[英]Infix To Prefix Mathematical Expression Converter in Java
我正在嘗試編寫一種將Infix轉換為Prefix Mathematical Expression的方法,並為此使用了堆棧。 但是在某些情況下我會出錯,而且我不知道這是什么問題。
碼:
import java.io.BufferedReader;
import java.io.FileReader;
import java.util.Stack;
import java.util.StringTokenizer;
public class Prefixer {
public static void main(String[] args) {
String fileName;
boolean reduce = false;
if (args.length == 1){
fileName = args[0];
}
else if (args.length >= 2){
reduce = args[0].equals("-r");
fileName = args[1];
}
else
{
return;
}
BufferedReader reader;
String line = "";
try {
reader = new BufferedReader(new FileReader (fileName));
line = reader.readLine();
line = line.trim();
} catch (Exception e) {
e.printStackTrace();
}
System.out.println(infixToPrefixConvert(line,reduce));
}
public static boolean isOperand(String s) {
return !(s.equals("+") || s.equals("-") || s.equals("/") || s.equals("*") || s.equals("(") || s.equals(")"));
}
public static boolean isNumber(String s){
try {
Integer.parseInt(s.trim());
} catch (Exception e){
return false;
}
return true;
}
public static String operationCombine(Stack<String> operatorStack, Stack<String> operandStack, boolean reduce){
String operator = operatorStack.pop();
String rightOperand = operandStack.pop();
String leftOperand = operandStack.pop();
if (reduce && isNumber(rightOperand) && isNumber(leftOperand)){
int left = Integer.parseInt(leftOperand);
int right = Integer.parseInt(rightOperand);
int result = 0;
if (operator.equals("+")){
result = left + right;
}else if (operator.equals("-")){
result = left - right;
}else if (operator.equals("*")){
result = left * right;
}else if (operator.equals("/")){
result = left / right;
}
return "" + result;
}
String operand = "(" + operator + " " + leftOperand + " "+ rightOperand + ")";
return operand;
}
public static int rank(String s) {
if (s.equals("+") || s.equals("-"))
return 1;
else if (s.equals("/") || s.equals("*"))
return 2;
else
return 0;
}
public static String infixToPrefixConvert(String infix, boolean reduce) {
Stack<String> operandStack = new Stack<String>();
Stack<String> operatorStack = new Stack<String>();
StringTokenizer tokenizer = new StringTokenizer(infix);
while (tokenizer.hasMoreTokens()) {
String token = tokenizer.nextToken();
if (isOperand(token)) {
operandStack.push(token);
}
else if (token.equals("(") || operatorStack.isEmpty()
|| rank(token) > rank(operatorStack.peek())) {
operatorStack.push(token);
}
else if (token.equals(")")) {
while (!operatorStack.peek().equals("(")) {
operandStack.push(operationCombine(operatorStack, operandStack,reduce));
}
operatorStack.pop();
}
else if( rank(token) <= rank(operatorStack.peek())){
while(!operatorStack.isEmpty() && rank(token) <= rank(operatorStack.peek())){
operandStack.push(operationCombine(operatorStack, operandStack,reduce));
}
operatorStack.push(token);
}
}
while( !operatorStack.isEmpty() ) {
operandStack.push(operationCombine(operatorStack, operandStack,reduce));
}
return (operandStack.peek());
}
}
該文件應該是您要轉換的infix表達式的第一行。 如果要在轉換為前綴時減少簡單的算術表達式,則使用-r標志。 例如在test.txt中,如果我有
3 * 9 + ( 9 + y ) / 4 - x
它工作正常。 但是如果我有
12 / c + c * p ^ ( 8 + 9 )
它應該給我:12 c / cp 8 9 + ^ * +但我得到:(+ p(* ^(+ 8 9)))
問題是什么。 請幫我。 謝謝
瞥一眼您的代碼,我可能會說這行
String operand = "(" + operator + " " + leftOperand + " "+ rightOperand + ")";
是問題的根源,也是您沒有在operationCombine
函數中考慮運算符^
的事實。
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