Translation schemes

. A CFG where semantic actions occur within the rhs of production

. A translation scheme to map infix to postfix

E T R

R addop T {print(addop)} R | e

T num {print(num)}

parse tree for 9 - 5 + 2

. Assume actions are terminal symbols

. Perform depth first order traversal to obtain 9 5 - 2 +

. When designing translation scheme, ensure attribute value is available when referred to

. In case of synthesized attribute it is trivial (why ?)

In a translation scheme, as we are dealing with implementation, we have to explicitly worry about the order of traversal. We can now put in between the rules some actions as part of the RHS. We put this rules in order to control the order of traversals. In the given example, we have two terminals (num and addop). It can generally be seen as a number followed by R (which necessarily has to begin with an addop). The given grammar is in infix notation and we need to convert it into postfix notation. If we ignore all the actions, the parse tree is in black, without the red edges. If we include the red edges we get a parse tree with actions. The actions are so far treated as a terminal. Now, if we do a depth first traversal, and whenever we encounter a action we execute it, we get a post-fix notation. In translation scheme, we have to take care of the evaluation order; otherwise some of the parts may be left undefined. For different actions, different result will be obtained. Actions are something we write and we have to control it. Please note that translation scheme is different from a syntax driven definition. In the latter, we do not have any evaluation order; in this case we have an explicit evaluation order. By explicit evaluation order we have to set correct action at correct places, in order to get the desired output. Place of each action is very important. We have to find appropriate places, and that is that translation scheme is all about. If we talk of only synthesized attribute, the translation scheme is very trivial. This is because, when we reach we know that all the children must have been evaluated and all their attributes must have also been dealt with. This is because finding the place for evaluation is very simple, it is the rightmost place.