usedEncoding:nil
                                                             error:nil];
         NSLog(@"content: %@" , content)

      [content release];

} 

Source:  

http://howtomakeiphoneapps.com/2009/06/reading-and-writing-text-files-in-iphone-os-3-0/