如何使用 ofstream C++ 替换文本文件中的特定值？
How replace specific value from textfile using ofstream c++?
您好，我在自动售货机上工作，我想通过更新文本文件来更新商品数量。我一直在尝试使用 ofstream 和 ifstream 但没有用。
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
这是自动售货机检查用户输入的地方，也是我要更新文件的地方
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});
}
}
}
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
- 将自动售货机的商品内容写入文件
修改 以某种方式 如何工作？由于您无法使用任意新数据在线更改文件，因此您需要这样做：
将文件读入内存-->对内存中的数据进行操作-->将修改后的数据保存到文件中
以上有2种方法。
- 打开文件-->读取数据-->关闭文件-->修改内存中的数据-->打开文件覆盖原文件输出-->保存数据-->关闭文件
或者，更安全一点：
- 打开文件-->读取数据-->关闭文件-->修改内存中的数据-->打开临时文件输出-->保存数据到临时文件-->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，在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，您可以在构造函数中“加载”数据，然后在析构函数中“保存”数据。然后一切都会自动工作。
关于“加载”功能。您需要逐行读取源文件，然后将该行拆分为您需要的数据成员。我已经回答了一个问题 ，它描述了 4 种不同的分割线的方法。在下面给出的示例中，我使用 std::regex_match 基于 std::regex 的解决方案。这将确保数据采用预期格式。
请注意，您还应该覆盖提取器和插入器运算符 >> 和 <<，以便更轻松地处理流。
最后但同样重要的是，所有内容都应封装在 类.
中
请查看部分实现的自动售货机功能的有效且经过测试的示例代码。在此代码中，我使用了 C++17 功能，例如 if 和初始化程序。所以，如果你想编译，请为你的编译器启用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。最后你需要想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(), 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;
}
您好，我在自动售货机上工作，我想通过更新文本文件来更新商品数量。我一直在尝试使用 ofstream 和 ifstream 但没有用。
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
这是自动售货机检查用户输入的地方，也是我要更新文件的地方
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});
}
}
}
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
- 将自动售货机的商品内容写入文件
修改 以某种方式 如何工作？由于您无法使用任意新数据在线更改文件，因此您需要这样做：
将文件读入内存-->对内存中的数据进行操作-->将修改后的数据保存到文件中
以上有2种方法。
- 打开文件-->读取数据-->关闭文件-->修改内存中的数据-->打开文件覆盖原文件输出-->保存数据-->关闭文件
或者，更安全一点：
- 打开文件-->读取数据-->关闭文件-->修改内存中的数据-->打开临时文件输出-->保存数据到临时文件-->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，在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，您可以在构造函数中“加载”数据，然后在析构函数中“保存”数据。然后一切都会自动工作。
关于“加载”功能。您需要逐行读取源文件，然后将该行拆分为您需要的数据成员。我已经回答了一个问题 std::regex_match 基于 std::regex 的解决方案。这将确保数据采用预期格式。
请注意，您还应该覆盖提取器和插入器运算符 >> 和 <<，以便更轻松地处理流。
最后但同样重要的是，所有内容都应封装在 类.
中请查看部分实现的自动售货机功能的有效且经过测试的示例代码。在此代码中，我使用了 C++17 功能，例如 if 和初始化程序。所以，如果你想编译，请为你的编译器启用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。最后你需要想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(), 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;
}