问题描述
嗨,我正在使用自动售货机,我想通过更新文本文件来更新商品的数量。我一直在尝试使用ofstream和ifstream,但无法正常工作。
这是我的文本文件。
Water:1:1.99:D1
Coke:4:2.79:D2
Milk:6:3.15:D3
Ham Sandwitch:9:4.50:L1
Lunchables:3:6.00:L2
Cereal:2:3.59:L3
M&M:8:1.75:C1
SourPatch:0:2.10:C2
Twix:6:2.99:C3
void vendingWorking(Item &item) {
if(item.quantity == 0) {
cout << endl;
cout << "------------------------------------" << "\n";
cout << "" << item.name << " (OutStock)" << endl;
cout << "------------------------------------" << "\n";
}
else {
//Check if itemCode is same as product ID
if(itemCode == item.productId) {
//HERE I WANT TO UPDATE THE QUANTITY OF THE ITEM IF USER HAS PICKED ONE
//EXAMPLE: (Old) Water:2:2.50:D1 -> (New) Water:1:2.50:D1
//Message for user
cout << endl;
cout << "------------------------------------" << "\n";
cout << "" << item.name << ",$" << fixed << setprecision(2)<< item.price << " (InStock)" << "\n" ;
//Pass value to vector
tempBasket.push_back({item.name,item.price});
}
}
}
解决方法
您想做的是:
- 从文件中读取自动售货机的产品内容
- 以某种方式修改数据
- 将自动售货机的产品内容写入文件
如何修改 的工作方式?由于您无法使用任意新数据在线更改文件,因此需要执行以下操作:
将文件读入内存->处理内存中的数据->将修改后的数据保存在文件中
以上介绍了两种方法。
- 打开文件->读取数据->关闭文件->修改内存中的数据->通过覆盖原始文件打开文件以供输出->保存数据->关闭文件
或者,更安全一些:
- 打开文件->读取数据->关闭文件->在内存中修改数据->打开临时文件进行输出->将数据保存在临时文件中->关闭临时文件->如果一切正常,删除原始文件->将临时文件重命名为原始文件名
但是关键是要处理内存中的数据。
您还可以创建“加载”和“保存”功能。因此,在任何时候,更改内存中的数据后,您都可以“保存”修改后的数据。使用上述方法之一。
或者,您可以将数据“加载”到构造函数中,然后“保存”在析构函数中。一切都会自动运行。
关于“加载”功能。您需要逐行读取源文件,然后将该行拆分为所需的数据成员。我已经回答了一个问题here,该问题描述了有关如何分割线的4种不同方法。在下面给出的示例中,我使用基于std::regex
的基于std::regex_match
的解决方案。这样可以确保数据采用预期的格式。
请注意,您还应该覆盖提取器和插入器运算符>>
和<<
,以更轻松地使用流。
最后但并非最不重要的一点是,所有内容都应封装在类中。
请参阅有效且经过测试的示例代码,以了解部分实现的自动售货机功能。在这段代码中,我使用的是C ++ 17功能,例如带有初始化程序的if
。因此,如果要编译,请为编译器启用C ++ 17。
此外,这只是一些代码来说明上面的解释。有100万个解决方案。最后,您需要提出一些符合要求的东西。
#include <iostream>
#include <fstream>
#include <string>
#include <iterator>
#include <vector>
#include <regex>
#include <algorithm>
#include <numeric>
const std::regex re{ R"(^([^:]+):(\d+):(\d+\.\d+):([A-Z]+\d+))" };
class VendingMachine {
// Local definition of item struct
struct Item {
// Item attributes
std::string name{};
unsigned long quantity{};
double price{};
std::string productID{};
// Simple overwrite of extractor operator
friend std::istream& operator >> (std::istream& is,Item& it) {
// Read a complete line and check,if that worked
if (std::string line{}; std::getline(is,line)) {
// Check,if the input line,is in the expected format
if (std::smatch sm{}; std::regex_match(line,sm,re)) {
it.name = sm[1];
it.quantity = std::stoul(sm[2]);
it.price = std::stod(sm[3]);
it.productID = sm[4];
}
else std::cerr << "\n***Error while reading: '" << line << "'\n'";
}
return is;
}
// Simple overwrite of inserter operator
friend std::ostream& operator << (std::ostream& os,const Item& it) {
return os << it.name << ':' << it.quantity << ':' << it.price << ':' << it.productID;
}
};
// All products in vending machine
std::vector<Item> products{};
// Filename for saving and loading
std::string fileName{ "products.txt" };
public:
// Constructor and Destructor
// Constructor will load the data from a file
VendingMachine() { load(); }; // Default constructor
VendingMachine(const std::string& fn) : fileName(fn) { load(); }; // Constructor + file name
// Destructor will automatically save product file
~VendingMachine() { save(); };
// Simple overwrite of extractor operator
friend std::istream& operator >> (std::istream& is,VendingMachine& vm) {
// Delete all existing products
vm.products.clear();
// Copy all data from stream into internal structure
std::copy(std::istream_iterator<Item>(is),{},std::back_inserter(vm.products));
return is;
}
// Simple overwrite of extractor operator
