This document describes shift-reduce parsing. Shift-reduce parsing is a bottom-up parsing approach where the input string is collapsed by reducing parts of the string according to production rules until the start symbol is reached, as opposed to top-down parsing which expands symbols. It uses two main data structures: an input buffer and a stack. Initially, it puts the input string in the buffer and a start symbol on the stack. It then performs the basic operations of shift, which moves symbols from the buffer to the stack, and reduce, which replaces symbols on the stack according to production rules. It halts when the start symbol remains on the stack and the buffer is empty, indicating successful parsing.

1. SHIFT REDUCE PARSER
SUBMITTED BY
TEJVEER SINGH
MCA 2nd Year
SUBMITTED TO
Dr. VANDANA RANA

2. DEFINITION AND EXPLANATION
Bottom-up parsing is based on the reverse process to top-down.
Instead of expanding successive non-terminals according to
production rules, a current string form is collapsed each time until
the start non-terminal is reached to predict the legal next symbol.
This can be regarded as a series of reductions. This approach is also
known as shift-reduce parsing.
This is the primary parsing method for many compilers, mainly
because of its speed and the tools which automatically generate a
parser based on the grammar.

3. Shift reduce parser requires mainly two
data structures:
1. A input buffer for storing the input string.
2. A stack for storing and accessing the production rules.

4. WORKING
Initially, shift-reduce parser is present in the following
configuration
1. Stack contains only the $ symbol.
2. Input buffer contains the input string with $ at its
end.

5. .
After achieving this configuration,
• The parser stops / halts.
• It reports the successful completion of parsing.
• An error is detected.
• Or stack is left with only the start symbol and the input buffer
becomes empty.

6. Basic Operations –
• Shift: This involves moving of symbols from input buffer onto the top of
the stack.
• Reduce: The handle appearing on the stack top is
replaced with the appropriate non-terminal symbol.

7. • Accept: If only start symbol is present in the stack and the
input buffer is empty then, the parsing action is called accept.
When accept action is obtained, it means successful parsing is
done.
• Error: In this situation parser becomes confused.
Since the parser can neither perform shift action nor reduce
action and not even accept action.

8. Rules To Remember
• If the priority of incoming operator is more than the priority of in stack
operator, then shift action is performed.
It is important to remember the following rules while performing the
shift-reduce action
• If the priority of in stack operator is same or less than the priority of in
stack operator, then reduce action is performed.

9. Problem-01 Consider the following grammar-
E → E – E
E → E * E
E → id
Parse the input string “ id – id * id “ using a shift-reduce parser.
Solution :- The priority order is: id > * > –
$ id – id * id $ Shift
$ id – id * id $ Reduce E →
id$ E – id * id $
Shift$ E – id * id $ Shift
$ E – id * id $ Reduce E → id
$ E – E * id $
Shift$ E – E * id $ Shift
$ E – E * id $ Reduce
E → id$ E – E * E $ Reduce
E → E * E

10. Problem 02: – Consider the grammar
S –> S + S
S –> S * S
S –> id
Perform Shift Reduce parsing for input string “id + id + id”.
SOLUTION:-

11. Problem 03: – Consider the grammar
E –> 2E2
E –> 3E3
E –> 4
Perform Shift Reduce parsing for input string “32423”.
Solution:-

12. THANK YOU