java 从左往右遍历，计算算式的值

Posted 2021-05-08

package com.example.a24107.computeapplication2;

/**
 * Created by 24107 on 2017/12/8.
 */

public class StrToNum {
    /**
     * 从左往右遍历，先把括号找出来，再计算括号中的值，去掉括号，再计算*和/，再计算+和-，
     */

    public static void main(String[] args){
        float answer = LeftToRightCompute("2+2×5.5-2");
        System.out.println(answer);
    }

    public static float LeftToRightCompute(String str) {
        int numcount = -1;
        float firstnum = 1;
        float answer = 0;
        for (int i = 0; i < str.length(); i++) {
            if (str.charAt(i) >= '0' && str.charAt(i) <= '9') {
                if (numcount==-1) {
                    numcount = i;
                }
                firstnum = Float.parseFloat(str.substring(numcount, i+1));
                answer = firstnum;
                //return answer;
            }
            else {
                switch (str.charAt(i)) {
                    case '+':
                        numcount = -1;
                        answer=firstnum + LeftToRightCompute(str.substring(i + 1));
                        return answer;
                    //break;
                    case '-':
                        numcount = -1;
                        answer=firstnum - LeftToRightCompute(str.substring(i + 1));
                        return answer;
                    //break;
                    case '×':
                        numcount = -1;
                        float secondnummulti = FirstnumLeftToRight(str.substring(i+1));
                        float answer1 = firstnum * secondnummulti;
                        String newstringmuti;
                        if (secondnummulti==(int)secondnummulti) {
                            newstringmuti = str.substring(i + 1 + Float.toString(secondnummulti).length()-2);
                        }
                        else{
                            newstringmuti = str.substring(i + 1 + Float.toString(secondnummulti).length());
                        }
                        newstringmuti = answer1+ newstringmuti;
                        answer = LeftToRightCompute(newstringmuti);
                        return answer;
                        //break;
                    case '÷':
                        numcount = -1;
                        float secondnumchu = FirstnumLeftToRight(str.substring(i+1));
                        float answer2 = firstnum*secondnumchu;
                        String newstringchu = answer2+str.substring(i+1+Float.toString(answer2).length());
                        answer = LeftToRightCompute(newstringchu);
                        return answer;
                        //break;
                    case '(':
                        numcount = -1;
                        float answer3 = LeftToRightCompute(str.substring(i+1));
                        break;
                    case ')':
                        numcount = -1;
                        return answer;
                        //break;
                }
            }
        }
        return answer;
    }

    public static float FirstnumLeftToRight(String str) {
        int numcount = -1;
        float firstnum = 0;
        if (str.charAt(0)=='('){
            return LeftToRightCompute(str.substring(1));
        }
        else {
            for (int i = 0; i < str.length(); i++) {
                if (str.charAt(i) >= '0' && str.charAt(i) <= '9'||str.charAt(i)=='.') {
                    if (numcount==-1) {
                        numcount = i;
                    }
                        firstnum = Float.parseFloat(str.substring(numcount, i + 1));
                }
                else {
                    return firstnum;
                }
            }
        }
        return firstnum;
    }
}