[英]How replace specific value from textfile using ofstream c++?
嗨,我正在使用自動售貨機,我想通過更新文本文件來更新商品的數量。 我一直在嘗試使用 ofstream 和 ifstream 但不起作用。
這是我的文本文件。
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
這是自動售貨機檢查用戶輸入以及我想更新文件的地方
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});
}
}
}
你想做的是:
修改如何以某種方式工作? 由於您無法使用任意新數據在線更改文件,因此您需要這樣做:
將文件讀入內存 --> 對內存中的數據進行操作 --> 將修改后的數據保存到文件中
對於上述有兩種方法。
或者,更安全一點:
但關鍵是,要處理內存中的數據。
您還可以創建“加載”和“保存”功能。 因此,在更改內存中的數據后,您可以隨時“保存”修改后的數據。 使用上述方法之一。
或者,您可以在構造函數中“加載”數據並將其“保存”在析構函數中。 然后一切都會自動運行。
關於“加載”功能。 您需要逐行讀取源文件,然后將該行拆分為所需的數據成員。 我在這里回答了一個問題,它描述了 4 種不同的分割線的方法。 在下面給出的示例中,我使用基於std::regex
的解決方案std::regex_match
。 這將確保數據采用預期的格式。
請注意,您還應該覆蓋提取器和插入器運算符>>
和<<
以便更輕松地處理流。
最后但並非最不重要的一點是,一切都應該封裝在類中。
請參閱部分實現的自動售貨機功能的工作和測試示例代碼。 在這段代碼中,我使用了 C++17 特性,比如if
with initializer。 所以,如果你想編譯,那么請為你的編譯器啟用 C++17。
此外,這只是一些代碼來說明上述解釋。 有100萬個解決方案。 最后,您需要提出一些符合要求的東西。
#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(), products.end(),[&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;
}
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