friend std::ostream& operator << (std::ostream& os,const VendingMachine& vm) {
// Copy all data to stream
std::copy(vm.products.begin(),vm.products.end(),std::ostream_iterator<Item>(os,"\n"));
return os;
}
// Load file from file
void load() {
// Open file and check,if it could be opened
if (std::ifstream ifs(fileName); ifs) {
// Use existing extractor operator
ifs >> *this;
}
else std::cerr << "\n***Error: Could not open file '" << fileName << "' for reading\n";
}
// Save products to file
void save() {
// Open file and check,if it could be opened
if (std::ofstream ofs(fileName); ofs) {
// Use existing inserter operator
ofs << *this;
}
else std::cerr << "\n***Error: Could not open file '" << fileName << "' for writing\n";
}
// Show the complete content of the vending machine. Even if one product category quantity is 0
void displayContent() {
// Some header line
std::cout << "\nNumber of selections in vending machine: " << products.size() << "\n\nProducts:\n\n";
// All Items wit their attributes
for (const Item& item : products)
std::cout << item.productID << "\t Quantity: " << item.quantity << "\t Price: " << item.price << "\t --> " << item.name << '\n';
}
// Select an item and the decrease quatnity
void getItem() {
// COunt the number of overall items in the vending maschine
const unsigned long overallItemQuantity = std::accumulate(products.begin(),products.end(),0UL,[](size_t sum,const Item& it) {return sum + it.quantity; });
// If there are at all products in the machine and not all item quantity is 0
if (products.size() && overallItemQuantity > 0UL ) {
// Instruction from user
std::cout << "\n\nGet item\nPlease select from below list:\n\n";
// Show list of possible selections
for (const Item& item : products) {
if (item.quantity > 0UL) std::cout << item.productID << " \tPrice " << item.price << " \t--> " << item.name << '\n';
}
// Get user input. What item does the user want to have
std::cout << "\n\nPlease select product by typing the ID: ";
if (std::string id{}; std::getline(std::cin,id)) {
// FInd the selected item in the product list
if (std::vector<Item>::iterator iter{ std::find_if(products.begin(),[&id](const Item& i) {return i.productID == id && i.quantity > 0UL; }) };iter != products.end())
// In my example I do not handle payment. Simply decrease quantity
--iter->quantity;
else
std::cerr << "\n\n***Error: Unknown product ID\n"; // Wrong input
}
}
else std::cerr << "\n\n***Error: Vending machine empty\n";
}
// Run the machine. Main menu and actions. At the moment kust get items without payment
// Needs to be extended for real application
void run() {
// We run the main menu in a loop as long as the machine is active
bool active{ true };
while (active) {
// Show main menu
std::cout << "\n\n\nMain menu. Please select:\n 1 --> Get Item\n 0 --> Exit\n\nOption: ";
// Get user selection
unsigned int option; std::cin >> option;
std::cin.ignore(std::numeric_limits<std::streamsize>::max(),'\n');
// Depending on the user selected action
switch (option) {
case 0:
// Leave function.
active = false;
std::cout << "\n\nExiting . . .\n";
break;
case 1:
// Get an item
std::cout << "\n";
getItem();
break;
default:
std::cout << "\n\n\nError: Wrong selection. Please try again\n";
break;
}
}
}
};
int main() {
// Define a Vending Machine. Read data from disk
VendingMachine vendingMachine;
// SHow what is in initially
vendingMachine.displayContent();
// Run the machine
vendingMachine.run();
// Show,what is now in the machine
vendingMachine.displayContent();
// Destructor of vendingMachine will be called and file automatically saved
return 0;
}