Translator – Operand Stack

When processing operators, it is necessary check the data types of it's operands already in the RPN list and it may be necessary to insert hidden conversion operators into the RPN list. The location of the first operand may not be the second to the last item added to the RPN list. Take the example A% * B% + C * D; the + operator will be the last operator to be processed, at which point the RPN list will contain A% B% MulInt C D MulDbl. For the + operator, which needs to be the add double operator also needs a CvtDbl inserted after the * operator, which has been changed to an multiple integer operator.

The Translator needs to save the location of where the previous operands are located, for both checking data types and for inserting conversion operators. This will be accomplished with another stack named the operand stack. When an operand is added to the RPN list, it's the pointer to element containing the operand in the RPN list will also be pushed onto the operand stack. When an operator is processed, it will pop it's operands off of the operand stack and use them to check data types and insert necessary conversion operator. After the operator is added to the RPN list, it's element pointer into the RPN list is pushed onto the operand stack for a future operator.

The operand stack parallels the run-time stack except instead of holding results of operands/operators, it will hold pointers to the items the will create the results during run-time.

Looking back at the example, when the + operator is processed, on the operand stack, there will be a pointer to the MulInt operator and a pointer to the MulDbl operator. It will insert a CvtDbl after the MulInt operator,  add an AddDbl to the RPN list and push it's element pointer onto the operand stack. The resulting RPN list will contain  A% B% AddInt CvtDbl C D MulDbl AddDbl. When this expression is executed during run-time, no time wasting decisions or checks need to be made.