Can't initialize array with pre-defined variable int

Question

int shifts[c] = {0};

The c is defined. Error: shifts maybe not be initialized. Why? What am I doing wrong here? I think it has something to do with c, but I am not sure. If I can't use the current declaration, how do I get around it? I tried it with vectors first:

vector<bool> shifts(c, false)

But, it didn't work. Had bad_alloc error.

int max = 0, min = 10000;
for (int i = 0; i != 2*no_shifts; ++i) {
    int x;
    fin >> x;
    time.push_back(x);

    if (max < x)
        max = x;

    if (min > x)
        min = x;
}
c = max - min + 1;

c has to be a compile-time constant for the array declaration. Is it? — juanchopanza
– juanchopanza, Commented May 29, 2014 at 21:24
Yes. Umm, I don't know. 'c = max - min + 1' max and min are int. Is there something wrong with this? — Ishaan Singh
– Ishaan Singh, Commented May 29, 2014 at 21:26
@IshaanSingh How are max and min defined? Can you show more code, at least enough to see exactly what c gets set to, ideally an entire but short program — simonc
– simonc, Commented May 29, 2014 at 21:28
c is not a constant expression since it involves max and min which are non-const. Variable-length arrays are not part of C++ and your compiler may have restrictions on how they can be used. (In C99, variable-length arrays may not be initialized.) — Brian Bi
– Brian Bi, Commented May 29, 2014 at 21:36

AJG85 · Accepted Answer · 2014-05-29 21:48:11Z

3

Based solely on c = max - min + 1; it seems that you are trying statically initialize an array without a compile time constant. Either use a dynamically allocated array or std::vector if you need something that can have it's size determined at runtime.

Edit: You may actually want to consider using std::bitset instead as an alternative container.

edited May 29, 2014 at 21:48

answered May 29, 2014 at 21:37

AJG85

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3 Comments

Ishaan Singh Over a year ago

So, if I use vector<bool> shifts(c, false), will it work? How do you get the grey padding around the code (inline)?

AJG85 Over a year ago

std::vector<bool> shifts(c, false); should make a vector with c amount of bool all set to false with the default allocator. You use the ` key to inline code snippets.

M.M Over a year ago

Make sure you create the vector after calculating the value of c to use

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