Initialize array with extra element using a macro

Question

I am initializing an array in two different ways depending a macro:

# if feature_enabled
const int v[4] = {1, 2, 3, 4};
#else
const int v[5] = {0, 1, 2, 3, 4};
#endif

The problem is that the data in the assignment is actually large matrices, and for various reasons it's not a good solution to just copy the data with a minor modification (just one more element at the beginning of the array.)

I was wondering if there is a way to do the same thing that I did here, without essentially duplicating the last n-1 elements.

Do you know that the array size does not match the number of elements in the initializer list? — mch
– mch, Commented Dec 6, 2019 at 14:17
@mch fixed it now, it was just something I typed up for the question. — C. E.
– C. E., Commented Dec 6, 2019 at 14:31
@C.E. "and for various reasons it's not a good solution to just copy the data with a minor modification" - Could you give us a reference for that, what is your worry about? — RobertS supports Monica Cellio
– RobertS supports Monica Cellio, Commented Dec 6, 2019 at 17:05
@RobertS-ReinstateMonica Why, in general, is duplicating code bad? That's a rather broad topic. Luckily, I got my answer anyway. — C. E.
– C. E., Commented Dec 6, 2019 at 18:13
@C.E. You do not need to make an over-detailed explanation, it were just be fine if you could give a reference at least to what you are pointing to and which is the base of that question. So, people who aren´t confirmed with these things can understand the purpose of that question. — RobertS supports Monica Cellio
– RobertS supports Monica Cellio, Commented Dec 6, 2019 at 18:20

Wai Ha Lee · Accepted Answer · 2019-12-06 16:39:42Z

16

If you don't specify the size on the array but let it be auto-deduced, you can just add the 0 in the front conditionally:

const int v[] = {
# if feature_enabled
  0,
#endif
  1, 2, 3, 4
};

edited Dec 6, 2019 at 16:39

Wai Ha Lee

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answered Dec 6, 2019 at 14:16

Turtlefight

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Comments

mik · Accepted Answer · 2019-12-06 15:09:55Z

3

If you need to keep the array size, then:

# if feature_enabled
const int v[4] = {
#else
const int v[5] = {0,
#endif
  1, 2, 3, 4
};

answered Dec 6, 2019 at 15:09

mik

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