Evaluating a math expression given in string

问题

Here is my code:

String test = "12+23-42-53+4-31";
        int counter = 0;
        Stack<Integer> numb= new Stack<Integer>();
        Stack<String> op = new Stack<String>();
        for(int i = 0; i<test.length();i++){
            if(test.charAt(i)=='-' || test.charAt(i)=='+'){
                int number = Integer.parseInt(test.substring(counter, i));
                counter=i+1;
                numb.push(number);
                String oper = Character.toString(test.charAt(i));
                op.push(oper);
            }

        }

If I loop through numb stack then the last number of test string is missing. Are there any solutions?

回答1:

When you exit the for loop, you don't add the last number because there is no more "-" or "+" symbol at the end of your test String.

Just push the last number into the numb stack once you exit the for loop.

Hint: counter is the position where the last number starts, and the last position is the String length.

If you do it correct and print the stacks:

//Print the stacks to see result
System.out.println(numb.toString());
System.out.println(op.toString());

You will obtain following output:

[12, 23, 42, 53, 4, 31]
[+, -, -, +, -]

回答2:

In your if statement, you test if the char is a '-' or a '+'. Then, you parse the number that comes before. So you should parse the last number after your for-loop finishes.

回答3:

ScriptEngine engine = new ScriptEngineManager().getEngineByName("javascript");
Object o = engine.eval("12+23-42-53+4-31");
System.out.println(o);

来源：https://stackoverflow.com/questions/28500031/evaluating-a-math-expression-given-in-string