Thread: Help with calculator

I'm creating a calculator but I need help. The user is supposed to write for example 2+2 and get the answer 4 in a dialogue window but I'm stuck on how to make it work.

here is my code

String s = JOptionPane.showInputDialog(null, "Type in number");
        int num = Integer.parseInt(s);
 
        ArrayList<String> matte = new ArrayList<String>();
        matte.add("+");
        matte.add("-");
        matte.add("/");
        matte.add("*");
        matte.get(0);
        System.out.println(matte);

Is that all your code? It needs a lot more to be able to compile and execute - import statements, class definition, main method, etc

Before writing any more code you need to do some design work laying out the steps the program needs to take to solve the problem.
Once you have the steps, then try writing the code to implement the steps.

It only has to show in one message dialogue.

If that is all the code is supposed to do, then you have it done since it already uses a JOptionPane class method to show a message in a dialog.

we don't have to design it

By design I meant a list of the steps the program must take to solve the problem. All programs need that.

What steps must the program take to solve the problem? Can you make a list of the steps?
Once the steps are known, then the arrays, selections and iterations, etc can be used to do them.

For example, a list of steps at a very high level:
ask user for input
read user's input
process user's input >>>>>>> this needs to have lots more detailed steps
display results

Originally Posted by Norm

What steps must the program take to solve the problem? Can you make a list of the steps?
Once the steps are known, then the arrays, selections and iterations, etc can be used to do them.

For example, a list of steps at a very high level:
ask user for input
read user's input
process user's input >>>>>>> this needs to have lots more detailed steps
display results

My program has to be able to read mathimatical expression from one single dialogue input. For example 2+4, 6*2, 100-50, 6/3.
Show error message if something is missing, or things that are not allowed for example: 10%12, 176. It's also not allowed to input such as 1+5*2, only two is allowed like 1+5.
Show error message if you try to divide with 0.
In order to solve this we have to use arrays, selections, iterations, string, double and character.

I've only managed to create this now

String s = JOptionPane.showInputDialog("number");
        int n = Integer.parseInt(s);
        int sum = 0;
        int i=1;
        while (i <= n) {
            sum = n+n;
            sum = n-n;
            sum = n*n;
            i= i + 1;
        }
        JOptionPane.showMessageDialog(null, "Summan = " + sum);

Though you can only input one number and it will be read as n*n

read mathimatical expression from one single dialogue input. For example 2+4

Ok, what steps does the code need to take to break that expression up into its 3 parts?

The posted code looks like it will only read and process a single number, like 2 vs an expression like 2+4.

Please edit your post and wrap your code with code tags:

[code]
**YOUR CODE GOES HERE**
[/code]

to get highlighting and preserve formatting.

make it so that it can process two and whatever count type the user wants input like +

Ok, what does the program need to do to get the two operands and the operator from the expression the user entered?

It could be done with Regular Expressions, but I suspect that is not how the instructor wants it done.

Another way is to use String class methods like indexOf and substring.

try to get tal1 but it won't work.

Please explain in English how you are trying to get the 3 parts of the expression.

What if there are no spaces in the expression? For example: "3+4"

want to capture tal1 in the substring

What is tal1?

Given the String: "3+4" what steps can the code take to separate it into the three Strings: "3", "+" and "4"?
Can you make a list of the steps in English the code needs to take to do that?

I am assuming that "3+4" is the type of String the user will enter. If it is not that way, please post some examples of user input.

Ok, how is the code going to separate the String: "3+4" into the three Strings: "3", "+" and "4"?

For now assume that "+" is the only operator to worry about. Later work on getting the others.
First the code must find the location of the "+"
Then what?

Some info on using the substring method:
substring(0, index) returns String from start of String
substring(index) returns String through the end of the String
Read the API doc for exact details.

Another useful String method is trim - removes spaces before and after

Originally Posted by Norm

Ok, how is the code going to separate the String: "3+4" into the three Strings: "3", "+" and "4"?

For now assume that "+" is the only operator to worry about. Later work on getting the others.
First the code must find the location of the "+"
Then what?

Some info on using the substring method:
substring(0, index) returns String from start of String
substring(index) returns String through the end of the String
Read the API doc for exact details.

Another useful String method is trim - removes spaces before and after

Okay I've been trying to do that with "+" for example with an index of code, but I don't understand the result when I run the code in the terminal I just get 1.

String num = "1+2";
int tecken = num.indexOf('+');
        System.out.println(tecken);
String operator = "";
        for (int o = 0; o <= num.length(); o++) {
            if (num.contains("+"))
                operator = "+";
 
            else if (num.contains("-"))
                operator = "-";
            else if (num.contains("*")){
                operator = "*";
        } else {
            System.out.println("There is no operator");
            return;
        }
    }

  String num = "1+2";
  int tecken = num.indexOf('+');
 System.out.println(tecken);     //<<< This prints a 1, the location of the +

tecken now has the location of the + in the String
Use the substring method to get the value before the + >> "1"
Use substring to get the value after the + >> "2"
The code now has the three parts of the expression: "1", "+" , "2"
convert the two operands "1" and "2" to int values using the parseInt method
now do the + operation by adding the two int values together

No. The substring method should use the value returned by the indexOf method. The args for the method should not be numbers.

There are two Strings you want to extract: "1" and "2"

Earlier I said:
substring(0, index) returns String from start of String
substring(index) returns String through the end of the String

To get the "1" which starts at the beginning of the String use this syntax: substring(0, index)
To get the "2" which is in the String after the "+", use this syntax: substring(index)

The value in index may need to be adjusted so the String does not include the +. If you print the values of the Strings returned by substring you will see which one needs to be adjusted.

Not quite right. The indexOf method should find the location of the "+" like it did in posts #17 and #18
What happened when you compiled and executed that code? You should always compile and execute code BEFORE posting it so you can copy any error messages and post them with the code.

Did you print out the values in num1 and num2 to check?

Originally Posted by Norm

Not quite right. The indexOf method should find the location of the "+" like it did in posts #17 and #18
What happened when you compiled and executed that code? You should always compile and execute code BEFORE posting it so you can copy any error messages and post them with the code.

Did you print out the values in num1 and num2 to check?

String num2 = num.substring(tecken, 2);
        String num1 = num.substring(0, tecken);

it only print out 1

What is printed out when you compile and execute the code.
Be sure to print the values in num1 and num2

  String num2 = num.substring(tecken, 2);

The substring does not need the 2 if you want all of the rest of the String returned.

it only print out 1

What is that the value of?