Question

I am trying to calculate an arithmetic expression, which is entered as a string (for example, ( 5+4*5-1/8 ), which will give the result 3). I enter an expression and convert it into an array. First; the result will start with the first element and it will change in the loop. But the problem is operator precedence. How can I use the operator presedence in a loop? Here is my code:

import java.util.Scanner;


public class HesapMakinesi {

    private char value[];

    private int count;

    private Scanner str = new Scanner(System.in);

    private String process;

    HesapMakinesi() {

        System.out.print("Enter the process ");

        process = str.next();

        //System.out.println(islem);

        Initializer(process);

    }

    private void Initializer(String process) {

        count = process.toCharArray().length;

        value = new char [count];

        int i;

        System.arraycopy(process.toCharArray(), 0, value, 0, count);

        //System.out.println(value);

        if(value[0]=='-' || value[0]=='+' || value[0]=='/' || value[0]=='*' ||  // A process cannot start with an operator
                value[count-1]=='-' || value[count-1]=='+' || value[count-1]=='/' || value[count-1]=='*') {

            System.out.println("You have entered a wrong process.Please enter again!!!");

            System.out.print("Enter the process: ");

            process = str.next();

            Initializer(process);

        }



        for(i=0; i<count; i++) { // A process cannot include a character except operators

            if( value[i]!='+' && value[i]!='-' && value[i]!='*' && value[i]!='/' && value[i]!='(' && value[i]!=')' && !Character.isDigit(value[i]) ) {

                System.out.println("You have entered a wrong process.Please enter again!!!");

                System.out.print("Enter the process: ");

                process = str.next();

                Initializer(process);

            }


        }


        for(i=0; i<count-1; i++) { // A process cannot have operators sequantially

            if( !Character.isDigit(value[i]) && !Character.isDigit(value[i+1]) ) {

                if( (value[i] == '+' && value[i+1] == '+' ) || (value[i] == '+' && value[i+1] == '-' ) || (value[i] == '+' && value[i+1] == '*' ) || 
                        (value[i] == '+' && value[i+1] == '/' ) ) {

                    System.out.println("You have entered a wrong process.Please enter again!!!");

                    System.out.print("Enter the process: ");

                    process = str.next();

                    Initializer(process);

                }

                else if( (value[i] == '-' && value[i+1] == '+' ) || (value[i] == '-' && value[i+1] == '-' ) || (value[i] == '-' && value[i+1] == '*' ) || 
                        (value[i] == '-' && value[i+1] == '/' ) ) {

                    System.out.println("You have entered a wrong process.Please enter again!!!");

                    System.out.print("Enter the process: ");

                    process = str.next();

                    Initializer(process);

                }

                else if( (value[i] == '*' && value[i+1] == '+' ) || (value[i] == '*' && value[i+1] == '-' ) || (value[i] == '*' && value[i+1] == '*' ) || 
                        (value[i] == '*' && value[i+1] == '/' ) ) {

                    System.out.println("You have entered a wrong process.Please enter again!!!");

                    System.out.print("Enter the process: ");

                    process = str.next();

                    Initializer(process);

                }

                else if( (value[i] == '/' && value[i+1] == '+' ) || (value[i] == '/' && value[i+1] == '-' ) || (value[i] == '/' && value[i+1] == '*' ) || 
                        (value[i] == '/' && value[i+1] == '/' ) ) {

                    System.out.println("You have entered a wrong process.Please enter again!!!");

                    System.out.print("Enter the process: ");

                    process = str.next();

                    Initializer(process);

                }


            }

        }

        //sCount();

    }

    /*private void Count(){

        double result,temp;

        int i;

        for(i=0; i<count; i++) {

            if( value[i]!= )

        }


    }*/

}

Answer 1

Following on from my comment… If you’re dealing with a simple expression where you can only have numbers and signs +-/* then you can have a simple approach:

1. split your expression by lowest precedence operators first (+-) remembering the signs.
2. Compute each piece – as now the precedence isn’t important, since everything is of the same level
3. sum those computed pieces taking into account the sign of the piece from step 1.

In your example, you’ll end up with something like this:

1. (Split by +-) three pieces: (1) 5; (2) 4*5 with sign +; (3) 1/8 with sign –
2. Compute each of the three pieces: (1) 5; (2) 20; (3) 0.125
3. Sum the three pieces with their respective signs: 5+20-0.125 = 24.875