如何在不退出 function 并随后将其存储在 c++ 的情况下返回一个值

Question

Here I'm trying to mimic data feed streaming from stock exchange using random number and subsequently store it in array.

#include <iostream>
#include <array>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <vector>
#include <fstream>
#include <cstdlib>
#include <chrono>
#include <ctime>
#include <stdlib.h>

long int prevcntr=0;

using namespace std;

std::pair<long int, double>feedgenerator(long int len)
{
    srand(time(0));
    for(long int itr=1;itr<=len;itr++)
    {
        return {itr, (double)rand()/RAND_MAX};
        //should continue evaluating the function without exiting
        pause(0.001); //To allow some interval for 'makearray' to store it
    }
}

template<size_t nn>
std::array<double, nn> makearray(long int cntr, double value, long int len)
{
    std::array<double, nn> outarr; // should be able to pass the value of 'len' to 'nn'
    long int itr=0;
    begin:
    while(cntr <= prevcntr)goto begin; // should wait until the next update
    outarr[itr] = value;
    prevcntr = cntr;
    while(itr<len)
    {
        itr++;
        goto begin; // control goes back to the beginning until all the elements of the array are filled with value
    }
    //should return the array after it is fully filled
    return outarr;
}

int main()
{
    double *p=new double[];
    long int len = 100000;
    *p = makearray(feedgenerator(len), len)

    // I should be able to call these as nested functions as above

    for(int i=0;i<len;i++)
    cout<<*p[i]<<"\n";

    return 0;
}

Question is how do I return a value without exiting the feedgenerator function. If I try to get all the values at once then it wouldn't mimic the data feed. Data feed is essentially, the same value being updated, sequentially.

To store the data, makearray is being used ( vector shouldn't be used as it is extremely slow).

Overall the idea is, feedgenerator should update the same value with an increasing counter (in the real scenario counter will be the time and value will be price etc.) and makearray should store the data (Unless I store the data, the data would be lost, as in the case of data feed from stock exchange) for subsequent analysis.

In makearray , I want to pass the length of the array as an argument of the function, so that it can be controlled by another program. How can it be done is not clear to me.

The code in it's present form, doesn't compile.

Answer 1

Question is how do I return a value without exiting the feedgenerator function

Because you are trying to mimic the data feed stream from stock exchange, so, I think you should consider to use thread. Example:

class FeedGenerator {
  long int max_length;
  std::deque<double> queue;
  std::mutex mutex;
  std::thread thread;
public:
  FeedGenerator(long int len) : max_length(len) {
    thread(&FeedGenerator::run, this);
  }

  // Get the latest `len` values
  int getData(std::vector<double>& vec, int len) {
    std::lock_guard<std::mutex>(mutex);
    int nlen = std::min(len, queue.size());
    vec.resize(nlen);
    std::copy(queue.end() - nlen, queue.end(), vec.begin());
    return nlen;
  }

private:
  void run() {
    srand(time(0));
    while (TRUE)
    {
      {
        std::lock_guard<std::mutex>(mutex);
        queue.push_back((double)rand()/RAND_MAX);
        if (queue.size() >= len) {
          queue.pop_front();
        }
      }
      pause(0.001);
    }
  }
};

int main()
{
  long int len = 100000;
  FeedGenerator feedgenerator(len);

  std::vector<double> p;
  feedgenerator.getData(p, 10); // get the latest 10 values

  for(int i=0;i<p.size();i++)
  std::cout << p[i] << std::endl;

  return 0;
}