Question

I have been building a simple formula calculator and have gotten stuck with addition and subtraction. As you should know, when calculating an equation, you follow the arithmetic rules of precedence, i.e. brackets, order: power functions, division, multiplication, addition and subtraction. The problem is that addition and subtraction are given equal priority, so therefore you can read it from left to right. Here is my code so far:

{
ArrayList<String> equation = java.util.Arrays.asList({"2","-","2","+","5"});

  while(equation.contains("+")){
          addMe(equation);
  }
  while(equation.contains("-")){
          minusMe(equation);            
  }
}

public static void addMe(ArrayList<String> numberList){
 for (int i = 0, n = numberList.size(); i < n; i++) { 
   String value = (String) numberList.get(i); 
   if(value.equals("+")){

    String wordBefore = (String) numberList.get(i-1);
    String wordAfter = (String) numberList.get(i+1);
    System.out.println("This is the word before " + wordBefore);
    System.out.println("This is the word after " + wordAfter);
    double doubleFromBefore = Double.parseDouble(wordBefore);
    double doubleFromAfter = Double.parseDouble(wordAfter);
    double answer = doubleFromBefore + doubleFromAfter;
    System.out.println("This is the answer: " + answer);
    String stringAnswer = String.valueOf(answer);
    String newNum2 = value.replace(value, stringAnswer);
    numberList.set(i,newNum2);
    numberList.remove(i-1);
    numberList.remove(i);
    break;
  }
 }   
}

The minusMe method is exactly the same as the addMe method except with “-” in relevant places. The problem I am having is getting the equation read from left to right one item at a time and either doing the add or subtract method. Ideally I think I need to combine my 2 while loops with an iterator, to solve the problem but my attempts haven’t worked. Any idea as to if this will solve my problem? If so please provide amended loop.

Regards

Answer 1

Have a look at this

java.uti.ArrayList<String> equation = java.util.Arrays.asList({"2","-","2","+","5"});
java.util.Iterator<String> equIterator = equation.iterator();
int result = 0;
int multiplier = 1;
while(equIterator.hasNext()){
     String operandOrOperator = equIterator.next();
     if(operandOrOperator.equals("+")){
          multiplier=1;
     }else if(operandOrOperator.equals("-")){
          multiplier=-1;
     }else if(operandOrOperator.equals("*")){
          result*=Integer.parseInt(equIterator.next()); // Assuming that next element will be there always after operator.
     }else{
          result+=(multiplier * Integer.parseInt(operandOrOperator));
     }
}
System.out.println("Final result : " + result);