Initialize std::array with a random size

Question

I have two macros:

#define length(array) sizeof(array)/sizeof(array[0])
#define randRange(x,y) x + ( std::rand() % ( y - x + 1 ) )

I Can do this:

int data[] = {2, 3, 34, 10, 3, 12, 30 };
const int arr_length = length(data);
std::array<int,arr_length> data_std_array;

I Can initialize with a constant:

const int arr_length = 500;
std::array<int,arr_length> data_std_array;

But I can't initialize an std::array with my rand function:

float a = randRange(2, 200);
const int arr_length = (int)a;
std::array<int,arr_length> data_std_array;

float a = randRange(2, 200);
int counter = 0;
for(int i =0; i < a; i++){
    counter++;
}
const int arr_length = counter;
std::array<int,arr_length> data_std_array;

float a = randRange(2, 200);
const int arr_length = static_cast<int>(a);

None of these work.

I've tried all sorts of variables like size_t and unsigned ints, constants, pointers etc. I always get this error message

Error msg I always receive: float a = randRange(2, 200); const int arr_length = static_cast(a);

/home/martin/Documents/github/on-line-mean-calculation/tests/test.cpp: In member function ‘virtual void algorithmTest_TestAlgorithmRandomArray_Test::TestBody()’:
/home/martin/Documents/github/on-line-mean-calculation/tests/test.cpp:87:20: error: the value of ‘arr_length’ is not usable in a constant expression
     std::array<int,arr_length> data_std_array;
                    ^
/home/martin/Documents/github/on-line-mean-calculation/tests/test.cpp:76:15: note: ‘arr_length’ was not initialized with a constant expression
     const int arr_length = static_cast<int>(a);
               ^
/home/martin/Documents/github/on-line-mean-calculation/tests/test.cpp:87:30: error: the value of ‘arr_length’ is not usable in a constant expression
     std::array<int,arr_length> data_std_array;
                              ^
/home/martin/Documents/github/on-line-mean-calculation/tests/test.cpp:76:15: note: ‘arr_length’ was not initialized with a constant expression
     const int arr_length = static_cast<int>(a);
               ^
/home/martin/Documents/github/on-line-mean-calculation/tests/test.cpp:87:30: note: in template argument for type ‘long unsigned int’ 
     std::array<int,arr_length> data_std_array;

How can I initialize and std array with a random length in range?

The short answer: you can't. std::array's size is a compile time constant. Pretty sure it's very difficult to derive a compile-time constant from some random value that's determined at run-time. — Sam Varshavchik
Yeah, you definitely can make a compile-time pseudo-random number generator (I challenge you to replicate std::mt19937 :P), seed it with e.g. __TIME__ and get the value from it. — HolyBlackCat
Compile time size - std::array, run time size - std::vector. — Bo Persson
By the way, in C++17, there is std::size that should be used instead. See en.cppreference.com/w/cpp/iterator/size. — Phil1970

Yakk - Adam Nevraumont Yakk - Adam Nevraumont · Accepted Answer · 2018-03-31T21:20:14

No; array's length must be a compile time constant, and there are no compile time random number engines in std C++ library.

If you want a runtime determined contiguous buffer of data, use std::vector. That it what it is for, and what it does.

If you want a non-std random number compile-time constant, choose 4. I rolled it using a fair die.

In theory you could use macros that encode the compile time of your code and write a compile time pseudo-random number generator based on that. Or you could use continuation passing style and templated code with a bound to compile some large number K of programs and choose which one to execute at runtime. But all of these are quite frankly bad ideas.

Heck, you could make your C++ program print a C++ program to a file, include a C++ compiler in your program, link against the output of the program you print dynamically, and run that code.

Initialize std::array with a random size

2 Answers

Run-Time Random Number Generation

Compile-Time Random Number Generation