在将中缀表达式转换为后缀表达式时处理括号 [英] Handling parenthesis while converting infix expressions to postfix expressions
问题描述
我正在开发一个Java项目,需要我将中缀表达式转换为后缀表达式。我目前能够使用此方法将中缀表达式转换为postfix,只要它们不包含括号,但我无法弄清楚如何处理括号。
I'm working on a project in Java that requires me to convert an infix expression to a postfix expression. I am currently able to convert infix expressions to postfix with this method as long as they don't contain parenthesis, but I can't figure out how to handle parenthesis.
基本上,我有两个堆栈,其中包含称为令牌的对象。令牌是一个包装类,它包含一个字符串,它是一个数字,变量(被评估为数字,等待用户输入),运算符(运算符具有与之关联的优先级,以便我的方法可以确定如何处理'+',' - ','*'和'/'之间的操作顺序,或括号(括号有一种方法可以确定它是一个左括号还是一个闭括号)。
Basically, I have two stacks that hold objects that are called 'Token'. A Token is a wrapper class that holds a string that is either a number, variable (which gets evaluated as a number, pending on user input), operator (the operator has a priority level associated with it so that my method can determine how to handle order of operations between '+', '-', '*' and '/'), or a parenthesis (the parenthesis has a way to determine if it is a open parenthesis or a closed parenthesis).
我该如何处理括号?那么多个括号层呢?
How should I handle parenthesis? What about multiple layers of parenthesis?
public String toPostFix() {
StringBuilder postfixstr = new StringBuilder();
Stack<Token> in_fix = new Stack<>();
Stack<Token> post_fix = new Stack<>();
for (int i = tokens.length - 1; i >= 0; i--) {
t = new Token(tokens[i]);
in_fix.push(t);
}
//there are still tokens to process
while (!in_fix.empty()) {
//is a number
if (in_fix.peek().type == 1) {
postfixstr.append(in_fix.pop().toString());
}
//is an operator and the stack is empty
else if (in_fix.peek().type == 3 && post_fix.empty()) {
post_fix.push(in_fix.pop());
}
// is an operator that has higher priority than the operator on the stack
else if (in_fix.peek().type == 3 && in_fix.peek().isOperator() > post_fix.peek().isOperator()) {
post_fix.push(in_fix.pop());
}
// is an operator that has lower priority than the operator on the stack
else if (in_fix.peek().type == 3 && in_fix.peek().isOperator() <= post_fix.peek().isOperator()) {
postfixstr.append(post_fix.pop());
post_fix.push(in_fix.pop());
}
//puts the rest of the stack onto the output string
if (in_fix.empty()) {
while (!post_fix.empty()) {
postfixstr.append(post_fix.pop());
}
}
}
return postfixstr.toString();
}
推荐答案
你需要向左推括号到堆栈上,并在遇到右括号时处理堆栈:
You need to push the left parenthesis onto the stack, and process the stack like so when you encounter a right parenthesis:
// opening (
if (in_fix.peek().type == 4) {
post_fix.push(in_fix.pop());
}
//closing )
if(in_fix.peek().type == 5){
while(!(post_fix.isEmpty() || post_fix.peek().type == 4)){
postfixstr.append(post_fix.pop());
}
if (post_fix.isEmpty())
; // ERROR - unmatched )
else
post_fix.pop(); // pop the (
in_fix.pop(); // pop the )
}
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