[Question]Simple Calculator problem?

[AlleDingeAndern]

I'm an amateur Java programmer.

Started learning, but then had some issues, and proceeded to learn Flash. Flash isn't exactly what I'm looking for, so I'm trying Java again. I have a problem with a calculator I'm attempting to make.
I was wondering if you could help me point out the flaw(s) in my calculator... THe source code is as follows, and it's pretty basic. I just can't get it to display the final output number...

import java.util.Scanner;


public class SimpleCalculator {


public static void main(String[] args) {
// TODO: Input/Output integers - only 1 scanner variable needed


System.out.println("Please enter a number below, can be decimal or whole");
Scanner scan = new Scanner(System.in);
Scanner scan2 = new Scanner(System.in);
double num1 = scan.nextDouble();

System.out.println("Number one equals "+ num1);
System.out.println("Please enter a second number below");

double num2 = scan.nextDouble();
System.out.println("Number two equals "+ num2);

System.out.println("Please enter an operator for these numbers: +,-,*,/");
String oper = scan2.nextLine();

System.out.println("Your chosen operator is " + oper);

if (oper == "/")
{double num3 = num1/num2;}
else if (oper == "+")
{double num3 = num1+num2;}
else if (oper == "-")
{double num3 =num1 - num2;}
else if (oper == "*")
{double num3 = num1*num2;}

System.out.println("The finalized output for your numbers are: " + num3);
}
}
It always says, in the last line, num3 cannot be resolved to a variable.

I just can't seem to get this to work, I've tried setting double num3 = 0; then running the code, I've also tried removing it completely...

Any help would be gladly appreciated, thanks, Joe.

[mastal]

Read about variable visibility and lifetime ;)

Also check how to compare Strings in Java.

[ace84]

Hey,

instead of testing a value with the '=' sign, use the .equals comparison instead. Also declare 'num3' before the if statement.

For example:
double num3 = 0;

if( oper.equals("-" ) )
num3 = num1 - num2;
else if( oper.equals("*" ) )
num3 = num1 * num2;
else if( oper.equals("/" ) )
num3 = num1 / num2;
else
num3 = num1 + num2;

that should give you the correct results.

[AlleDingeAndern]

@Mastal
Will do, Thanks!

@Ace84
That worked! Thanks! Two hours trying to fix this, solved in a matter of minutes. Thanks again!
~Joe

[sunde887]

If you don't understand/didn't find the explanation of what caused your problem here is a quick one:

The double num3 is declared inside of curly braces, and thus it has a scope of only inside the curly braces, once the braces are exited, the declared variable no long exists.