Translator – First and Last Operand

When an error occurs, the inclusive range between the first and last tokens of the offending expression will be reported. Take the previous example statement Z$=A$+B$>C$. The >$ token will be on top of the done stack when the expression type error is detected (the >$ operator returns an integer and cannot be assigned to a string variable). The entire expression from A$ to C$ should be highlighted.

This will be accomplished by saving the first and last operand for each operator as the expression is processed. When an expression type error is detected, these operands will be available. When assigning the first operand to an operator, if the operand on done stack has a first operand assigned to it, then that operand will be used, otherwise the operand token on the done stack will be assigned. The same for the last operand.

In the statement above, the +$ operator will have A$ for its first operand (since A$ does not have a first operand assigned to it) and B$ for its last operand (B$ does not have a last operand). At the >$ operator, its first operand will be assigned to A$ (which is the +$ operand's first operand) and C$ for its last operand (C$ does not have a last operand).

The first operand is no longer saved on the done stack except for string operands (a specific type operator has already been selected), the first operand needs to be attached to the token on the hold stack where the operator is residing until put into the output list. Therefore, the hold stack needs to hold structures containing a token pointer and a first operand token pointer instead of just a token pointer – a new hold stack item will be defined to hold these.

The done stack currently holds output list element pointers (the output list consists of RPN items each containing a token pointer, number of operands and attached operand array, if any). The done stack will also need to hold the first and last operand token pointers. Therefore, the done stack needs to hold structures containing an output list element item pointer along with the first and last token pointers – a new done stack item will be defined to hold these.