从使用链表计算的操作中发出返回值的问题

Question

我目前正在研究旨在将字符串解析为链表的代码，并使用该列表作为计算操作的方法。 如所见，我当前将操作字符串设置为"1 + 2"，运行时，它返回整数 0。

非常感谢所有帮助。

import java.awt.Dimension;
import java.util.LinkedList;
import java.util.List;

public class LinkedListCalculator {
static List<String> CalcInput = new LinkedList<String>();
static String operation = "1+2";
static int equationLength = operation.length();
static int result = 0;
static int num = 0;
static int mul = 1;
static int finalNum = 0;
static char lastOperator = 0;

public static void addToList() {
    String finalNumString;
    if (lastOperator == '+') {
        finalNumString = Integer.toString(finalNum);
        CalcInput.add(finalNumString);
        CalcInput.add("+");
    } else if (lastOperator == '-') {
        finalNumString = Integer.toString(finalNum);
        CalcInput.add(finalNumString);
        CalcInput.add("-");
    } else if (lastOperator == '*') {
        finalNumString = Integer.toString(finalNum);
        CalcInput.add(finalNumString);
        CalcInput.add("*");
    } else if (lastOperator == '/') {
        finalNumString = Integer.toString(finalNum);
        CalcInput.add(finalNumString);
        CalcInput.add("/");
    }
}
public static void computeList() {
    int size = CalcInput.size();
    String lastIndex = null;
    for (int i = 0; i > size; i++) {
        String currentIndex = CalcInput.get(i);
            if (i > 0) {
                lastIndex = CalcInput.get(i - 1);
            }
            if (currentIndex == "+") {
                int lastIndexInt = Integer.valueOf(lastIndex);
                result += lastIndexInt;
            }
            if (currentIndex == "-") {
                int lastIndexInt = Integer.valueOf(lastIndex);
                result -= lastIndexInt;
            }
            if (currentIndex == "*") {
                int lastIndexInt = Integer.valueOf(lastIndex);
                result *= lastIndexInt;
            }
            if (currentIndex == "/") {
                int lastIndexInt = Integer.valueOf(lastIndex);
                result /= lastIndexInt;
            }
    }
}
public static void main(String [] args) {
    for (int currEqnCharNdx = 0; currEqnCharNdx < equationLength ; currEqnCharNdx++) {
        char j = operation.charAt(currEqnCharNdx);
        if (j >= '0' && j <= '9') {
            num = j - '0';
            finalNum *= mul;
            finalNum += num;
            mul = 10;
        } else if (operation.charAt(currEqnCharNdx) == '+') {
            addToList();
            lastOperator = '+';
            mul = 1;
            finalNum = 0;
        } else if (operation.charAt(currEqnCharNdx) == '-') {
            addToList();
            lastOperator = '-';
            mul = 1;
            finalNum = 0;
        } else if (operation.charAt(currEqnCharNdx) == '*') {
            addToList();
            lastOperator = '*';
            mul = 1;
            finalNum = 0;
        } else if (operation.charAt(currEqnCharNdx) == '/') {
            addToList();
            lastOperator = '/';
            mul = 1;
            finalNum = 0;
        } 
    }
    int i = CalcInput.size();
    computeList();
    System.out.println("result: " + result);
}
}

Answer 1

你的代码有很多问题。以下是一些指向错误的指针：

String lastIndex = null;
    for (int i = 0; i > size; i++) {

这将永远不会执行（更改为 i <大小）>

我也怀疑addToList（）应该在设置lastOperator等之后发生。

 else if (operation.charAt(currEqnCharNdx) == '+') {
                addToList();
                lastOperator = '+';
                mul = 1;
                finalNum = 0;
            }

另外，在这里（在addToList（）中）：

if (lastOperator == '+') {
        finalNumString = Integer.toString(finalNum);
        CalcInput.add(finalNumString);
        CalcInput.add("+");
    }

你加上最后一个数字，加上 + 号，但没有什么会导致 2 被添加，所以当它在最后求和时（如果你修复我提到的其他事情），它将对 1+ 求和，结果为 1（而不是 1+2 使结果为 3）。

这是它输出 1（见上面最后一点）供 OP 重现的代码：

import java.awt.Dimension;
import java.util.LinkedList;
import java.util.List;
import java.util.StringTokenizer;

public class LinkedListCalculator {
static List<String> CalcInput = new LinkedList<String>();
static String operation = "1+2";
static int equationLength = operation.length();
static int result = 0;
static int num = 0;
static int mul = 1;
static int finalNum = 0;
static char lastOperator = 0;

public static void addToList() {
    String finalNumString;
    if (lastOperator == '+') {
        finalNumString = Integer.toString(finalNum);
        CalcInput.add(finalNumString);
        CalcInput.add("+");
    } else if (lastOperator == '-') {
        finalNumString = Integer.toString(finalNum);
        CalcInput.add(finalNumString);
        CalcInput.add("-");
    } else if (lastOperator == '*') {
        finalNumString = Integer.toString(finalNum);
        CalcInput.add(finalNumString);
        CalcInput.add("*");
    } else if (lastOperator == '/') {
        finalNumString = Integer.toString(finalNum);
        CalcInput.add(finalNumString);
        CalcInput.add("/");
    }
}
public static void computeList() {
    int size = CalcInput.size();
    String lastIndex = null;
    for (int i = 0; i < size; i++) {
        String currentIndex = CalcInput.get(i);
            if (i > 0) {
                lastIndex = CalcInput.get(i - 1);
            }
            if (currentIndex == "+") {
                int lastIndexInt = Integer.valueOf(lastIndex);
                result += lastIndexInt;
            }
            if (currentIndex == "-") {
                int lastIndexInt = Integer.valueOf(lastIndex);
                result -= lastIndexInt;
            }
            if (currentIndex == "*") {
                int lastIndexInt = Integer.valueOf(lastIndex);
                result *= lastIndexInt;
            }
            if (currentIndex == "/") {
                int lastIndexInt = Integer.valueOf(lastIndex);
                result /= lastIndexInt;
            }
    }
}
public static void main(String [] args) {

    for (int currEqnCharNdx = 0; currEqnCharNdx < equationLength ; currEqnCharNdx++) {
        char j = operation.charAt(currEqnCharNdx);
        if (j >= '0' && j <= '9') {
            num = j - '0';
            finalNum *= mul;
            finalNum += num;
            mul = 10;
        } else if (operation.charAt(currEqnCharNdx) == '+') {
            lastOperator = '+';
            addToList();
            mul = 1;
            finalNum = 0;
        } else if (operation.charAt(currEqnCharNdx) == '-') {
            addToList();
            lastOperator = '-';
            mul = 1;
            finalNum = 0;
        } else if (operation.charAt(currEqnCharNdx) == '*') {
            addToList();
            lastOperator = '*';
            mul = 1;
            finalNum = 0;
        } else if (operation.charAt(currEqnCharNdx) == '/') {
            addToList();
            lastOperator = '/';
            mul = 1;
            finalNum = 0;
        } 
    }
    int i = CalcInput.size();
    computeList();
    System.out.println("result: " + result);
}
}