Example_10.py

#!/usr/bin/python3

# Implement Elimination of Left Recursion
gram = {}

def add(str):                               #to rules together
    x = str.split("->")
    y = x[1]
    x.pop()
    z = y.split("|")
    x.append(z)
    gram[x[0]]=x[1]

def removeDirectLR(gramA, A):        
	"""gramA is dictonary"""
	temp = gramA[A]
	tempCr = []
	tempInCr = []
	for i in temp:
		if i[0] == A:
			#tempInCr.append(i[1:])
			tempInCr.append(i[1:]+[A+"'"])
		else:
			#tempCr.append(i)
			tempCr.append(i+[A+"'"])
	tempInCr.append(["e"])
	gramA[A] = tempCr
	gramA[A+"'"] = tempInCr
	return gramA


def checkForIndirect(gramA, a, ai):
	if ai not in gramA:
		return False 
	if a == ai:
		return True
	for i in gramA[ai]:
		if i[0] == ai:
			return False
		if i[0] in gramA:
			return checkForIndirect(gramA, a, i[0])
	return False

def rep(gramA, A):
	temp = gramA[A]
	newTemp = []
	for i in temp:
		if checkForIndirect(gramA, A, i[0]):
			t = []
			for k in gramA[i[0]]:
				t=[]
				t+=k
				t+=i[1:]
				newTemp.append(t)

		else:
			newTemp.append(i)
	gramA[A] = newTemp
	return gramA

def rem(gram):
	c = 1
	conv = {}
	gramA = {}
	revconv = {}
	for j in gram:
		conv[j] = "A"+str(c)
		gramA["A"+str(c)] = []
		c+=1

	for i in gram:
		for j in gram[i]:
			temp = []	
			for k in j:
				if k in conv:
					temp.append(conv[k])
				else:
					temp.append(k)
			gramA[conv[i]].append(temp)


	#print(gramA)
	for i in range(c-1,0,-1):
		ai = "A"+str(i)
		for j in range(0,i):
			aj = gramA[ai][0][0]
			if ai!=aj :
				if aj in gramA and checkForIndirect(gramA,ai,aj):
					gramA = rep(gramA, ai)

	for i in range(1,c):
		ai = "A"+str(i)
		for j in gramA[ai]:
			if ai==j[0]:
				gramA = removeDirectLR(gramA, ai)
				break

	op = {}
	for i in gramA:
		a = str(i)
		for j in conv:
			a = a.replace(conv[j],j)
		revconv[i] = a

	for i in gramA:
		l = []
		for j in gramA[i]:
			k = []
			for m in j:
				if m in revconv:
					k.append(m.replace(m,revconv[m]))
				else:
					k.append(m)
			l.append(k)
		op[revconv[i]] = l

	return op

n = int(input("Enter No of Production: "))
for i in range(n):
    txt=input()
    add(txt)
   
result = rem(gram)

for x,y in result.items():
    print(f'{x} -> {y}')