fill vector with random elements

Question

Why my lambda doesn't put elements in vector?

void generate_N_numbers_in_the_vector(std::vector<int> &vec, int &M,
                                      const int &N) {
  std::random_device rand;
  static std::mt19937 gen(rand());
  static std::uniform_int_distribution<int> distribution(0, M);
  vec.reserve(N);

  std::generate(vec.begin(), vec.end(), []() { return distribution(gen); });

  for (auto elem : vec) std::cout << elem << std::endl;
}

Does this answer your question? [Choice between vector::resize() and vector::reserve()](https://stackoverflow.com/questions/7397768/choice-between-vectorresize-and-vectorreserve) — Evg, Jan 08 '20 at 19:21
In modern C++ you better return that vector rather than pass it by reference - due to move semantics it is not less efficient but much more readable. For example it may solve your issue as a side effect (you probably would use proper ctor with size) — Slava, Jan 08 '20 at 19:26
@Slava, it can be less efficient if this function is called repeatedly on the same vector. — Evg, Jan 08 '20 at 19:29
@Evg then subject to the question should be "add random elements to a vector" instead of "fill vector..." — Slava, Jan 08 '20 at 19:33

n314159 · Answer 1 · 2020-01-08T19:31:19.367

4

When you use reserve the number of elements in the vector does not increase. That means that the difference between begin(vec) and end(vec) is 0 (or whatever the size of your vector was at the start). There are two possibilities:

vec.resize(N);
std::generate(vec.begin(), vec.end(), [](){return distribution(gen);});
// or
vec.reserve(N); // Not necessary but sensible.
std::generate_n(std::back_inserter(vec), N, [](){return distribution(gen);});

edited Jan 08 '20 at 19:31

answered Jan 08 '20 at 19:23

n314159

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[Can't capture a static variable in a lambda](https://stackoverflow.com/q/45930423) – Evg Jan 08 '20 at 19:26
"That means that the difference between begin(vec) and end(vec) is 0" How do you know this?... – Aykhan Hagverdili Jan 08 '20 at 19:26
@Ayxan *When you use reserve the number of elements in the vector does not increase.* – NathanOliver Jan 08 '20 at 19:28
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@NathanOliver how do you know the vector was empty to begin with? – Aykhan Hagverdili Jan 08 '20 at 19:28
From the name of the of the function, I assume OP wants the vector to have n random numbers. In which case your second suggestion is outright wrong. – Aykhan Hagverdili Jan 08 '20 at 19:28
1

@Ayxan Because if it was, the user would be complaining that it only generated some numbers. – NathanOliver Jan 08 '20 at 19:29
Thx @Evg I missed the static. I already wondered why OPs code even compiles. – n314159 Jan 08 '20 at 19:31
In the second solution I suggest you clear it first. The function name suggests it should be cleared. – Evg Jan 08 '20 at 19:35
@NathanOliver there could be other reasons. For instance, `std::cout` could have already failed. – Aykhan Hagverdili Jan 08 '20 at 19:35

score 1 · Answer 2 · answered Jan 08 '20 at 19:22

1

std::vector::reserve only reserves space, meaning increases the capacity, not the size. Use std::vector::resize instead to change the vector's size.

answered Jan 08 '20 at 19:22

Aykhan Hagverdili

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fill vector with random elements

2 Answers2