UMBC CMSC 331 ONLINE
LECTURE 3
SD Vick
Mini Lectures

Left Recursion defined and explained with and caveats
(see
http://www.csee.umbc.edu/~vick/331/online/Left_Rec
_Rem_1.swf)

Left recursion removal example (see
http://www.csee.umbc.edu/~vick/331/online/Left_Rec
_rem_2.swf)

(Full text of mini-lectures
http://www.csee.umbc.edu/~vick/331/online/L3.ppt )
What’s the General Rule?
A -> A a1 | A a2 | … A am | b1 | b2 | bn
________________________________________
A-> b1 A’ | b2 A’ … bn A’
A’ -> a1 A’ | a2 A’ | … am A’ | e
Removing Left Recursion

Consider the following Grammar
<z> -> <w> ? <x> |
<z> ! <y> |
a <z>
<x> -> <z> * <x> |
<x> / <y>
|
<w>
<w> ->
e | f
Why is this Left
Recursive?
 Now we must just pick
our a’s and our b’s

The Mechanics
<z> -> <w> ? <x> |
<z> -> <w> ? <x> <z’> |
a <z> <z’>
<z’> -> ! <y> <z’> | e
b1
<z> ! <y> |
a1
a <z>
b2
Try the second rule on your own
 Here we have 2 b‘s and 1 a,
what if the situation was
reversed?
