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Compiler Design LL(1) Parser in Python

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Prerequisite: Construction of LL(1) Parsing Table, Classification of top-down parsers, FIRST Set, FOLLOW Set 

In this article, we are going to see how to design LL(1) Parser compiler using Python.

LL(1) grammar

The first ‘L’ in LL(1) stands for scanning the input from left to right, the second ‘L’ stands for producing a leftmost derivation, and the ‘1’ for using one input symbol of lookahead at each step to make parsing action decisions. LL(1) grammar follows Top-down parsing method. For a class of grammars called LL(1) we can construct grammars predictive parser. That works on the concept of recursive-descent parser not requiring backtracking.

Elimination of Left Recursion

A grammar is left recursive if it has a nonterminal A such that there is a derivation A → A α | β. Top-down parsing methods cannot handle left-recursive grammars, so a transformation is needed to eliminate left recursion. Left recursion can be eliminated by modifying the rule as follows: (A’ is new non-terminal and ε is representing epsilon).

A → β A’
A’ → α A’ | ε

Left Factoring

It is a grammar transformation that is useful for producing grammar suitable for predictive or top-down parsing. When the choice between two alternative productions is not clear, we rewrite the productions to defer the decision to make the right choice.  

For example, if we have grammar rule A → α β1 | α β2
A → α A’
A’ → β1 | β2

Concept of First and Follow

The construction of a top-down parser is aided by FIRST and FOLLOW functions, that are associated with a grammar G. During top-down parsing, FIRST and FOLLOW allow us to choose which production to apply, based on the next input symbol.

Rules for First computation:

1) If x is terminal, then FIRST(x)={x}

2) If X→ ε is production, then add ε to FIRST(X)

3) If X is a non-terminal and X → PQR then FIRST(X)=FIRST(P)

   If FIRST(P) contains ε, then  

        FIRST(X) = (FIRST(P) – {ε}) U FIRST(QR)

Rules for Follow computation:

Note: Epsilon (ε) can never be present in FOLLOW of any non-terminal symbol.

1) For Start symbol, place $ in FOLLOW(S)

2) If A→ α B, then FOLLOW(B) = FOLLOW(A)

3) If A→ α B β, then

  If ε not in FIRST(β),

       FOLLOW(B) = FIRST(β)

  else do,

       FOLLOW(B) = (FIRST(β)-{ε}) U FOLLOW(A)

Parsing table

After the construction of the parsing table, if for any non-terminal symbol in the table we have more than one production rule for any terminal symbol in the table column the grammar is not LL(1). Otherwise, then grammar is considered as LL(1).  

Rules for construction of parsing table:

Step 1: For each production A → α , of the given grammar perform Step 2 and Step 3.  

Step 2: For each terminal symbol ‘a’ in FIRST(α), ADD A → α in table T[A,a], where ‘A’ determines row & ‘a’ determines column.

Step 3:  If ε is present in FIRST(α) then find FOLLOW(A), ADD A → ε, at all columns ‘b’, where ‘b’ is FOLLOW(A).  (T[A,b])

Step 4: If ε is in FIRST(α) and $ is the FOLLOW(A), ADD A → α to T[A,$].

The assumption made in code:  

a) LHS symbol of First rule is considered as start symbol.

b) ‘#’ represents epsilon symbol.


  • There are seven functions and the driver code, they together execute the calculations. Code takes grammar rules as input. The user is required to determine which symbols are terminals (list: term_userdef) and which are non-terminals (list: nonterm_userdef).
  • Code is executed on the sample set 5 for demo purpose as shown in code, this grammar has left recursion, function computeAllFirsts() is called. This function is responsible for calling functions removeLeftRecursion() and LeftFactoring(). These functions work on the above-mentioned rules respectively. Now, first() function is called for every non-terminal. The recursive logic implemented is purely based on mentioned rules of FIRST computation only.
  • Base condition of first() is whether an epsilon or terminal symbol is present, for every non-terminal code enters a recursive logic and if the FIRST has multiple symbols, a list is returned else a string is returned. Therefore, in between lines of code are just making the program type-safe.
  • FIRST computation is the prerequisite for FOLLOW computation as follow() function has multiple calls to first() function. A start_symbol is the LHS symbol of First rule given in the ‘rules’ list. (This can be modified), For FOLLOW computation computeAllFollows() function is called. This leads to the calling of follow() function on all non-terminals. follow() function has recursive logic with a base condition being, the function called on start_symbol which will return $ symbol.
  • Rest all conditions are handled as per the above-mentioned rules in FOLLOW computation. For the target non-terminal all rules are traversed and required firsts and circular follows are calculated. All of the intermediate results are accumulated during traversal and the final list of computed follow is returned at the traversal end. Here also, the base case returns a string, but if there are multiple symbols in the result, a list is returned. Therefore, in between extra lines of code are added for type safety.
  • After FIRSTs and FOLLOWs are calculated, createPaseTable() function is called. Here firsts and follow are outputted in a formatted way. Then for parse table 2D list named ‘mat’ is prepared, non-terminals form rows and terminals form columns, with ‘$’ as an extra column. Above mentioned rules for the construction of the parsing table forms the foundation for logic implemented.

