Perform arithmetic operations using elements in list

Question

I need to perform arithmetic operations using the elements in a list. For example, if I have a list of elements [25, +, 5, -, 8, /, 4] how can I convert this as an arithmetic operation in the form 25+5-(8/4) and print its result (28 in this example)? The list I used is of type String. Anyone, please help me to solve this problem. I am new to Arraylist and all.

Answer 1

An expression as List:

List<String> expression = new ArrayList();
Collections.addAll(expression, "25", "+", "5", "-", "8", "/", "4");

Now you have to do operations till there is just one term.

You can do the evaluation as a human: first look at * and /, then at + and -. There are really many variants to write this.

while (expression.size() > 1) {
    int terms = expression.size();

    // ... reduce expression:

    // 1. * and /:
    ...
    if (expression.size() < terms) {
        continue;
    }

    // 2. + and -:
    for (int i = 1; i + 1 < experesssion.size(); i += 2) {
        String operator = experesssion.get(i);
        switch (operator) {
        case "+":
            int lhs = Integer.parseInt(expression.get(i - 1));
            int rhs = Integer.parseInt(expression.get(i + 1));
            int n = lhs + rhs;
            expression.remove(i);
            expression.remove(i +1);
            expression.set(i - 1, String.valueOf(n));
            i -= 2; // Keep at this operator position.
            break;
        ...
        }
    }

    if (expression.size() == terms) {
        System.out.println("Could not reduce expression: " + expression);
        break;
    }
}
String result = expression.get(0);
System.out.println(result);

Write your own loop(s). The code aboveis not necessarily very logical.