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Thread: Help with calculator

  1. #26
    Super Moderator Norm's Avatar
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    Default Re: Help with calculator

    Looks like you are making progress. As I said in post#20, the index value needs to be adjusted so that the String does not have the "+"
    For example use index+1 instead of just index as the argument to substring
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help with calculator

    Quote Originally Posted by Norm View Post
    Looks like you are making progress. As I said in post#20, the index value needs to be adjusted so that the String does not have the "+"
    For example use index+1 instead of just index as the argument to substring
    Yes thank you! (tecken+1) only prints our the last number now.

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    Default Re: Help with calculator

    (tecken+1) only prints our the last number now!
    Yes that is all that should be in num2 - the second operand: "2"

    Change the expression to something with values that will not be confused with index values. For example: "321+44"
    The problem with "1+2" is that when 1 is printed, you do not know if it is the index of "+" or the value of the first operand.
    When the second operand's value is printed, you should see "44" not "2"
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help with calculator

    Quote Originally Posted by Norm View Post
    Yes that is all that should be in num2 - the second operand: "2"

    Change the expression to something with values that will not be confused with index values. For example: "321+44"
    The problem with "1+2" is that when 1 is printed, you do not know if it is the index of "+" or the value of the first operand.
    When the second operand's value is printed, you should see "44" not "2"
    I understand I changed my string to 341+44 and tecken+1 prints out 44 and 0, tecken prints out 321

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    Default Re: Help with calculator

    Ok, now the expression has been broken into three parts. Now convert the operands to int values and do the addition to get the results.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help with calculator

    Quote Originally Posted by Norm View Post
    Ok, now the expression has been broken into three parts. Now convert the operands to int values and do the addition to get the results.
    Okay
    int num4 = Integer.parseInt(num2);
            int num5 = Integer.parseInt(num1);
    What am I going to put in my print code?

  7. #32
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    Default Re: Help with calculator

    What do you want to see on the console when the program executes?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help with calculator

    Quote Originally Posted by Norm View Post
    What do you want to see on the console when the program executes?
    The result of what 341+44 is

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    Default Re: Help with calculator

    Ok, if those two values are in variables, can you add the variables together to get the desired result?
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    Default Re: Help with calculator

    Quote Originally Posted by Norm View Post
    Ok, if those two values are in variables, can you add the variables together to get the desired result?
    I tried making them into one string and convert it to int to print out but then it prints out 34144

    String result = num1 + num2;
            int num4 = Integer.parseInt(result);
     
            System.out.println(result);

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    Default Re: Help with calculator

    making them into one string
    Leave them as int values.
    Use an assignment statement to assign their sum to another int variable.
    Then print the value of that int variable.

    The + operator with two Strings concatenates their values: "asd" + "def" gives "asddef"
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help with calculator

    Quote Originally Posted by Norm View Post
    Leave them as int values.
    Use an assignment statement to assign their sum to another int variable.
    Then print the value of that int variable.

    The + operator with two Strings concatenates their values: "asd" + "def" gives "asddef"
    okay what is assignment statement?

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    Default Re: Help with calculator

    what is assignment statement?
    A statement that assigns a value to a variable. The variable is on the left, followed by =, followed by a value
    Examples from your code:
    String result = num1 + num2;
    int num4 = Integer.parseInt(result);
    int tecken = num.indexOf('+');

    See the tutorial: https://docs.oracle.com/javase/tutor...bolts/op1.html
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help with calculator

    Quote Originally Posted by Norm View Post
    A statement that assigns a value to a variable. The variable is on the left, followed by =, followed by a value
    Examples from your code:
    String result = num1 + num2;
    int num4 = Integer.parseInt(result);
    int tecken = num.indexOf('+');

    See the tutorial: https://docs.oracle.com/javase/tutor...bolts/op1.html
    like this?
    int result = (num4 + num5);
    System.out.println(result);

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    Default Re: Help with calculator

    Did you try that? What happened?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help with calculator

    Quote Originally Posted by Norm View Post
    Did you try that? What happened?
    Yes it worked!! I only need now to make error messages when you try to /0 and if you try to use more than to numbers. So when you type in for example 2+2-1, you're supposed to get an error message. I've tried with 0 like
    if (s.contains("/0")){
    JOptionPane.showMessageDialog(null, "error");
    }
    but it doesn't work and for the other I don't know....

  17. #42
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    Default Re: Help with calculator

    Do you know about try{}catch for trapping exceptions? If so you could put the parseInt method calls inside of a try{}catch block and let the parseInt method throw an exception if either of the two operands (before and after the opcode) are not valid int values.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help with calculator

    Quote Originally Posted by Norm View Post
    Do you know about try{}catch for trapping exceptions? If so you could put the parseInt method calls inside of a try{}catch block and let the parseInt method throw an exception if either of the two operands (before and after the opcode) are not valid int values.
    No I haven't learn about it yet and I'm not sure if my teacher will allow it but I'll ask him. So I can't use if statement or a for loop?

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    Default Re: Help with calculator

    Yes, you can make a method to scan a String and test if it contains only digits. The String class has methods for getting at each character in the String.
    The Character class has methods to test a character to see if it is a digit.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help with calculator

    Quote Originally Posted by Norm View Post
    Yes, you can make a method to scan a String and test if it contains only digits. The String class has methods for getting at each character in the String.
    The Character class has methods to test a character to see if it is a digit.
    okay something like this?
    for (i=0; i<s.length(); i++)
    			if (s.charAt(i) == '')

  21. #46
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    Default Re: Help with calculator

    That looks like a start. Now add the method definition and use a Character class method.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help with calculator

    Quote Originally Posted by Norm View Post
    That looks like a start. Now add the method definition and use a Character class method.
    Okay I have solved all of it and I don't have much time on this assignment. How do I make error message if there is a letter in the input?
    Last edited by Dragonfly11; November 6th, 2019 at 09:18 AM.

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    Default Re: Help with calculator

    The test inside of the if statement should be about the char being a digit or not. See the Character class for method to use.
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Help with calculator

    Quote Originally Posted by Norm View Post
    The test inside of the if statement should be about the char being a digit or not. See the Character class for method to use.
    okay I solved it like this

    int count=0;

    for ( int i=0; i<s.length(); i++) {
    if (s.charAt(i) == '*' || s.charAt(i) == '/' || s.charAt(i) == '+' || s.charAt(i) == '-') {


    count++;
    }

    } if (count > 1) {
    System.out.println("error");
    System.exit(0);
    }

    I missed making an error message if there is a letter, how do I make it?

  25. #50
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    Default Re: Help with calculator

    If you want to test if all the characters in a String are digits, use the Character class's method to test each character in a loop.

    Another way would be to test if each character is >= '0' and <= '9'
    If you don't understand my answer, don't ignore it, ask a question.

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