How to Draw a Syntax Tree in Compiler Design

What is Syntax Tree?

tree in which each leaf node describes an operand & each interior node an operator. The syntax tree is shortened form of the Parse Tree.

Example1 − Draw Syntax Tree for the string a + b ∗ c − d.

Rules for constructing a syntax tree

Each node in a syntax tree can be executed as data with multiple fields. In the node for an operator, one field recognizes the operator and the remaining field includes a pointer to the nodes for the operands. The operator is known as the label of the node. The following functions are used to create the nodes of the syntax tree for the expressions with binary operators. Each function returns a pointer to the recently generated node.

mknode (op, left, right) − It generates an operator node with label op and two field including pointers to left and right.
mkleaf (id, entry) − It generates an identifier node with label id and the field including the entry, a pointer to the symbol table entry for the identifier.
mkleaf (num, val) − It generates a number node with label num and a field including val, the value of the number. For example, construct a syntax tree for an expression a − 4 + c. In this sequence, p₁, p₂, … . . p₅are pointers to the symbol table entries for identifier 'a' and 'c' respectively.

p₁− mkleaf (id, entry a); p₂− mkleaf (num, 4); p₃− mknode ( ′−′, p₁, p₂) p₄− mkleaf(id, entry c) p₅− mknode(′+′, p₃, p₄);

The tree is generated in a bottom-up fashion. The function calls mkleaf (id, entry a) and mkleaf (num 4) construct the leaves for a and 4. The pointers to these nodes are stored using p₁and p₂. The call mknodes (′−′, p₁, p₂ ) then make the interior node with the leaves for a and 4 as children. The syntax tree will be

Syntax Directed Translation of Syntax Trees

Production	Semantic Action
E → E⁽¹⁾ + E⁽²⁾	{E. VAL = Node (+, E⁽¹⁾. VAL, E⁽²⁾. VAL)}
E → E⁽¹⁾ ∗ E⁽²⁾	{E. VAL = Node (∗, E⁽¹⁾. VAL, E⁽²⁾. VAL)})
E → (E⁽¹⁾)	{E. VAL = E⁽¹⁾. VAL}
E → E⁽¹⁾	{E. VAL = UNARY (−, E⁽¹⁾. VAL}
E → id	{E. VAL = Leaf (id)}