Added push down automata implementation

Author: DeepankarAcharyya

Automaton/PushDown-Automata.py

@@ -0,0 +1,90 @@
+# A Simple Implementation of Push Down Automaton:
+##    Defining the 6 tuples:
+#>  *  Q-->finite set of sets
+#>  *  Sigma-->finite set of input alphabet
+#>  *  Gamma-->finite set of stack alphabet
+#>  *  Delta-->transition relation
+#>  *  Q0-->start state
+#>  *  Z-->initial stack symbol
+#>  *  F-->set of accepting states
+
+# Defining the states:
+
+#* state 0: starting state
+
+#* state 1:From state 0 whenever it sees a 1 or 2, it moves to state 1+pushes the element onto the stack
+#* state 1:From state 1 whenever it sees a 1 or 2, it remains in state 1+pushes the element onto the stack
+
+#* state 2:From state 1 whenever it sees a 0, it moves to state 2+pops from the stack
+#* state 2:From state 1 whenever it sees a 0, it remains in state 2+pops from the stack
+
+#* state 3:From state 0, if it sees a 0,it moves to state 3,the rejected state
+#* state 3:From state 2, if it sees a 1 or 2 , it moves to state 3, the rejected state
+#* state 3:If at the end, the stack is not empty, it moves to state 3,the rejected state
+
+#stack functions
+def push(a,list1):
+    #pushing to the stack/adding to the top of the stack
+    list1.append(a)
+    return 1
+
+def pop(list1):
+    #for poping from the stack/removing the top element of the stack
+    index=len(list1)-1
+    if (index>0):
+        list1.pop(index)
+        return 1
+    else:
+        return 0
+
+#    Q={0,1,2,3}
+#    Sigma={0,1,2}
+#    Starting state={0}
+#    Z=#
+#    F={2}
+
+#setting the initial stack symbol
+stack=['#']
+#setting the starting state
+state=0
+
+#taking the input
+input_string=input('Enter the String:')
+
+#performing the operations
+l=len(input_string)
+i=0
+if l%2==0:
+    while i<l//2:
+        letter=int(input_string[i])
+        if letter in [1,2]:
+            push(letter,stack)
+            state=1
+        else :
+            state=3
+            break
+        i+=1
+    
+    while i<l:
+        letter=int(input_string[i])
+        if state==3:
+            break
+        if letter==0:
+            state=2
+            pop(stack)
+        else:
+            state=3
+            break
+        i+=1
+else:
+    state=3
+    
+if state==2 and len(stack)!=1:
+    state=3
+    
+
+#checking the final state and displaying the result
+if(state==2):
+    print("The String is accepted")
+else:
+    print("The String is rejected")