Approach for String Validation

  • After the parsing table is generated, we must validate the input string for the given grammar. We make use of the stack and buffer for this purpose. If grammar is not LL(1) String validation cannot be performed. Enter the follow of START SYMBOL, ‘$’ in both stack and buffer. Firstly, enter the input string in reverse order into the buffer. Then add the START SYMBOL to the top of the stack.
  • Then we iteratively traverse over the current state of stack and buffer and match the entries of Table[x][y], where ‘x’ is a symbol at top of the stack and ‘y’ is a symbol at the rear of the buffer. We take the corresponding rule from the table and expand Non-terminal at TOS in the following iteration.
  • If x and y both symbols are the same terminals, then we pop them from both stack and buffer and continue computation. If the stack and buffer remain with only ‘$’ symbol, this shows that all input string symbols have been matched and the string belongs to the given grammar. If Rule is not present in the parsing table or x and y are unequal terminal symbols, then string does not belong to the given grammar.

Below is the complete implementation:


# LL(1) parser code in python
def removeLeftRecursion(rulesDiction):
    # for rule: A->Aa|b
    # result: A->bA',A'->aA'|#
    # 'store' has new rules to be added
    store = {}
    # traverse over rules
    for lhs in rulesDiction:
        # alphaRules stores subrules with left-recursion
        # betaRules stores subrules without left-recursion
        alphaRules = []
        betaRules = []
        # get rhs for current lhs
        allrhs = rulesDiction[lhs]
        for subrhs in allrhs:
            if subrhs[0] == lhs:
        # alpha and beta containing subrules are separated
        # now form two new rules
        if len(alphaRules) != 0:
            # to generate new unique symbol
            # add ' till unique not generated
            lhs_ = lhs + "'"
            while (lhs_ in rulesDiction.keys()) \
                    or (lhs_ in store.keys()):
                lhs_ += "'"
            # make beta rule
            for b in range(0, len(betaRules)):
            rulesDiction[lhs] = betaRules
            # make alpha rule
            for a in range(0, len(alphaRules)):
            # store in temp dict, append to
            # - rulesDiction at end of traversal
            store[lhs_] = alphaRules
    # add newly generated rules generated
    # - after removing left recursion
    for left in store:
        rulesDiction[left] = store[left]
    return rulesDiction
def LeftFactoring(rulesDiction):
    # for rule: A->aDF|aCV|k
    # result: A->aA'|k, A'->DF|CV
    # newDict stores newly generated
    # - rules after left factoring
    newDict = {}
    # iterate over all rules of dictionary
    for lhs in rulesDiction:
        # get rhs for given lhs
        allrhs = rulesDiction[lhs]
        # temp dictionary helps detect left factoring
        temp = dict()
        for subrhs in allrhs:
            if subrhs[0] not in list(temp.keys()):
                temp[subrhs[0]] = [subrhs]
        # if value list count for any key in temp is > 1,
        # - it has left factoring
        # new_rule stores new subrules for current LHS symbol
        new_rule = []
        # temp_dict stores new subrules for left factoring
        tempo_dict = {}
        for term_key in temp:
            # get value from temp for term_key
            allStartingWithTermKey = temp[term_key]
            if len(allStartingWithTermKey) > 1:
                # left factoring required
                # to generate new unique symbol
                # - add ' till unique not generated
                lhs_ = lhs + "'"
                while (lhs_ in rulesDiction.keys()) \
                        or (lhs_ in tempo_dict.keys()):
                    lhs_ += "'"
                # append the left factored result
                new_rule.append([term_key, lhs_])
                # add expanded rules to tempo_dict
                ex_rules = []
                for g in temp[term_key]:
                tempo_dict[lhs_] = ex_rules
                # no left factoring required
        # add original rule
        newDict[lhs] = new_rule
        # add newly generated rules after left factoring
        for key in tempo_dict:
            newDict[key] = tempo_dict[key]
    return newDict
# calculation of first
# epsilon is denoted by '#' (semi-colon)
# pass rule in first function
def first(rule):
    global rules, nonterm_userdef, \
        term_userdef, diction, firsts
    # recursion base condition
    # (for terminal or epsilon)
    if len(rule) != 0 and (rule is not None):
        if rule[0] in term_userdef:
            return rule[0]
        elif rule[0] == '#':
            return '#'
    # condition for Non-Terminals
    if len(rule) != 0:
        if rule[0] in list(diction.keys()):
            # fres temporary list of result
            fres = []
            rhs_rules = diction[rule[0]]
            # call first on each rule of RHS
            # fetched (& take union)
            for itr in rhs_rules:
                indivRes = first(itr)
                if type(indivRes) is list:
