Setting an array's size to the value of a const derived from the division of another constant

Question

I have these two supposed constants:

int const MATRIX_SIZE = 1000;

int const LONGEST_LR_LINK = (int)floor(MATRIX_SIZE/2);

I am attempting to declare an array:

int lrLinkArray [LONGEST_LR_LINK];

I get the error:

error: array bound is not an integer constant before ‘]’ token

It would not be that much trouble to manually plug in whatever half MATRIX_SIZE is for LONGEST_LR_LINK, but I am curious as to what a solution to this would be. Is it bad practice to do any kind of calculation for a const?

@chris please no, [rolling your own floor is hard](http://stackoverflow.com/a/24348037/1708801) and not advised. — Shafik Yaghmour, Jul 03 '14 at 17:37
@chris too bad most of the highly scored answers there have broken solutions and not enough downvotes. — Shafik Yaghmour, Jul 03 '14 at 17:48

score 7 · Accepted Answer · answered Jul 03 '14 at 17:37

7

Integer division in C++ automatically rounds down. So if you have the definition

int const LONGEST_LR_LINK = MATRIX_SIZE/2;

then LONGEST_LR_LINK will indeed be a constant expression, and

int lrLinkArray [LONGEST_LR_LINK];

should be fine. Besides, floor(MATRIX_SIZE/2) doesn't do what you think it does, anyway; you'd have to write floor(MATRIX_SIZE/2.0) in order to force non-integer division.

answered Jul 03 '14 at 17:37

Brian Bi

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Thank you. I guess I had it in my head that integer division would return a float remainder. – Gerald Jul 03 '14 at 17:57

score 3 · Answer 2 · edited Jul 03 '14 at 17:39

3

int const LONGEST_LR_LINK = MATRIX_SIZE/2;

Integer division already rounds down.

edited Jul 03 '14 at 17:39

jwodder

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answered Jul 03 '14 at 17:38

Alan Stokes

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Shafik Yaghmour · Answer 3 · 2014-07-04T01:40:00.367

Integer division should do what you want, we can see this from the draft C++ standard, in section 5.6 Multiplicative operators it says:

[...]For integral operands the / operator yields the algebraic quotient with any fractional part discarded;⁸¹[...]

To understand why it does not you have to realized that even though you initialize MATRIX_SIZE with a literal which makes it a constant expression std::floor is not a constexpr function and therefore LONGEST_LR_LINK is also not a constant expression and can not be used for array bounds which require an integral constant expression.

Also, as far I understand old style C casts are also not allowed in constant expressions either.

Setting an array's size to the value of a const derived from the division of another constant

3 Answers3