Android Calcluate字符串从左到右 我使用这个代码来计算一个字符串，但是这个代码计算字符串而不考虑优先级的顺序。 Exp 3+3*3输出12.0 但是我想得到18.0的输出 计算应该从左到右我如何修改这段代码我真的不知道为什么有人需要这段代码，但给你。我有一个方法可以让你从左到右计算一个算术字符串，方法是使用一个正则表达式，将双数字中的元素和四个基本运算符+，-，*。/。然后，它通过遍历所有部分来计算结果，保存实际操作员，并将其与结果和实际数字一起使用 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; } } } }

我认为您可以在

onEqual（）

方法中使用它

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;
}

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由于您的计算逻辑在对库方法

evaluate（）

的调用中，除非我们看到该代码，否则我们无法为您提供太多帮助。显然，有很多方法可以使用字符串输入进行数学计算，但是您必须滚动自己的类，而不是使用库。

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