[英]reversed polish giving wrong answer
我需要制作一個使用中綴表達式並使用rpn對其求值的計算器。
Java代碼:
public RpnCalculator() {
}
public float eval(float arg1, float arg2, String operator) {
switch (operator) {
case PLUS:
return arg1 + arg2;
case MINUS:
return arg2 - arg1;
case MULTIPLICATION:
return arg1 * arg2;
case DIVISION:
return arg2 / arg1;
default:
return 0;
}
}
public String evaluateInfixExpression(String expression) {
Stack<String> operators = new Stack<>();
String[] args = expression.split(SPACE);
Stack<String> values = new Stack<>();
for (String arg : args) {
if (isANumber(arg)) {
values.push(arg);
continue;
}
if (operators.isEmpty()) {
operators.push(arg);
} else if (precedence(arg) <= precedence(operators.peek())) {
float result = eval(Float.parseFloat(values.pop()), Float.parseFloat(values.pop()), operators.pop());
values.push(String.valueOf(result));
operators.push(arg);
} else if (precedence(arg) > precedence(operators.peek())) {
operators.push(arg);
}
}
while (!operators.isEmpty()) {
float result = eval(Float.parseFloat(values.pop()), Float.parseFloat(values.pop()), operators.pop());
values.push(String.valueOf(result));
}
return expression;
}
public int precedence(String operator){
if (operator.equals(PLUS) || operator.equals(MINUS)){
return 1;
}
return 2;
}
public boolean isANumber(String number) {
if (number.matches("-?\\d+")) {
return true;
}
return false;
}
}
而且效果很好,但有時它會給出錯誤的答案...在我看來,我遵循的是調車場算法原理,但是如您所見,我實際上並未將infix轉換為postfix,但我嘗試對走,也許那是個問題。
例如,表達式-2 + 6 * 8/3 * 18-33/3-11的計算結果是286,而不是264。應該有一些我無法注意到的錯誤,而且已經有兩天了,所以請幫助我。 我還在堆棧上閱讀了很多有關RPN的線程,但是似乎每個人都有不同的問題,所以我沒有找到適合我的情況的答案。
謝謝。
我不是RPN專家,但是我注意到您正在按從右到左的順序評估參數,因此,在評估了乘法和除法之后,您最終得到以下結果:
operators = + - -
values = -2 288 11 11
然后,您執行(從右到左的順序):
11 - 11 = 0 // would expect -22 here
288 - 0 = 288
-2 + 288 = 286
這不能給您正確的結果。
如果按從左到右的順序進行評估,則會得到:
-2 + 288 = 286
286 - 11 = 275
276 - 11 = 264
因此,我對您的代碼做了一些更改:
public String evaluateInfixExpression(String expression) {
Deque<String> operators = new LinkedList<>();
String[] args = expression.split(SPACE);
Deque<String> values = new LinkedList<>();
for (String arg : args) {
if (isANumber(arg)) {
values.push(arg);
continue;
}
if (operators.isEmpty()) {
operators.push(arg);
} else if (precedence(arg) <= precedence(operators.peek())) {
float result = eval(Float.parseFloat(values.pop()), Float.parseFloat(values.pop()), operators.pop());
values.push(String.valueOf(result));
operators.push(arg);
} else if (precedence(arg) > precedence(operators.peek())) {
operators.push(arg);
}
}
while (!operators.isEmpty()) {
String v1 = values.removeLast();
String v2 = values.removeLast();
float result = eval(Float.parseFloat(v2), Float.parseFloat(v1), operators.removeLast());
values.addLast(String.valueOf(result));
}
return expression;
}
對於RPN,首先應將中綴形式轉換為后綴形式。 為此,您可以使用Dijkstra的Shunting-yard算法 。
此算法的示例實現:
public class ShuntingYard {
private static boolean isHigerPrec(String op, String sub) {
return (ops.containsKey(sub) && ops.get(sub).precedence >= ops.get(op).precedence);
}
public static Stack<String> postfix(String infix) {
Stack<String> output = new Stack<>();
Deque<String> stack = new LinkedList<>();
for (String token : infix.split("\\s")) {
if (ops.containsKey(token)) {
while ( ! stack.isEmpty() && isHigerPrec(token, stack.peek()))
