將文件數據存儲到結構中的數組成員之一

Question

我有一個文本文件：

3 2.50 15.00 1.20

2 26.70 5.30

5 40.00 3.50 2.90 71.20 5.30

1 3.86

4 232.30 39.29 9.30 94.32

第一條垂直線是商品數量，然后是每件商品的價格。

我有一個結構：

struct Buyers
{
    int iItemCount;
    double arItemPrice[]; //array stores all items purchased by a buyer
};

問題：

1. 如何將買家的所有商品價格存儲到arItemPrice數組中？

Answer 1

您需要使用動態增長的容器，例如買家結構中的std::vector 。

這是因為你事先不知道一行中有多少個價格。 此外，您需要在文本文件中了解這一點； 元素的數量是冗余信息。 我們可以看到，有多少價格在排隊。 std::vector也知道它的大小。 因此，結構中的“itemCount”也是多余的。

使用 iostream 函數讀取很簡單。 我們為 Buyer 結構重載了提取器操作符。 在此，我們首先讀取一行，然后讀取該行中的單個元素。

請在此處查看許多可能的解決方案之一。

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <vector>

// Define a struct for buyers. With number of prices and proces
struct Buyer {
    unsigned int itemCount{};

    // Becuase we do not know in advance, how many elements will be in the container, we use a vector
    // This can grow dynamically
    std::vector<double> itemPrice{};
};

// This will read one line of Buyer information into our Buyer struct
std::istream& operator >> (std::istream& is, Buyer& b) {

     // Read excatly one line from the stream
    if (std::string line{}; getline(is, line)) {

        // Put the string into a std::istringstream for further extraction of data
        std::istringstream iss(line);

        // Initialize our internal vector to empty
        b.itemPrice.clear();

        // Read the item count
        iss >> b.itemCount;

        // Read in a loop all prices and append them to our vector
        for (double price{}; iss >> price; b.itemPrice.push_back(price))   ;
    }
    return is;
}

// The source text file with data (is the same handling as an fstream). Like a file in memory.
std::istringstream textFile{ R"(3 2.50 15.00 1.20
2 26.70 5.30
5 40.00 3.50 2.90 71.20 5.30
1 3.86
4 232.30 39.29 9.30 94.32
)" };


int main() {
    // We will save all buyers in this  dynamically growing vector
    std::vector<Buyer> buyers{};

    // Read all byuers in a loop
    for (Buyer b{}; textFile >> b; buyers.push_back(b));

    return 0;
}

添加了帶有new版本和不帶有std::vector的版本

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <vector>

// Define a struct for buyers. With number of prices and proces
struct Buyer {
    unsigned int itemCount{};
    double* itemPrice{};

    ~Buyer() { delete[] itemPrice; }
};

// This will read one line of Buyer information into our Buyer struct
std::istream& operator >> (std::istream& is, Buyer& b) {

     // Read excatly one line from the stream
    if (std::string line{}; getline(is, line)) {

        // Put the string into a std::istringstream for further extraction of data
        std::istringstream iss(line);

        // Initialize our internal vector to empty
        b = {};

        // Read the item count
        iss >> b.itemCount;

        // Allocate new memory
        b.itemPrice = new double[b.itemCount]();

        // Read in a loop all prices and append them to our vector
        for (unsigned int i{}; i < b.itemCount; ++i)
            iss >> b.itemPrice[i];
    }
    return is;
}

// The source text file with data (is the same handling as an fstream). Like a file in memory.
std::istringstream textFile{ R"(3 2.50 15.00 1.20
2 26.70 5.30
5 40.00 3.50 2.90 71.20 5.30
1 3.86
4 232.30 39.29 9.30 94.32
)" };

int main() {
    // Use a static array with some numbers
    Buyer buyers[20];

    // Read all buyers in a loop
    unsigned int i = 0;
    while (textFile >> buyers[i] && i<20) {
        std::cout << "Line " << i + 1 << ". Count: " << buyers[i].itemCount << "\n";
        for (unsigned int k = 0; k < buyers[i].itemCount; ++k) std::cout << buyers[i].itemPrice[k] << "  ";
        std::cout << "\n";
        ++i;
    }

    // Release allocated memory
    for (unsigned int k = 0; k < i-1; ++k) {
        buyers[i].~Buyer();
    }
    return 0;
}