I am trying to develop a calculator program that inputs an arithmetic expression of the form number operator number = and computes the result of the expression. The expression will be evaluated from left to right not considering regular operator precedence. For example, the expression 14 - 5 * 3 = will produce 27.0. The value = displays the final result and terminates the program.
I’ve been trying to fix this for a couple of days now, but it outputs the wrong answer whenever I enter an expression with more than two numbers. For instance, 2.8 + 2 - 9.5 should equal -4.7 but the program outputs -6.7. Any idea why that is the case?
import java.util.Scanner;
public class Calculator {
// Compute an arithmetic expression
public static void main(String[] args) {
// Declare the identifiers
final String END = "=";
String input;
double num1 = 0;
double num2 = 0;
char operator = 0;
Scanner scnr = new Scanner (System.in);
System.out.println("Enter your numeric expression in the following form: ");
System.out.println("number operator number operator number = ");
System.out.println("Leave a blank space after each number or operator.");
System.out.println("Example: 3.5 * 3 - 5 / 2.5 =");
// Input the first item
System.out.print("> ");
input = scnr.next();
// Process the first item and input and process the rest of the items
while (!input.equals(END)){
switch (input){
case "+":
operator = '+';
System.out.println("> Operator is: " + operator);
break;
case "-":
operator = '-';
System.out.println("> Operator is: " + operator);
break;
case "*":
operator = '*';
System.out.println("> Operator is: " + operator);
break;
case "/":
operator = '/';
System.out.println("> Operator is: " + operator);
break;
default: // a number was entered
if (num1 == 0) {
num1 = Double.parseDouble(input);
System.out.println("> Num1 is: " + num1);
}
else {
num2 = Double.parseDouble(input);
System.out.println("> Num2 is: " + num2);
}
} // end of switch
if (num1 != 0 && num2 != 0) {
System.out.println("Num2 before calc is " + num2);
switch (operator) {
case '+':
num2 = num1 + num2;
break;
case '-':
num2 = num1 - num2;
break;
case '*':
num2 = num1 * num2;
break;
case '/':
num2 = num1 / num2;
break;
default:
}
}
input = scnr.next();
} // end of while-loop
// Display the answer
System.out.println("> Answer is: " + num2);
System.out.println("Have a nice day!");
}
}
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Answer
In order to make it work, try to:
- in your 2nd switch statement, change
num2 = num1 + num2;intonum1 = num1 + num2;. Do this for all cases; - I added an
isOperatorboolean to skip computing the operation if input is an operator.
Full code below:
import java.util.Scanner;
public class Calculator {
// Compute an arithmetic expression
public static void main(String[] args) {
// Declare the identifiers
final String END = "=";
String input;
double num1 = 0;
double num2 = 0;
char operator = 0;
boolean isOperator;
Scanner scnr = new Scanner (System.in);
System.out.println("Enter your numeric expression in the following form: ");
System.out.println("number operator number operator number = ");
System.out.println("Leave a blank space after each number or operator.");
System.out.println("Example: 3.5 * 3 - 5 / 2.5 =");
// Input the first item
System.out.print("> ");
input = scnr.next();
// Process the first item and input and process the rest of the items
while (!input.equals(END)){
isOperator = true;
switch (input){
case "+":
operator = '+';
System.out.println("> Operator is: " + operator);
break;
case "-":
operator = '-';
System.out.println("> Operator is: " + operator);
break;
case "*":
operator = '*';
System.out.println("> Operator is: " + operator);
break;
case "/":
operator = '/';
System.out.println("> Operator is: " + operator);
break;
default: // a number was entered
isOperator = false;
if (num1 == 0) {
num1 = Double.parseDouble(input);
System.out.println("> Num1 is: " + num1);
}
else {
num2 = Double.parseDouble(input);
System.out.println("> Num2 is: " + num2);
}
} // end of switch
// do not compute the operation if the input is an operator and num1,num2 != 0
if (num1 != 0 && num2 != 0 && !isOperator) {
System.out.println("Num2 before calc is " + num2);
switch (operator) {
case '+':
num1 = num1 + num2;
break;
case '-':
num1 = num1 - num2;
break;
case '*':
num1 = num1 * num2;
break;
case '/':
num1 = num1 / num2;
break;
default:
}
}
input = scnr.next();
} // end of while-loop
// Display the answer
System.out.println("> Answer is: " + num1);
System.out.println("Have a nice day!");
}
}
Edit: As mentioned in the comments, the code does not treat the cases when the user inputs 0. Below, I removed the if(num1 == 0) and if (num1 != 0 && num2 != 0) conditions:
import java.util.Scanner;
public class Calculator {
// Compute an arithmetic expression
public static void main(String[] args) {
// Declare the identifiers
final String END = "=";
String input;
double result = 0;
double num = 0;
char operator = 0;
boolean isOperator;
Scanner scnr = new Scanner (System.in);
System.out.println("Enter your numeric expression in the following form: ");
System.out.println("number operator number operator number = ");
System.out.println("Leave a blank space after each number or operator.");
System.out.println("Example: 3.5 * 3 - 5 / 2.5 =");
// Input the first item
System.out.print("> ");
input = scnr.next();
// Process the first item and input and process the rest of the items
while (!input.equals(END)){
isOperator = true;
switch (input){
case "+":
operator = '+';
System.out.println("> Operator is: " + operator);
break;
case "-":
operator = '-';
System.out.println("> Operator is: " + operator);
break;
case "*":
operator = '*';
System.out.println("> Operator is: " + operator);
break;
case "/":
operator = '/';
System.out.println("> Operator is: " + operator);
break;
default: // a number was entered
isOperator = false;
num = Double.parseDouble(input);
System.out.println("> Num is: " + num);
} // end of switch
// do not compute the operation if the input is an operator
if (!isOperator) {
System.out.println("Result before calc is " + result);
switch (operator) {
case '+':
result += num;
break;
case '-':
result -= num;
break;
case '*':
result *= num;
break;
case '/':
result /= num;
break;
default:
result += num;
}
}
input = scnr.next();
} // end of while-loop
// Display the answer
System.out.println("> Answer is: " + result);
System.out.println("Have a nice day!");
}
}