output.push(stack.pop());
stack.push(token);
} else {
output.push(token);
}
}
while ( ! stack.isEmpty())
output.push(stack.pop());
return reverse(output);
}
private static Stack<String> reverse(Stack<String> original) {
Stack<String> reverse = new Stack<>();
while(!original.isEmpty()) reverse.push(original.pop());
return reverse;
}
}
和操作類:
public enum Operator {
ADD(1), SUBTRACT(1), MULTIPLY(2), DIVIDE(2);
final int precedence;
Operator(int p) { precedence = p; }
public static Map<String, Operator> ops = new HashMap<String, Operator>() {{
put("+", Operator.ADD);
put("-", Operator.SUBTRACT);
put("*", Operator.MULTIPLY);
put("/", Operator.DIVIDE);
}};
public static Operator fromString(String str){
return ops.get(str);
}
}
最后,您的課程進行了修改:
public class RpnCalculator {
private static Float eval(float arg1, float arg2, Operator operator) {
switch (operator) {
case ADD:
return arg1 + arg2;
case SUBTRACT:
return arg2 - arg1;
case MULTIPLY:
return arg1 * arg2;
case DIVIDE:
return arg2 / arg1;
default:
throw new IllegalArgumentException("Operator not supported: " + operator);
}
}
public static Float evaluateInfixExpression(String expression) {
Stack<String> stack = ShuntingYard.postfix(expression);
Stack<Float> result = new Stack<>();
while(!stack.isEmpty()){
String nextElement = stack.pop();
if(isANumber(nextElement)){
result.push(new Float(nextElement));
} else {
result.push(eval(result.pop(), result.pop(), Operator.fromString(nextElement)));
}
}
return result.pop();
}
private static boolean isANumber(String number) {
return number.matches("-?\\d+");
}
}
資源:
這是一個簡潔的解決方案,可以即時進行計算:
public class RpnCalculator {
public static Float evaluateInfixExpression(String inflixExpression) {
Stack<Float> operands = new Stack<>();
Stack<Operator> operators = new Stack<>();
for (String token : inflixExpression.split("\\s")) {
if (isOperator(token)) {
while (!operators.isEmpty() && operators.peek().hasHigherPrecedenceThan(token))
operands.add(eval(operands.pop(), operands.pop(), operators.pop()));
operators.push(fromString(token));
} else {
operands.add(new Float(token));
}
}
while (!operators.isEmpty())
operands.add(eval(operands.pop(), operands.pop(), operators.pop()));
return operands.pop();
}
private static Float eval(float arg2, float arg1, Operator operator) {
switch (operator) {
case ADD:
return arg1 + arg2;
case SUBTRACT:
return arg1 - arg2;
case MULTIPLY:
return arg1 * arg2;
case DIVIDE:
return arg1 / arg2;
default:
throw new IllegalArgumentException("Operator not supported: " + operator);
}
}
}
和Operator
類:
public enum Operator {
ADD(1), SUBTRACT(1), MULTIPLY(2), DIVIDE(2);
final int precedence;
Operator(int p) { precedence = p; }
private static Map<String, Operator> ops = new HashMap<String, Operator>() {{
put("+", Operator.ADD);
put("-", Operator.SUBTRACT);
put("*", Operator.MULTIPLY);
put("/", Operator.DIVIDE);
}};
public static Operator fromString(String token){
return ops.get(token);
}
public static boolean isOperator(String token) {
return ops.containsKey(token);
}
public boolean hasHigherPrecedenceThan(String token) {
return isOperator(token) && this.precedence >= fromString(token).precedence;
}
}
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