[英]Prefix Calculator — won't work for some cases — C++
我正在創建一個前綴計算器,用戶輸入前綴表達式,程序對其進行評估。 它適用於某些情況,例如“+43”輸出7應該是,但“/ -421”輸出“2”,當它應輸出“1”,而“/ + 421”輸出“6”而不是“ 3“,這樣的事情。 有任何修復建議嗎? 我稍后會添加例外,所以他們暫時被注釋掉了。
PrefixCalculator.cpp
#pragma once
#include <sstream>
using namespace std;
template<class T>
class PrefixCalculator {
public:
PrefixCalculator(void){
numOperator = 0;
numOperand = 0;
};
~PrefixCalculator(void){};
T eval(istringstream&);
int getNumOperator() {
return numOperator;
};
int getNumOperand() {
return numOperand;
};
private:
//if you feel you need private helper functions and/or helper data
int numOperator;
int numOperand;
};
template<class T>
T PrefixCalculator<T>::eval(istringstream& input) {
//this function needs to throw an exception if there's a problem with the expression or operators
char nextChar = input.peek();
//this while loop skips over the spaces in the expression, if there are any
while(nextChar == ' ') {
input.get(); //move past this space
nextChar = input.peek(); //check the next character
}
if(nextChar == '+') {
input.get(); //moves past the +
numOperator++;
return eval(input) + eval(input); //recursively calculates the first expression, and adds it to the second expression, returning the result
}
/***** more operators here ******/
if(nextChar == '-') {
input.get();
numOperator++;
return eval(input) - eval(input);
}
if(nextChar == '*') {
input.get();
numOperator++;
return eval(input) * eval(input);
}
if(nextChar == '/') {
input.get();
numOperator++;
return eval(input) / eval(input);
}
/****** BASE CASE HERE *******/
//it's not an operator, and it's not a space, so you must be reading an actual value (like '3' in "+ 3 6". Use the >> operator of istringstream to pull in a T value!
input>>nextChar;
T digit = nextChar - '0';
numOperand++;
return digit;
//OR...there's bad input, in which case the reading would fail and you should throw an exception
}
driver.cpp
#include <sstream>
#include <string>
#include <iostream>
#include "PrefixCalculator.h"
using namespace std;
int main(int argc, char** argv) {
PrefixCalculator<int> calc;
string expression;
cout << "Give a prefix expression to evaluate, or q to quit." << endl;
getline(cin,expression);
while(expression[0] != 'q') {
//try {
int result = calc.eval(istringstream(expression));
cout << result << endl;
//}
//catch { //will not compile, you have to finish this!
//
//}
cout << "Give a prefix expression to evaluate or q to quit." << endl;
getline(cin,expression);
}
return 0;
}
我糊塗了。 根據您提供的輸出,您的程序運行正常,但您的期望不正確。
讓我們評估表達式“/ -421”:
1.操作員檢測到'/',推入堆棧:
+----+
| / |
+====+
操作員檢測到' - ',推入堆棧:
+---+
| - |
+---+
| / |
+===+
檢測到的數字,因為之前的操作員需要2個參
+---+
| 4 |
+---+
| - |
+---+
| / |
+===+
檢測到的數字,兩個運營商中的第二個。
將剩余的操作數彈出堆棧,4。
從堆棧彈出操作評估參數:
result = 4 - 2 --> 1
將結果推到堆棧上。
+---+
| 2 |
+---+
| / |
+===+
檢測到的數字,除法運算符的第2個參數2。
彈出堆棧中的數字,這將成為除法運算的第一個參數。
從堆棧彈出操作並使用參數進行評估:
result = 2 / 1 --> 2
將結果推入堆棧。
+---+
| 2 |
+===+
達到結束表達式,彈出結果並打印:
2
編輯1:
您可以在分析下一個令牌之前打印堆棧(每行一個項目)來確認程序的操作。
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