字符串的四则运算表达式

jiezi
5 年前
    public static void main(String[] args) {
        // 支持括号 小数 负数
        String statement = "-10/(4.5+5.5)*(-4-6+20)/-2"; // 10/(-2) 也行
        System.out.println(calculate(statement));
    }

    @SuppressWarnings("unchecked")
    private static double calculate(String statement){Object[] result = filter(statement);
        // 运算数字
        List<Double> numList = (List) result[0];
        // 运算符
        List<Character> symbolList = (List) result[1];
        while (!symbolList.isEmpty()) {int index = symbolList.indexOf('(');
            if (index == -1) {
                // 没有括号正常运算
                realCalculate(numList, symbolList);
            } else {int right = symbolList.indexOf(')');
                if (right == index + 1) {
                    // 括号内的值都取完了，删除括号
                    symbolList.remove(index);
                    symbolList.remove(index);
                    continue;
                }
                // 左右括号齐全 先算括号内的
                if (right != -1) {List<Double> doubles = numList.subList(index, right);
                    List<Character> subChars = symbolList.subList(index + 1, right);
                    realCalculate(doubles, subChars);
                }
            }
        }
        return numList.get(0);
    }

    /**
     * @return 一个包含数字的列表和一个包含运算符的列表
     */
    private static Object[] filter(String statement) {
        // 形式 123,456,789 可能会有空字符串
        StringBuilder nums = new StringBuilder();
        // 符号列表
        List<Character> symbolList = new LinkedList<>();
        for (int i = 0; i < statement.length(); i++) {char c = statement.charAt(i);
            if (c == '-' && (i == 0 || statement.charAt(i - 1) == '(' 
            || statement.charAt(i - 1) == '*' || statement.charAt(i - 1) == '/')) {nums.append(c).append(statement.charAt(i + 1));
                i++;
            } else if (Character.isDigit(c) || c == '.') {nums.append(c);
            } else {symbolList.add(c);
                nums.append(',');
            }
        }

        String[] ss = nums.toString().split(",");
        List<Double> numList = new ArrayList<>();
        for (String num : ss) {if (!num.isEmpty()) {numList.add(Double.parseDouble(num));
            }
        }
        return new Object[]{numList, symbolList};
    }

    private static void realCalculate(List<Double> numList, List<Character> symbolList) {while (!symbolList.isEmpty()) {int index = symbolList.indexOf('*'), tmp;
            double value = 0.0D;
            if (index != -1 && (tmp = symbolList.indexOf('/')) != -1) {
                // 同时出现 * / 从左至右运算
                if (index < tmp) {value = numList.remove(index) * numList.remove(index);
                } else {
                    index = tmp;
                    value = numList.remove(index) / numList.remove(index);
                }
            } else if (index != -1) {value = numList.remove(index) * numList.remove(index);
            } else if ((index = symbolList.indexOf('/')) != -1) {value = numList.remove(index) / numList.remove(index);
            } else if ((index = symbolList.indexOf('+')) != -1 && (tmp = symbolList.indexOf('-')) != -1) {
                // 同时出现 + - 从左至右运算
                if (index < tmp) {value = numList.remove(index) + numList.remove(index);
                } else {
                    index = tmp;
                    value = numList.remove(index) - numList.remove(index);
                }
            } else if (index != -1) {value = numList.remove(index) + numList.remove(index);
            } else if ((index = symbolList.indexOf('-')) != -1) {value = numList.remove(index) - numList.remove(index);
            }
            // 删除运算符
            symbolList.remove(index);
            // 将计算结果放回列表，待下次计算
            numList.add(index, value);
        }
    }
我的方法是先从括号的算起，根据运算符索引查找运算数索引，从而进行计算，算完后删除运算符和运算数，并将运算结果放回待运算的列表
总结一下
