I am trying to convert this C program to Java.

double expression();

char token() { 
    char ch;
    std::cin >> ch;
    return ch;
}

double factor() { 
    double val = 0;
    char ch = token();
    if (ch == '(') {
        val = expression();
        ch = token();
        if (ch != ')') {
            std::string error = std::string("Expected ')', got: ")   ch;
            throw std::runtime_error(error.c_str());
        }
    }
    else if (isdigit(ch)) {
        std::cin.unget();
        std::cin >> val;
    }
   else throw std::runtime_error("Unexpected character");
    return val;
}

double term() { 
    int ch;
    double val = factor();
    ch = token();
    if (ch == '*' || ch == '/') {
        double b = term();
        if (ch == '*')
            val *= b;
        else
            val /= b;
    }
    else std::cin.unget(); // here it is
    return val;
}

double  expression() {
    double val = term();
    char ch = token();
    if (ch == '-' || ch==' ') {
        double b = expression();
        if (ch == ' ')
            val  = b;
        else
            val -= b;
    }
    else std::cin.unget();
    return val;
}

The program can evaluate (or simplify) mathematical expressions (i.e. (5 6-(6*2)/3) ) In term(), "unget" the character if the character is equivalent to any digit. But I can't find "unget()" in Java. And I am a little bit confused regarding this "Unget" operation.

What can I replace with the equivalent of "unget()" in C for Java?

CodePudding user response：

InputStream interface in Java supports two methods mark and reset, subclasses may or may not support them.

if (!stream.markSupported())
  stream = new BufferedInputStream(stream);  // Supports mark/reset

// If a C   function calls unget(), call mark() before token()
// in a Java function, or let token() call mark(),
// and call reset() instead of unget().
stream.mark();  // Remember current position in stream
// read data, process data
stream.reset();  // Rewind to the remembered position

CodePudding user response：

I suggest to use the scanner class to read from console. It can check if for example the next chars on the console are a int, or a given pattern (the math expression) and if so it loads the input.

For example:

// Using Scanner for Getting Input from User
String mathematicalOperation = "\([0123456789 -]*\)" ;
Pattern pattern = Pattern.compile(mathematicalOperation);
Scanner in = new Scanner(System.in);
if(in.hasNextInt){ 
   val = in.nextInt();}
else if (in.hasNext(Pattern pattern) ) {
   String expresionRaw = in.next(Pattern pattern);
   val = expreion(expresionRaw);
}

Or this : if unget moves the input pointer back then if you are using in java this can be done with the reset() methid of the BufferedReader class