Generate valid infix arithmetic expression randomly

Question

For my program I need to generate valid infix expressions with customizable complexity. The problem is that I cant figure out a way to prevent division by zero, floating point answers and negative answers.

To prevent negative answers, I'm taking a dirty approach. That is, generate an expression, evaluate it, if it turns out to be negative, generate again.Here are some things you should know:

1. inToPost() is a method that converts the generated infix expression to postfix for evaluation.
2. complexityLevel <= DIVIDE implies that we are not supposed to put parenthesis in the expression.
3. complexityLevel == ARITHMETIC_PARENTHESIS implies that parenthesis are included.

How can I make sure that a) there are no divisions by zero b) no divisions result in floating point (figured out a dirty way to do this) c) final result is not negative Here is the code

public void generateRandom(int operandLimit, int operatorCount, int complexityLevel) {
        Random rand = new Random();
        infix.clear();

        int i = 0;
        infix.add( rand.nextInt(operandLimit) + 1 );

        while(i < operatorCount) {
            int operator;
            if(complexityLevel <= DIVIDE)
                operator = rand.nextInt(complexityLevel - 1)*1000 + 1000;
            else
                operator = rand.nextInt(complexityLevel - 3)*1000 + 1000;

            int operand = rand.nextInt(operandLimit) + 1;

            if( operator == Operator.DIVIDE ) {
                int lastNum = infix.get(infix.size() - 1);

                if( lastNum < operand) {
                    int temp = operand;
                    operand = lastNum;
                    lastNum = temp;
                }

                lastNum -= lastNum % operand;
                infix.set(infix.size() - 1, lastNum);
            }

            infix.add( operator );
            infix.add( operand );

            ++i;
        }

        if(complexityLevel == ARITMETIC_PARENTHESIS) {
            int braceOpen = rand.nextInt( operatorCount ) * 2;
            infix.add(braceOpen, Operator.BR_OPEN );
            infix.add(braceOpen + 4, Operator.BR_CLOSE);
        }

        inToPost();
        if(evaluate() < 0)
            generateRandom(operandLimit, operatorCount, complexityLevel);
    }

Answer 1

It looks like you've dealt with your conditions (b) and (c) already. Since your operands are never 0, I would guess that the only possible violation of (a) is if the added parentheses happen to wrap a zero value, and the operator before that is a division. You could check for that case if you modified your inToPost() to take subexpressions:

if(braceOpen > 0 && infix.get(braceOpen) == Operator.DIVISION && 
        evaluate(inToPost(infix.subList(braceOpen, braceOpen + 3))) == 0) {
    // Put parentheses elsewhere, or cancel
}