Defining the amount of elements in an array

Question

I was wondering if I do this:

int TOPE , vector[TOPE] ;
cin >> TOPE ;

Does vector get the amount of elements I input or does it have a number that I can't control?

**No**. Neither has C++ support for variable length arrays, and even your compiler has this feature, it will not resize the array after initializing. — user1810087, Jun 11 '21 at 13:52
You won't be able to make an arbitrary sized array, only something predefined (from a constant or a literal). Use `std::vector` for dynamic arrays — Alexey S. Larionov, Jun 11 '21 at 13:53

score 4 · Accepted Answer · answered Jun 11 '21 at 13:53

4

The variable vector is a Variable-Length Array (VLA). Standard C++ doesn't support that, but some compiler supports that as extension.

The variable TOPE is not initialized at the point where the variable vector is declared, so its size is indeterminate.

To set the size of VLA to the number entered, you should write like this:

int TOPE;
cin >> TOPE ;
int vector[TOPE] ;

To do it in more standard way, you should use std::vector instead of VLA:

int TOPE;
cin >> TOPE ;
std::vector<int> vector(TOPE);

answered Jun 11 '21 at 13:53

MikeCAT

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If use `#define TOPE 10` is TOPE still a variable or would it work that way? – Axel Lutzky Marshall Jun 11 '21 at 13:58
1

`#define` is like a string replacement. Using that, `TOPE` will be replaced with `10` and it is no longer a variable. – MikeCAT Jun 11 '21 at 13:59

score 1 · Answer 2 · answered Jun 11 '21 at 13:55

Firstly, VLAs, or variable-length array is not a C++ standard, although some compiler like gcc does support it. More info : Why aren't variable-length arrays part of the C++ standard?

Secondly, compilers read in a top-down fashion. Changing a variable after it has been used as length for an array isn't going to change the array in any way.

To declare it properly, declare it like this :

int TOPE; std::cin >> TOPE;
int vector[TOPE];

Defining the amount of elements in an array

2 Answers2