對元素執行計算，并檢索索引向量中元素的索引

在這里，我試圖對向量中的元素進行計算和比較，找到平均值、較低值、較高值和差異。我也有列印向量中元素索引的問題。

#include <iostream>
#include <string>
#include <vector>

enum class OrderBookType{bid, ask};

class OrderBookEntry{
public:
    double price;
    double amount;
    std::string timestamp;
    std::string product;
    OrderBookType orderType;
    
    double computeAveragePrice(std::vector<OrderBookEntry>& entries);
    
    OrderBookEntry(double _price,
                   double _amount,
                   std::string _timestamp,
                   std::string _product,
                   OrderBookType _orderType):
    price(_price),
    amount(_amount),
    timestamp(_timestamp),
    product(_product),
    orderType(_orderType) {
    }
        
//*    Other kinds of functions I hope to implement for this program. */
//    double computeLowPrice(std::vector<OrderBookEntry>& entries);
//    double computeHighPrice(std::vector<OrderBookEntry>& entries);
//    double computePriceSpread(std::vector<OrderBookEntry>& entries);

};

double OrderBookEntry::computeAveragePrice(std::vector<OrderBookEntry> &entries) {
        return 5.5; // Just trying to test a for an output here
}

int main() {
    // Declaring vector entries
    std::vector<OrderBookEntry> entries;
        
    // Adding entries into entries
    entries.push_back(OrderBookEntry{
        10000,
        0.001,
        "2020/03/17 17:01:24.88492",
        "BTC/USDT",
        OrderBookType::bid
    });
    entries.push_back(OrderBookEntry{
        20000,
        0.002,
        "2020/03/17 17:01:24.88492",
        "BTC/USDT",
        OrderBookType::bid
    });

    // Prints prices of all entries
    for (OrderBookEntry& entry : entries) {
        std::cout << "The price entry " << &entry << " is " << entry.price << std::endl;
    }
    
    std::cout << "The lower price is " << entries.computeAveragePrice() << std::endl;
        
    return 0;
}

我在這里遇到了三個問題。

首先，我無法呼叫 computeAveragePrice() 函式來獲取它回傳的值——在這種情況下，只有 5.5 的值用于測驗。

上圖是下面代碼行的錯誤訊息。

std::cout << "The lower price is " << entries.computeAveragePrice() << std::endl;

其次，當我嘗試列印向量中元素的索引時，我得到了一個奇怪的值。

以上是我得到的輸出。我期待獲得元素的相應索引，但得到了這個。這是我用來遍歷向量的代碼。

for (OrderBookEntry& entry : entries) {
    std::cout << "The price entry " << &entry << " is " << entry.price << std::endl;
}

Lastly, I could use some reference to any article that teaches me how to perform arithmetic on values within a vector such as averages, comparisons and finding differences.

Thank you in advance!

uj5u.com熱心網友回復：

computeAveragePrice是成員函式 inOrderBookEntry但條目是 a std::vector<OrderBookEntry>，而不是 aOrderBookEntry所以它沒有這樣的成員函式。

我建議你computeAveragePrice離開OrderBookEntry這個只有一個單身資訊的班級OrderBookEntry。您可以創建另一個類來保存一個集合OrderBookEntry并將其放在那里。這個類可以是一個簡單的包裝器std::vector<OrderBookEntry>：

例子：

class OrderBook {   // A class to store a collection of OrderBookEntry:s
public:
    // A function to add an `OrderBookEntry`. You'll have to make this in
    // in the way you want. This is just an example:

    template<class... Args>
    decltype(auto) add(Args&&... args) {
        return entries.emplace_back(std::forward<Args>(args)...);
    }
    
    double computeAveragePrice() const;              // move it here

    // some member functions that just call the vector's member functions:
    size_t size() const { return entries.size(); }

    OrderBookEntry& operator[](size_t idx) { return entries[idx]; }
    const OrderBookEntry& operator[](size_t idx) const { return entries[idx]; }

    auto cbegin() const { return entries.cbegin(); }
    auto cend() const { return entries.cend(); }
    auto begin() const { return entries.cbegin(); }
    auto end() const { return entries.cend(); }    
    auto begin() { return entries.begin(); }
    auto end() { return entries.end(); }

private:
    std::vector<OrderBookEntry> entries;    // put the vector in here
};

double OrderBook::computeAveragePrice() const {
    // you need to #include <numeric> to use the accumulate function:
    if(entries.empty()) return NAN;   // not-a-number
    return std::accumulate(entries.begin(), entries.end(), 0.,
                           [](double tot, const OrderBookEntry& e ) {
                               return tot   e.price;
                           }) / entries.size();
}

有了這個，你main會看起來像下面這樣：

int main() {
    OrderBook entries;            // use the new collection class
        
    // Adding entries into entries
    entries.add(                  // using the add() member function
        10000,
        0.001,
        "2020/03/17 17:01:24.88492",
        "BTC/USDT",
        OrderBookType::bid
    );

    entries.add(
        20000,
        0.002,
        "2020/03/17 17:01:24.88492",
        "BTC/USDT",
        OrderBookType::bid
    );

    // Prints prices of all entries
    for (OrderBookEntry& entry : entries) {
        std::cout << "The price entry " << &entry << " is " << entry.price << '\n';
    }
    
    std::cout << "The average price is " << entries.computeAveragePrice() << '\n';
        
    return 0;
}

請注意，列印&entry將列印OrderBookEntry. 如果您想列印一些有意義的事情，你需要添加的過載operator<<，它接受一個OrderBookEntry作為輸入。

例子：

std::ostream& operator<<(std::ostream& os, const OrderBookEntry& e) {
    return os << e.product;
}

然后您可以列印條目，但不能通過獲取其地址：

for (const auto& entry : entries) {
    std::cout << "The price entry " << entry << " is " << entry.price << '\n';
    //                                 ^^^^^
}

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