Android計算字符串從左到右

Question

package com.javahelps.calculator;

import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import net.objecthunter.exp4j.Expression;
import net.objecthunter.exp4j.ExpressionBuilder;


public class MainActivity extends ActionBarActivity {
    // IDs of all the numeric buttons
    private int[] numericButtons = {R.id.btnZero, R.id.btnOne, R.id.btnTwo, R.id.btnThree, R.id.btnFour, R.id.btnFive, R.id.btnSix, R.id.btnSeven, R.id.btnEight, R.id.btnNine};
    // IDs of all the operator buttons
    private int[] operatorButtons = {R.id.btnAdd, R.id.btnSubtract, R.id.btnMultiply, R.id.btnDivide};
    // TextView used to display the output
    private TextView txtScreen;
    // Represent whether the lastly pressed key is numeric or not
    private boolean lastNumeric;
    // Represent that current state is in error or not
    private boolean stateError;
    // If true, do not allow to add another DOT
    private boolean lastDot;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // Find the TextView
        this.txtScreen = (TextView) findViewById(R.id.txtScreen);
        // Find and set OnClickListener to numeric buttons
        setNumericOnClickListener();
        // Find and set OnClickListener to operator buttons, equal button and decimal point button
        setOperatorOnClickListener();
    }

    /**
     * Find and set OnClickListener to numeric buttons.
     */
    private void setNumericOnClickListener() {
        // Create a common OnClickListener
        View.OnClickListener listener = new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // Just append/set the text of clicked button
                Button button = (Button) v;
                if (stateError) {
                    // If current state is Error, replace the error message
                    txtScreen.setText(button.getText());
                    stateError = false;
                } else {
                    // If not, already there is a valid expression so append to it
                    txtScreen.append(button.getText());
                }
                // Set the flag
                lastNumeric = true;
            }
        };
        // Assign the listener to all the numeric buttons
        for (int id : numericButtons) {
            findViewById(id).setOnClickListener(listener);
        }
    }

    /**
     * Find and set OnClickListener to operator buttons, equal button and decimal point button.
     */
    private void setOperatorOnClickListener() {
        // Create a common OnClickListener for operators
        View.OnClickListener listener = new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // If the current state is Error do not append the operator
                // If the last input is number only, append the operator
                if (lastNumeric && !stateError) {
                    Button button = (Button) v;
                    txtScreen.append(button.getText());
                    lastNumeric = false;
                    lastDot = false;    // Reset the DOT flag
                }
            }
        };
        // Assign the listener to all the operator buttons
        for (int id : operatorButtons) {
            findViewById(id).setOnClickListener(listener);
        }
        // Decimal point
        findViewById(R.id.btnDot).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (lastNumeric && !stateError && !lastDot) {
                    txtScreen.append(".");
                    lastNumeric = false;
                    lastDot = true;
                }
            }
        });
        // Clear button
        findViewById(R.id.btnClear).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                txtScreen.setText("");  // Clear the screen
                // Reset all the states and flags
                lastNumeric = false;
                stateError = false;
                lastDot = false;
            }
        });
        // Equal button
        findViewById(R.id.btnEqual).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                onEqual();
            }
        });
    }

    /**
     * Logic to calculate the solution.
     */
    private void onEqual() {
        // If the current state is error, nothing to do.
        // If the last input is a number only, solution can be found.
        if (lastNumeric && !stateError) {
            // Read the expression
            String txt = txtScreen.getText().toString();
            // Create an Expression (A class from exp4j library)
            Expression expression = new ExpressionBuilder(txt).build();
            try {
                // Calculate the result and display
                double result = expression.evaluate();
                txtScreen.setText(Double.toString(result));
                lastDot = true; // Result contains a dot
            } catch (ArithmeticException ex) {
                // Display an error message
                txtScreen.setText("Error");
                stateError = true;
                lastNumeric = false;
            }
        }
    }
}

我正在使用此代碼來計算字符串，但是此代碼在不考慮優先順序的情況下計算字符串。 Exp 3 + 3 * 3輸出12.0但我想獲得輸出18.0計算應該從左到右進行如何修改此代碼

Answer 1

我真的不知道為什么有人需要這個，但是你去了。

我為您提供了一種方法，該方法通過使用正則表達式從左到右評估算術字符串，該正則表達式將元素拆分為double數和四個基本運算符+,-,*./ 。 然后，通過遍歷所有部分來保存實際運算符，並將其與結果和實際編號一起使用來計算結果。

private double evalLeftToRight(String text){
    double result = 0.0f;
    char actualOperant = ' ';
    Pattern pattern = Pattern.compile("((\\d*\\.\\d+)|(\\d+)|([\\+\\-\\*/]))");
    Matcher m = pattern.matcher(text);  
    while(m.find()) {
        String part = m.group();
        if     (part.equals("+")) actualOperant = '+';
        else if(part.equals("-")) actualOperant = '-';
        else if(part.equals("*")) actualOperant = '*';
        else if(part.equals("/")) actualOperant = '/';
        else{
            double actualNumber = Double.parseDouble(part);
            switch(actualOperant){
                case ' ': result = actualNumber; break;
                case '+': result += actualNumber; break;
                case '-': result -= actualNumber; break;
                case '*': result *= actualNumber; break;
                case '/': result /= actualNumber; break;
            }
        }
    }
    return result;
}

我認為您可以在onEqual()方法中使用它。

private void onEqual() {
    [...]
    String txt = txtScreen.getText().toString();
    double result = evalLeftToRight(txt);
    txtScreen.setText(Double.toString(result));
    [...]
}