Thread: Visual Studio C++, syntax error : ']'

Originally Posted by Sparrowhawk

Making another array is easy enough with a loop, that would then get destroyed I assume because its scope is only inside the reverse function.

Yes, you can replace memcpy with a loop.
And no, it won't die because it would be allocated with malloc and malloc requires free.

I was just looking for a more efficient way I suppose. I hate throwing more lines of code at stuff if there's a neater/easier/cleaner way to do it.

C is not exactly good at this generic stuff.
The closes thing is a macro, but they are ugly and not very type-safe and hard to debug.

Well, there are ways to do this, but malloc() is the best in terms of having easiness, neatness. For your compiler which doesn't use the C99 standard I suppose.

If you cannot use malloc() then the next best thing is to declare a maximum size of the
array. Then you can just use temp[maximum_size].

If you are uncertain of the size or it is too big then you should pass a duplicate of the array inside the function. Like void reverse(int a[], int temp[], int size). You would have the duplicate array in main of course.

MORE importantly you have to think this. How are the values of a passed to the program. If you have a[] = {2,1,3,4; then you now that you have 4 elements in a. So you you declare int temp[4]. The point of passing size in the reverse function is pointless! It has no use, you know the size. It is 4.
Now. If the values where passed from a user then you would have to declare a with the size of the input entered in the first place before worrying about anything else! This can only be done with malloc() or by declaring again the array with a maximum size.
In any case, whatever you do with a you can do with temp. If you declare a as a 4-element array you already did, you should do that for temp

EDIT:
you would do:

Code:

int* temp;
temp = malloc(size*sizeof(int));

Thus allocating space. Not that since temp is a pointer you can use []. Then you use it as you did. Before the end of the function you would have to do:

Code:

free(temp);

To free the memory (not that it won't work if you don't)
Google malloc()/free and search for more details

Originally Posted by C_ntua

Well, there are ways to do this, but malloc() is the best in terms of having easiness, neatness. For your compiler which doesn't use the C99 standard I suppose.

If you cannot use malloc() then the next best thing is to declare a maximum size of the
array. Then you can just use temp[maximum_size].

If you are uncertain of the size or it is too big then you should pass a duplicate of the array inside the function. Like void reverse(int a[], int temp[], int size). You would have the duplicate array in main of course.

MORE importantly you have to think this. How are the values of a passed to the program. If you have a[] = {2,1,3,4; then you now that you have 4 elements in a. So you you declare int temp[4]. The point of passing size in the reverse function is pointless! It has no use, you know the size. It is 4.
Now. If the values where passed from a user then you would have to declare a with the size of the input entered in the first place before worrying about anything else! This can only be done with malloc() or by declaring again the array with a maximum size.
In any case, whatever you do with a you can do with temp. If you declare a as a 4-element array you already did, you should do that for temp

EDIT:
you would do:

Code:

int* temp;
temp = malloc(size*sizeof(int));

Thus allocating space. Not that since temp is a pointer you can use []. Then you use it as you did. Before the end of the function you would have to do:

Code:

free(temp);

To free the memory (not that it won't work if you don't)
Google malloc()/free and search for more details

I'm getting an error from this:

error C2440: '=' : cannot convert from 'void *' to 'int *'

I know malloc is what is causing it, I'm just not sure how to remedy the problem. Casting perhaps? I vaguely remember reading somewhere that there's a way to do this..

[edit] - Casted it and the program works perfectly now. Is casting the proper way though?

[edit2] - Another question-- is if temp is a pointer (it points to the memory that malloc made), how come I didn't need to dereference it to get the index in my loop? I think my instructor said that brackets automatically dereference or something... but the proper way to write it would be:

a[j] = i[*temp]

instead of

a[j] = temp[i]


Any input on this?

Originally Posted by tabstop

That's exactly what we're saying. In C you don't cast. But because you named your file whatever.cpp, your compiler thinks it's C++, so it's using C++ rules, which state that you must cast.

Ah ok, I understand. I believe you mean that in C you don't have to cast, but you can... Just in this case it isn't required in C... correct?

I'm aware that casting as an int will end up screwing me later if I pass the array something besides an int... I know that in C the purpose of doing malloc (sizeof(whateveryourarraynameishere)) is to avoid that, but it wouldn't accept that so there you are .

Yes, the correct way is to cast.
I would always like to recommend casting, even if it's C, since there are tools that can detect such mistakes.
C++ is very type strict, which means that unlike C, if your types don't match (eg assign void* to int*), the compiler will produce an error. You can get around this by telling the compiler that you really meant it, and you do that with a cast.
Don't worry about using a C++ compiler to compile your C code - it won't harm you. You just need to be aware that you may need casts where you wouldn't need in C (but casts are a good thing, if you ask me).