C & C++ string confusion

[Jerry Feldman]

On 02/06/2009 01:23 PM, bruce.labitt at autoliv.com wrote:
> Maybe some on the list might know the answer to this...  I am trying to 
> read n files, one at a time, and appending the data to a different file. 
> Since the files are so large, I need to delete each of the n files, once I 
> have captured the data. 
>
> Why on earth am I doing this?  My arrays are too large to fit in memory 
> all at once (I used up all 32GB!!) so I have to process each row of the 
> matrix separately.  (It slows stuff way down...)
>
> I find string manipulation in C to be a bit arcane.  This is what I have 
> come up with so far.  Unfortunately, (maybe fortunately?) the compiler 
> does not like my coding.  Oh yes, this has to be in a C or C++ dialect. 
> (No "I can do this in x lines of your favorite language" comments. :) )
>
> The code will be compiled using g++ on YDL to run on a QS22 (Cell 
> Processor) = Linux content :)
>
> /start code snippet
>
> main()
> {
>  string filename;
>  string shelldelcmd;
>  string mydelstr;
>  char filenum[4];
>  char filenamec[20];
>  FILE * fidjj;
>
>  shelldelcmd.assign("rm -f ");
>
>  for (jj=0; jj<1000; jj++)
>  {
>   filename.assign("out");
>   sprintf(filenum, "%04d", jj); //generate string for file number, like 
> "0010"
>   filename.append(filenum);  // filename = "outxxxx", where xxxx = jj
>   filenamec = filename.c_str;           // <=====  COMPILER DIES HERE 
> ==========
>
>   fidjj = fopen(filenamec, "rb");       // <===== location of second error
>   if (fidjj==NULL) {fputs ("File error, does not exist\n", stderr); 
> exit(1);}
>
>   fread some stuff...
>   fclose(fidjj);
>
>   mydelstr.assign(shelldelcmd);
>   mydelstr.append(filename);
>   mydelstr.append("\n");
>
>   cout << "my delete string is : " << mydelstr << endl;
>   system(mydelstr);     // delete the file I just read... !!!
>   fwrite data to a different file...
>  }
> }
>
> /end code snippet
>
> Compiler error is: error; incompatible types of assignment of '<unresolved 
> overloaded function type>' to 'char[20]'
>
> If I just use the string "filename" instead of "filenamec" in fopen I get 
> two errors, first the one in the previous paragraph, and second is: 
>
> error: cannot convert 'std::string' to const char * for argument '1' to 
> 'FILE * fopen(const char *, const char*)'
>
> If you think I should step away from the keyboard, well, unfortunately 
> that is not an option.  I have to learn this stuff as I go along...  And 
> no, I have never taken a class in C++.  I barely have the hang of C... 
> FWIW, I tried it in C and suffered some string craziness like unexpected 
> overwriting.  It was ugly...  This approach seems cleaner, except I do not 
> know how to convert the C++ strings to be able to use ordinary C fopens...
>
> Any tips or insight would be greatly appreciated...  (Awesome tips are 
> rewarded with beer!) 
>
>   
C++ strings (from the STL) are strictly C++ template classes. The 
std::string function, c_str() is what you want. You are not using it as 
a function.

filenamec = filename.c_str;           // <=====  COMPILER DIES HERE 

  strcpy(filenamec,filename.c_str());   // This is the correct construct 
in your code.
Or:
char *filenamec = filename.c_str());
You may need to use const char *.

-- 
Jerry Feldman <gaf at blu.org>
Boston Linux and Unix
PGP key id: 537C5846
PGP Key fingerprint: 3D1B 8377 A3C0 A5F2 ECBB  CA3B 4607 4319 537C 5846


-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 251 bytes
Desc: OpenPGP digital signature
Url : http://mail.gnhlug.org/mailman/private/gnhlug-discuss/attachments/20090206/b995d7d6/attachment.bin