# Category Archives: Theory of Computation & Automata

Prerequisite – Chomsky Hierarchy, Regular Languages As we all are aware that languages accepted by finite automata are called regular languages and those which are… Read More
Decidable Problems A problem is decidable if we can construct a Turing machine which will halt in finite amount of time for every input… Read More
Prerequisite – Pushdown Automata and Context Free Languages .  Suppose we have a context free grammar G with production rules: S->aSb|bSa|SS|ℇ   Left most derivation (LMD)… Read More
Moore Machines: Moore machines are finite state machines with output value and its output depends only on present state. It can be defined as (Q, q0, ∑, O, δ, λ) where: Q is finite set of states. q0 is the initial state. ∑ is the input alphabet. O is the output alphabet. δ is transition function which maps Q×∑ → Q. λ is the output function which maps Q → O. Figure 1 In the moore machine shown in Figure 1, the output is represented with each input state separated by /. The length of output for a moore machine is greater than input by 1. Input: 11 Transition: δ (q0,11)=> δ(q2,1)=>q2 Output: 000 (0 for q0, 0 for q2 and again 0 for q2)  Mealy Machines: Mealy machines are also finite state machines with output value and its output depends on present state and current input symbol. It can be defined as (Q, q0, ∑, O, δ, λ’) where: Q is finite set of states. q0 is the initial state. ∑ is the input alphabet. O is the output alphabet. δ is transition function which maps Q×∑ → Q. ‘λ’ is the output function which maps Q×∑→ O. Figure… Read More
The definition of context free grammars (CFGs) allows us to develop a wide variety of grammars. Most of the time, some of the productions of… Read More
Identifying languages (or problems*) as decidable, undecidable or partially decidable is a very common question in GATE. With correct knowledge and ample experience, this question… Read More
Turing Machine was invented by Alan Turing in 1936 and it is used to accept Recursive Enumerable Languages (generated by Type-0 Grammar).  A turing machine… Read More