Hello everyone! Today, I will guide you through implementing the powerful nested function feature in Excel using C++.
1. Basic Principles of Nested Excel Functions
Nested Excel functions have three characteristics:
- Hierarchical Calls: The parameter of one function can be another function
- Recursive Calculation: Calculation starts from the innermost function and gradually moves outward
- Type Matching: Ensure that the function parameter types are correct
To implement this functionality in C++, we need:
- Function Objects: Pass functions as parameters
- Recursive Parsing: Calculate nested expressions from the inside out
- Type System: Ensure parameter type matching
2. Basic Function Class Design
1. Base Function Class
#include <vector>
#include <memory>
#include <stdexcept>
class ExcelFunction {
public:
virtual ~ExcelFunction() = default;
// Calculate function result
virtual double evaluate() const = 0;
// Add argument
virtual void addArgument(std::shared_ptr<ExcelFunction> arg) {
arguments.push_back(arg);
}
protected:
std::vector<std::shared_ptr<ExcelFunction>> arguments;
};
2. Specific Function Implementation (Example: SUM)
class SumFunction : public ExcelFunction {
public:
double evaluate() const override {
double sum = 0.0;
for(const auto& arg : arguments) {
sum += arg->evaluate(); // Recursive calculation of each argument
}
return sum;
}
};
Tip:
- Use smart pointers to manage the lifecycle of function objects
<span>evaluate()</span>method is the core of recursion- Each argument is also a function object
3. Implementation of Function Nesting
1. Support for Constants and Cell References
class Constant : public ExcelFunction {
double value;
public:
Constant(double v) : value(v) {}
double evaluate() const override { return value; }
};
class CellReference : public ExcelFunction {
int row, col;
const std::vector<std::vector<double>>& sheet;
public:
CellReference(int r, int c, const std::vector<std::vector<double>>& s)
: row(r), col(c), sheet(s) {}
double evaluate() const override { return sheet[row][col]; }
};
2. Complex Nesting Example (AVERAGE Nested SUM)
int main() {
// Simulated worksheet data
std::vector<std::vector<double>> sheet = {
{1.0, 2.0},
{3.0, 4.0}
};
// Create nested function: AVERAGE(SUM(A1:B1), SUM(A2:B2))
auto avgFunc = std::make_shared<AverageFunction>();
// First SUM function
auto sum1 = std::make_shared<SumFunction>();
sum1->addArgument(std::make_shared<CellReference>(0, 0, sheet));
sum1->addArgument(std::make_shared<CellReference>(0, 1, sheet));
// Second SUM function
auto sum2 = std::make_shared<SumFunction>();
sum2->addArgument(std::make_shared<CellReference>(1, 0, sheet));
sum2->addArgument(std::make_shared<CellReference>(1, 1, sheet));
// Add to AVERAGE
avgFunc->addArgument(sum1);
avgFunc->addArgument(sum2);
// Calculate and output result
std::cout << "Calculation result: " << avgFunc->evaluate() << std::endl;
// Output: (1+2 + 3+4)/2 = 5
return 0;
}
4. Expression Parser
1. Simple Parser Implementation
#include <string>
#include <map>
class FunctionParser {
std::map<std::string, std::shared_ptr<ExcelFunction>> functionMap;
const std::vector<std::vector<double>>& sheet;
public:
FunctionParser(const std::vector<std::vector<double>>& s) : sheet(s) {
// Register known functions
functionMap["SUM"] = std::make_shared<SumFunction>();
functionMap["AVERAGE"] = std::make_shared<AverageFunction>();
// ...other functions
}
std::shared_ptr<ExcelFunction> parse(const std::string& expr) {
// Simple implementation: find function name and parameters
size_t paren = expr.find('(');
if(paren == std::string::npos) {
// Could be a constant or cell reference
if(expr[0] >= 'A' && expr[0] <= 'Z') {
// Parse cell reference like "A1"
int col = expr[0] - 'A';
int row = std::stoi(expr.substr(1)) - 1;
return std::make_shared<CellReference>(row, col, sheet);
} else {
// Parse constant
return std::make_shared<Constant>(std::stod(expr));
}
}
// Parse function call
std::string funcName = expr.substr(0, paren);
std::string argsStr = expr.substr(paren + 1, expr.size() - paren - 2);
// Create function object
auto func = functionMap.at(funcName);
// Parse arguments (simplified, does not consider nesting)
size_t start = 0, end;
while((end = argsStr.find(',', start)) != std::string::npos) {
std::string arg = argsStr.substr(start, end - start);
func->addArgument(parse(arg));
start = end + 1;
}
func->addArgument(parse(argsStr.substr(start)));
return func;
}
};
5. Advanced Functionality Extensions
1. Function Registration Mechanism
void registerFunction(const std::string& name,
std::function<std::shared_ptr<ExcelFunction>()> factory) {
functionFactories[name] = factory;
}
// Usage example
parser.registerFunction("MAX", []() {
return std::make_shared<MaxFunction>();
});
2. Cached Function Calculation
class CachedFunction : public ExcelFunction {
mutable double cache;
mutable bool hasCache = false;
public:
double evaluate() const override {
if(!hasCache) {
cache = ExcelFunction::evaluate();
hasCache = true;
}
return cache;
}
void invalidateCache() { hasCache = false; }
};
3. Circular Reference Detection
class SafeFunction : public ExcelFunction {
mutable std::unordered_set<ExcelFunction*> callStack;
public:
double evaluate() const override {
if(callStack.count(this)) {
throw std::runtime_error("Circular reference detected");
}
callStack.insert(this);
double result = ExcelFunction::evaluate();
callStack.erase(this);
return result;
}
};
6. Considerations
-
Memory Management:
- Use smart pointers to avoid memory leaks
- Circular references may prevent memory from being released
Performance Optimization:
// Pre-compile expression
auto compiledExpr = parser.parse("SUM(A1:A100)");
// Use the compiled expression directly for multiple calculations
Error Handling:
- Check if the number of parameters matches
- Validate if parameter types are legal
- Prevent infinite recursion
7. Hands-On Practice
Try these interesting exercises:
- Implement a logical judgment function that supports the IF function
- Add support for the VLOOKUP function
- Create an expression editor to visually build nested functions
- Implement a function calculation performance analysis tool
Conclusion
Today we learned about:
- The recursive implementation principles of function nesting
- The basic design of an expression parser
- How to avoid common pitfalls (such as circular references)
- Performance optimization and extension techniques
Function nesting is the core of Excel’s powerful calculation capabilities. Mastering this technique will make your programs more flexible. It is recommended to start with simple arithmetic functions and gradually add more complex features.