# NPDA for accepting the language L = {am bn cp dq | m+n=p+q ; m,n,p,q>=1}

Prerequisite – Pushdown automata, Pushdown automata acceptance by final state
Problem – Design a non deterministic PDA for accepting the language L = {    | m + n = p + q : m, n, p, q>=1}, i.e.,

L = {abcd, abbcdd, abbccd, abbbccdd, ......}

In each of the string, the total number of ‘a’ and ‘b’ are equal to the total number of ‘c’ and ‘d’.

Explanation –
Here, we need to maintain the order of ‘a’, ‘b’, ‘c’ and ‘d’. Thus, we need a stack along with the state diagram. The count of ‘a’, ‘b’, ‘c’ and ‘d’ is maintained by the stack. We will take 2 stack alphabets: = { 1, z }

Where, = set of all the stack alphabet
z = stack start symbol

Approach used in the construction of PDA –
As we want to design a NPDA, thus every time ‘a’, ‘b’, ‘c’ and ‘d’ will comes in proper order. When ‘a’ and ‘b’ comes then we will push ‘1’ into the stack. After that, when ‘c’ and ‘d’ comes then pop ‘1’ from the stack each time. So, at the end if the stack becomes empty then we can say that the string is accepted by the PDA.

Stack transition functions – (q0, a, z) (q0, 1z) (q0, a, 1) (q0, 11) (q0, b, 1) (q1, 11) (q1, b, 1) (q1, 11) (q1, c, 1) (q2, ) (q2, c, 1) (q2, ) (q2, d, 1) (q3, ) (q3, d, 1) (q3, ) (q3, , z) (qf, z)


Where, q0 = Initial state
qf = Final state = indicates pop operation So, this is our required non deterministic PDA for accepting the language L = {    | m + n = p + q ; m, n, p, q>=1 }. My Personal Notes arrow_drop_up Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.

Article Tags :

Be the First to upvote.

Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.