高中生程式解題系統：五則運算

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題目連結 http://zerojudge.tw/ShowProblem?problemid=a017。

此題用 Python 的 eval 函數會比較好解。

用C++的話可以先將中序運算式(Infix expression)轉成後序運算式(Postfix expression)在做求值，為Stack的應用，不過此處所提供的程式碼是直接對中序運算式做求值的。用Stack的細節可參考 四則運算 VS Stack 一文。

C++ 程式碼：

	#include <iostream>
	#include <string>
	using namespace std;
	void eatspaces(string &str); // Function to eliminate blanks
	int expr(string &str); // Function evaluating an expression
	int term(string &str, int& index); // Function analyzing a term
	int number(string &str, int& index); // Function to recognize a number
	string extract(string &str, int& index); //Function to extract a substring
	int main(void)
	{
	string buffer; // Input area for expression to be evaluated
	while( getline(cin, buffer) )
	{
	eatspaces(buffer); // Remove blanks from input
	cout << expr(buffer) // Output value of expression
	<< endl;
	//cout << buffer << endl;
	}
	}
	// Function to eliminate blanks from a string
	void eatspaces(string &str)
	{
	for(unsigned i = 0; i < str.length(); i++)
	{
	if(str[i] == ' ')
	str.erase(i, 1);
	}
	}
	// Function to evaluate an arithmetic expression
	int expr(string &str)
	{
	int value = 0; // Store result here
	int index = 0; // Keeps track of current character position
	value = term(str, index); // Get first term
	for(;;) // Infinite loop, all exits inside
	{
	switch(str[index++]) // Choose action based on current character
	{
	case '\0': // We're at the end of the string
	return value; // so return what we have got
	case '+': // + found so add in the
	value += term(str, index); // next term
	break;
	case '-': // - found so subtract
	value -= term(str, index); // the next term
	break;
	default: // If we reach here the string
	cout << endl // is junk
	<< "Arrrgh!*#!! There's an error"
	<< endl;
	exit(1);
	}
	}
	}
	// Function to get the value of a term
	int term(string &str, int& index)
	{
	int value = 0; // Somewhere to accumulate the result
	value = number(str, index); // Get the first number in the term
	// Loop as long as we have a good operator
	while( (str[index]=='*') || (str[index]=='/') || (str[index]=='%') )
	{
	if(str[index]=='*') // If it's multiply,
	value *= number(str, ++index); // multiply by next number
	if(str[index]=='/') // If it's divide,
	value /= number(str, ++index); // divide by next number
	if(str[index]=='%')
	value = value % number(str, ++index);
	}
	return value; // We've finished, so return what we've got
	}
	// Function to recognize an expression in parentheses
	// or a number in a string
	int number(string &str, int& index)
	{
	int value = 0; // Store the resulting value
	if(str[index] == '(') // Start of parentheses
	{
	string psubstr; // Pointer for substring
	psubstr = extract(str, ++index); // Extract substring in brackets
	//cout << "psubstr: " << psubstr << endl;
	value = expr(psubstr); // Get the value of the substring
	return value; // Return substring value
	}
	//cout << "number value: " << value << endl;
	while(isdigit(str[index])) // Loop accumulating leading digits
	value=10*value + ((str[index++]) - 48);
	// Not a digit when we get to here
	return value; // On loop exit we are done
	}
	// Function to extract a substring between parentheses
	// (requires string)
	string extract(string &str, int& index)
	{
	string pstr; // Pointer to new string for return
	int numL = 0; // Count of left parentheses found
	int bufindex = index; // Save starting value for index
	do
	{
	switch(str[index])
	{
	case ')':
	if(numL==0)
	{
	++index;
	pstr = str.substr(bufindex, index - bufindex-1);
	//str.erase(bufindex - 1, index - bufindex + 1);
	//cout << "extract pstr: " << pstr << endl;
	//cout << "str: " << str << endl;
	return pstr; // Return substring in new memory
	}
	else
	numL--; // Reduce count of '(' to be matched
	break;
	case '(':
	numL++; // Increase count of '(' to be matched
	break;
	}
	} while(index++ < str.length());// Loop - don't overrun end of string
	cout << "Ran off the end of the expression, must be bad input."
	<< endl;
	exit(1);
	return pstr;
	}

view raw a017.cpp hosted with ❤ by GitHub

Python 程式碼：

import sys

for s in sys.stdin:
    print(eval(s.replace("/", "//")))