计应191（西）宣大卫小学生口算

一、目的

学会用Java编写简单的四则运算。

二、步骤

思路分析：拿到表达式后，按照人的计算方法先乘除后加减，遍历运算符，得出结果。

三、核心代码

package sizeyunsuan;
 
import java.util.Scanner;
 
public class Szys {
        public static void main(String[] args) {
            Scanner str1 = new Scanner(System.in);
            System.out.println("请输入：");
            String str = str1.nextLine();
            String strs = str.replace("+","-");
            String[] s = strs.split("-");
            double temp=0;
            double sum=0;
            //遍历截取了的字符数组
            for(String a:s){
                //查找有*或/的数组
                if(a.contains("*")||a.contains("/")){
                    //遍历带符号的数组
                    for(int i=0;i<a.length();){
                        //设置一个变量，来控制下标，
                        int count = 1;
                        //需要截取的字符串长度
                        a:for(int j=i+1;j<a.length();j++){
                            //取出下一个字符
                            char c = a.charAt(j);
                            //System.out.println(c+"===");
                            //如果字符是*或/
                            if(c=='*'||c=='/'){
                                //终止a循环
                                break a;
                            }else{
                                //如果不是就长度加一；
                                count++;
                            }
                        }
                        //截取字符串
                        String strNun =a.substring(i,i+count);
//                      System.out.println(strNun+"=========");
                        //转变为数字
                        double d = Double.parseDouble(strNun);
                        //第一个数字
                        if(i==0){
                            temp = d;
                        }else{
                            //取出当前的数字前面的符号
                            char t =a.charAt(i-1);
                            //如果运算符是*
                            if(t=='*'){
                                //前一个数就乘它
                                temp*=d;
                            }else if(t=='/'){
                                temp/=d;
                            }
                        }
                        //把坐标向后移一个长度
                        i+=count+1;
                    }
                    str = str.replace(a, temp+"");
                }
            }
            //计算加减，与上面的思路一样
            for(int i = 0;i<str.length();){
                int count =1;
                a:for(int j=i+1;j<str.length();j++){
                    char c = str.charAt(j);
                    if(c=='+'||c=='-'){
                        break a;
                    }else{
                        count++;
                    }
                }
                String strNum = str.substring(i,i+count);
                double d  =Double.parseDouble(strNum);
                if(i==0){
                    sum=d;
                }else {
                    char s2 =str.charAt(i-1);
                    if(s2=='+'){
                        sum+=d;
                    }else if(s2=='-'){
                        sum-=d;
                    }
                }
                i+=count+1;
            }
            System.out.println(sum);
        }
    }