What is Instantaneous Description and Turnstile Notation?

Data Structure AlgorithmsComputer ScienceComputers

The instantaneous description (ID) of a push down automata (PDA) is represented by a triple (q,w,s)


  • q is the state.
  • w is unconsumed input.
  • s is the stack contents.

ID is an informal notation of how a PDA compares an input string and makes a decision that string is accepted or rejected.

Turnstile Notation

It is used for connecting pairs of ID's that represent one or more moves of a PDA.

The process of transition is denoted by the turnstile symbol "⊢"

⊢ it represents one move.

⊢ sign describes a sequence of moves.


(P,b,T) ⊢ (q,w,a)

While taking a transition from P to q the input symbol 'b' is consumed and top of the stack 'T' is represented.

Consider another example to understand more about ID and Turnstile Notation

Problem: Find out IDs for input string w = "aaabb" of PDA. and check whether string is accepted by PDA or not?

Solution: Let us see the Instantaneous Description for string w = "aaabb"

(q0,aaabb,Z0) |- (q0, aabb, aZ0) {based on Transition rule 1}

|-(q0,abb,aaZ0) {Based on transition rule 2}

|-(q0,bb,aaaZ0) {Based on transition rule 2}

|-(q1,b,aaZ0){ Based on transition rule 3}

|-(q1,λ,aZ0){ based on transition rule 3}

|- There is no defined move.

So finally the pushdown automaton stops at this move and the string is not accepted because the input string w is completed or input tape is empty, but the PDA stack is not empty.

So the string 'w' is not accepted.

Published on 16-Jun-2021 12:33:54