0

I'm currently working on a school project in Android Studio and so far I've written a code which generates a random equation, like "3+9/3", everytime you press a button on the screen. This equation shows up in a textview. Now I tried to evaluate the result of the equation with the "abs" command using a double. For that I stored the equation in a string and then tried to convert it to a double because the "abs" command doens't support strings. Here is the code:

String[] operationSet = new String[]{"+", "-", "/", "*"};

public void generate(View view) {
Random random = new Random();
int numOfOperations = random.nextInt(2) + 1;

List<String> operations = new ArrayList<>();

for (int i = 0; i < numOfOperations; i++) {
    String operation = operationSet[random.nextInt(4)];
    operations.add(operation);
}

int numOfNumbers = numOfOperations + 1;
List<Integer> numbers = new ArrayList<>();

for (int i = 0; i < numOfNumbers; i++) {
    int number = random.nextInt(10)+1;
    numbers.add(number);
}

String equation = "";
for (int i = 0; i < numOfOperations; i++) {
    equation += numbers.get(i);
    equation += operations.get(i);
}
equation += numbers.get(numbers.size() -1);

TextView TextEquation = (TextView)findViewById(R.id.textView);
TextEquation.setText(equation);

String stringResultOfEquation = String.valueOf(equation);

// Calculate the result of the equation

double doubleEquation = Double.parseDouble(equation);
double doubleResult = abs(doubleEquation);
String stringResult = String.valueOf(doubleResult);

TextView textResult = (TextView)findViewById(R.id.textView2);
textResult.setText(stringResult);

}

However, when I run the app in the emulator I just get an error message "NumberFormatException". So I guess there is something wrong with converting my string into a double. Is it possible that the quotation marks at my equation (for example: "5*3+6") cause problems? Is there a different way to store my equation from the string so I can use the "abs" command?

If anything is unclear in my question, feel free to aks and I will try to clarify the problem :)

Thank you already in advance!

ps. I asked a similar question a little while ago but it got falsely marked as a duplicate.

OneCricketeer
  • 179,855
  • 19
  • 132
  • 245
ztv
  • 134
  • 1
  • 9
  • The equation can only be stored as a string. You *could* convert the result to decimal, but you would need to calculate that result first. Maybe [this might be of interest](http://stackoverflow.com/questions/11993849/android-parse-simple-mathematical-formula-from-string) – musefan Oct 14 '16 at 14:43
  • Same request as previous question. Add the logcat, please – OneCricketeer Oct 14 '16 at 14:43
  • 2
    And how is this question different from the previous one you asked? Or the older ones? http://stackoverflow.com/questions/39979079/calculate-the-exact-value-of-an-equation You have to actually parse **and evaluate** the string. You can't just parse it as a Double... I believe that your previous question was correctly flagged – OneCricketeer Oct 14 '16 at 14:47

1 Answers1

0

I think the problem is this line here

double doubleEquation = Double.parseDouble(equation);

your "equation" is a String with operations, so you can't parse that String into a double. You will have to evaluate your "equation" first. To do so, you have to loop through each character in String operation, and when the char is for example "+", you perform addition. The String won't just evaluate into a number itself. Something like this

double i = Double.parseDouble("1");

will evaluate into 1.0

but this will result in error

double i = Double.parseDouble("1+1");
Pear
  • 785
  • 2
  • 10
  • 19