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Writing a String Evaluation Function

I am trying to write a String ie evaluation function

evaluate("4 + 1") ; // returns 5 
evaluate("4 + 1 + 3") ; // returns 8 
evaluate("4 + 1 * 3") ; // returns 7 (not 15) 

The operators are + - / and *

At first, I wanted to use regular expressions to collect operators and numbers, as they can be matched. And the later you find this information, somehow figure out a way to prioritize /*ove statements -+.

Here's how I started:

static String regex = "([\\+\\*-/])+";
static String digitRegex = "(\\d)+";

public static void main(String[] args) {
    System.out.println(getOperators("4 + 1 * 3"));
}

public static List<String> getOperators(String input) {
    Pattern p = Pattern.compile(regex);
    Matcher matcher = p.matcher(input);

    List<String> operatorList = new ArrayList<String>();

    int count = 0;
    while (matcher.find()){
        if (matcher.group(count) != null && matcher.group(count).trim().length() > 0) {
        operatorList.add(matcher.group(count));
        count++;
        }
    }

    return operatorList;
}

Now I can write another method for extracting numbers using the same logic.

public static List<Integer> getDigits(String input) {
        Pattern p = Pattern.compile(digitRegex);
        Matcher matcher = p.matcher(input);

        List<Integer> digitList = new ArrayList<Integer>();

        int count = 0;
        while (matcher.find()) {
            if (matcher.group(count) != null && matcher.group(count).trim().length() > 0) {
                digitList.add(Integer.valueOf(matcher.group(count)));
                count++;
            }
        }

        return digitList;
    }

Now this is the part where I am stuck. # 1 This method described above fails in the third example:

evaluate("4 + 1 * 3") ; // returns 7 (not 15)

And this is # 2. Even if I try the previous examples, I cannot figure out how to place them correctly.

I'm on the right track at all, does anyone have any good advice, please share?

+5
source
2

-... , - ...
, " ". "" - / StringTokenizer, ;)

public class NewClass {

    public static int evaluate(String str){
        if("".equals(str)){
            return 0;
        }
        else if(str.length() == 1){
            return Integer.valueOf(str);
        }
        else{
            String _a = String.valueOf(str.charAt(0));
            String _b = String.valueOf(str.charAt(1));
            if("+".equals(_b) || "-".equals(_b) ){
                if("+".equals(_b)){
                    return Integer.valueOf(_a) + evaluate(str.substring(2));
                }
                else{// "-"
                    return Integer.valueOf(_a) - evaluate(str.substring(2));
                }
            }
            else{// "*" or "/"
                boolean isMulti = ("*".equals(_b));
                String  _c = String.valueOf(str.charAt(2));                
                Integer tmp = 0;
                if(isMulti){
                    tmp = Integer.valueOf(_a) * Integer.valueOf(_c);
                }
                else{
                    tmp = Integer.valueOf(_a) / Integer.valueOf(_c);
                }
                String new_str = String.valueOf(tmp) + str.substring(3);                
                return evaluate(new_str);
            }
        }
    }

    public static void main(String[] args){        
        String e = "4+1*3";
        int t = evaluate(e);
        System.out.println(e + " = "+t);
    }

}
+2

. , , , . , , , ( ) .

OPP . .. .

edit - . .

.

2 -

c. .

.

, , . , , .

+1

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