Hello,
I have a structure like this:
However, I don't want to specify how many keys I can have in the array.Code:typedef struct { char *name; char keys[20][6]; } row;
Can I replace it with something like **keys and then reallocate memory when I add data ? Any suggestions how to do it ?
Best wishes,
D5
You can have either a pointer to a fixed array:
or a pointer to pointer and allocate two arrays.Code:char (*keys)[6]
In a 32-bit machine, if your key is always 6 chars, you loose 4 bytes per key if you go with the second approach.Code:char **keys;
--
Mats
Compilers can produce warnings - make the compiler programmers happy: Use them!
Please don't PM me for help - and no, I don't do help over instant messengers.
You can do it like this.
Code:#include <stdio.h> #include <string.h> typedef struct { char *name; char **keys; } row; int main() { row temp; int i; temp.name = malloc(50 * sizeof(char)); strcpy(temp.name, "Banana Man"); temp.keys = malloc(5 * sizeof(char*)); for ( i = 0; i < 5; ++i ) { temp.keys[i] = malloc(3 * sizeof(char)); memcpy(temp.keys[i], "1234567890" + i, 3); } puts(temp.name); for ( i = 0; i < 5; ++i ) { int j; for ( j = 0; j < 3; ++j ) { putchar(temp.keys[i][j]); } putchar('\n'); } return 0; }
The program never frees the memory. Bad.
Originally Posted by Adak
Forgive my ignorance, but how are you losing 4 bytes?
Because you'll have an extra pointer for every array element, that you won't need in this particular case since you have fixed-width keys.
I'm sure you have my best interests at heart and weren't just trying to be a know it all, so thanks for the nitpick. Anyway, my OS will free memory when the program ends and there's no point being anal about that kind of thing when it's just a little example for this thread. You might as well blast me for using // comments in C, or using magic numbers, or mixing tabs and spaces for indentation while you're at it.The program never frees the memory. Bad.
I could blame for all of it, but not freeing memory is not a good example for others, sooo...
I know the OS frees up resources at the end of a program, but I find it very bad practice to rely on that. There was a discussion about it before, and most agreed that it might be an "optimization," but certainly not a good idea just to be sloppy.
It's going to burn you someday, especially in C++.
I raised this point before, and while the opinion was divided, a general consensus is that we should teach best practices here. A simple solution is to show a code snippet instead, with the expectation that the reader should either know the rest, or better find out for himself.Anyway, my OS will free memory when the program ends and there's no point being anal about that kind of thing when it's just a little example for this thread.
Those are standard in C99.You might as well blast me for using // comments in C
Elysia is well known for doing that, I do it myself when I find the indentation unacceptable for reading.or mixing tabs and spaces for indentation while you're at it.
Look up a C++ Reference and learn How To Ask Questions The Smart Way
Because I relied on an "optimization" it means I was being sloppy?most agreed that it might be an "optimization," but certainly not a good idea just to be sloppy.
I doubt it, but you can believe whatever you want to.It's going to burn you someday, especially in C++.
Yes. It's sloppy not to free memory. There was no need for such an optimization here.Because I relied on an "optimization" it means I was being sloppy?
Really? How well do you actually know C++?I doubt it, but you can believe whatever you want to.
Yeah, but who dictates what best practices are?a general consensus is that we should teach best practices here
I thought "best practice" was not to use C99.Those are standard in C99.
Describe to me how not freeing memory is good practice.
Common sense: if you do not free what you allocate, you end up with a memory leak. That the OS may act like a garbage collector is irrelevant.Yeah, but who dictates what best practices are?
I do not entirely agree with this stand myself (it sounds a little too harsh), but I have to agree that since we are concentrating on standard C, and standard C is not concerned with whether the OS does such a reclamation of memory, we should teach as if this is not the case, and indeed it might not be the case.
You can read the discussion for yourself: Concerning delete/delete[] at program exit
Note that it is in the C++ programming forum, not here.
That's Prelude's opinion, but since that version of the C standard exists, to say that // is not part of C would be factually incorrect.I thought "best practice" was not to use C99.
Look up a C++ Reference and learn How To Ask Questions The Smart Way
Sorry, but I don't want to take over this thread until desmond5 says the original question is answered. I also think that you're not going to take any of my opinions seriously, so explaining them to you is pointless.Describe to me how not freeing memory is good practice.
