Decimal points in calculator android

Question

I am creating a calculator app in android. The issue is how can I check for existence of two decimals in a single numeric value . Currently my calculator allows inputs such as 1.2.3 which is illegal . It must allow a decimal if an operator has been used. Eg 1.1+2.2 is legal but 1.1.1+2.2 isn't. Here is my code for decimal handling:

public class TrialCalculator extends AppCompatActivity implements Button.OnClickListener {
    Button btn1, btn2, btnBack, btn3, btn4, btn5, btn6, btn7, btn8, btn9, btn0, btnPlus, btnMinus, btnMul, btnDiv, btnEquals, btnClear, btnDecimal, btnPercent;
    EditText calcResult;
    double number = 0;
    private char opcode = '1';

    private void handleDecimal() {
        if (opcode == 0) clear();
        if (calcResult.getText() == null) {
            calcResult.setText("0.");
            calcResult.setSelection(2);
        } else {
            String txt = calcResult.getText().toString();
            if (txt.lastIndexOf(".")<txt.length()-1) {
                calcResult.append(".");
            }
        }
    }
}

I am calling the buttonDot from onClick Method.

Answer 1

One solution is to have a flag which keeps track of the decimal:

class MyCalculator {
    private hasDecimal = false;

    // ...
}

Set this flag to true the first time that the user types a decimal. Then check the flag to see if a decimal has been previously typed.

Of course, this only works if you are responding to each key press directly rather than getting the entire input from a EditText after the user has typed the entire number.

Answer 2

You can use regex with matches function

\\\\d* mean match zero or more digits

(\\\\.\\\\d+)? match a . followed by one or more digits , ? mean matches between zero or one times of given group

Regex Demo : note with matches function in java, we don't need ^ start and $ ending anchors

Code

if (txt.matches("\\d*(\\.\\d+)?")) {
   // number has one decimal
  }
else{
   // number has more than one decimal
}

---

Note: if you don't want to allow values like .5 then use \\\\d+ instead of \\\\d* as

\\\\d+(\\\\.\\\\d+)?

As suggested by @Code-Apprentice , if you want to accept values like 4343. etc you can use

\\d*(\\.\\d*)?

Using text watcher

calcResult.addTextChangedListener(new TextWatcher() {
    @Override
    public void onTextChanged(CharSequence s, int start, int before, int count) {}

    @Override
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {}

    @Override
    public void afterTextChanged(Editable s) {
        boolean flag = s.toString().matches("\\d*(\\.\\d*)?");
        if(!flag){
            // calcResult.setError...
            // you display a toast 
        }             
    }
});

---

Update : To match multiple values with operators , you can use

(\\d*(\\.\\d*)?([+\\-*%\\]|$))*

RegEx demo

Test Cases

String pat="(\\d*(\\.\\d*)?([+\\-*%\\]|$))*";
System.out.println("4.5-3.3+3.4".matches(pat));
System.out.println(".5".matches(pat));
System.out.println("4".matches(pat));
System.out.println("4.5.5-4.4".matches(pat));
System.out.println("4.44.5+4.4.4".matches(pat));
System.out.println("4.".matches(pat));

Output :

true
true
true
false
false
true

Answer 3

lastIndexOf returns -1 if the character is not found in the string. So your condition txt.lastIndexOf(".")<txt.length()-1 is always true . You could change it to

txt.lastIndexOf(".") == -1

Answer 4

To check for existence of two decimals in a single no then split the string by decimal.

for eg,

String txt = calcResult.getText().toString();
String[] decimals = txt.split("\\.");
if(decimals.length > 2) {
// txt contains more than 1 decimal.
// Your logic ..
}