                    for i in indivRes:
            # if no epsilon in result
            # - received return fres
            if '#' not in fres:
                return fres
                # apply epsilon
                # rule => f(ABC)=f(A)-{e} U f(BC)
                newList = []
                if len(rule) > 1:
                    ansNew = first(rule[1:])
                    if ansNew != None:
                        if type(ansNew) is list:
                            newList = fres + ansNew
                            newList = fres + [ansNew]
                        newList = fres
                    return newList
                # if result is not already returned
                # - control reaches here
                # lastly if eplison still persists
                # - keep it in result of first
                return fres
# calculation of follow
# use 'rules' list, and 'diction' dict from above
# follow function input is the split result on
# - Non-Terminal whose Follow we want to compute
def follow(nt):
    global start_symbol, rules, nonterm_userdef, \
        term_userdef, diction, firsts, follows
    # for start symbol return $ (recursion base case)
    solset = set()
    if nt == start_symbol:
        # return '$'
    # check all occurrences
    # solset - is result of computed 'follow' so far
    # For input, check in all rules
    for curNT in diction:
        rhs = diction[curNT]
        # go for all productions of NT
        for subrule in rhs:
            if nt in subrule:
                # call for all occurrences on
                # - non-terminal in subrule
                while nt in subrule:
                    index_nt = subrule.index(nt)
                    subrule = subrule[index_nt + 1:]
                    # empty condition - call follow on LHS
                    if len(subrule) != 0:
                        # compute first if symbols on
                        # - RHS of target Non-Terminal exists
                        res = first(subrule)
                        # if epsilon in result apply rule
                        # - (A->aBX)- follow of -
                        # - follow(B)=(first(X)-{ep}) U follow(A)
                        if '#' in res:
                            newList = []
                            ansNew = follow(curNT)
                            if ansNew != None:
                                if type(ansNew) is list:
                                    newList = res + ansNew
                                    newList = res + [ansNew]
                                newList = res
                            res = newList
                        # when nothing in RHS, go circular
                        # - and take follow of LHS
                        # only if (NT in LHS)!=curNT
                        if nt != curNT:
                            res = follow(curNT)
                    # add follow result in set form
                    if res is not None:
                        if type(res) is list:
                            for g in res:
    return list(solset)
def computeAllFirsts():
    global rules, nonterm_userdef, \
        term_userdef, diction, firsts
    for rule in rules:
        k = rule.split("->")
        # remove un-necessary spaces
        k[0] = k[0].strip()
        k[1] = k[1].strip()
        rhs = k[1]
        multirhs = rhs.split('|')
        # remove un-necessary spaces
        for i in range(len(multirhs)):
            multirhs[i] = multirhs[i].strip()
            multirhs[i] = multirhs[i].split()
        diction[k[0]] = multirhs
    print(f"\nRules: \n")
    for y in diction:
    print(f"\nAfter elimination of left recursion:\n")
    diction = removeLeftRecursion(diction)
    for y in diction:
    print("\nAfter left factoring:\n")
    diction = LeftFactoring(diction)
    for y in diction:
    # calculate first for each rule
    # - (call first() on all RHS)
    for y in list(diction.keys()):
        t = set()
        for sub in diction.get(y):
            res = first(sub)
            if res != None:
                if type(res) is list:
                    for u in res:
        # save result in 'firsts' list
        firsts[y] = t
    print("\nCalculated firsts: ")
    key_list = list(firsts.keys())
    index = 0
    for gg in firsts:
        print(f"first({key_list[index]}) "
              f"=> {firsts.get(gg)}")
        index += 1
def computeAllFollows():
    global start_symbol, rules, nonterm_userdef,\
        term_userdef, diction, firsts, follows
    for NT in diction:
        solset = set()
        sol = follow(NT)
        if sol is not None:
            for g in sol:
        follows[NT] = solset
    print("\nCalculated follows: ")
    key_list = list(follows.keys())
    index = 0
    for gg in follows:
              f" => {follows[gg]}")
        index += 1
# create parse table
def createParseTable():
    import copy
    global diction, firsts, follows, term_userdef
    print("\nFirsts and Follow Result table\n")
    # find space size
    mx_len_first = 0
    mx_len_fol = 0
    for u in diction:
        k1 = len(str(firsts[u]))
        k2 = len(str(follows[u]))
        if k1 > mx_len_first:
            mx_len_first = k1
        if k2 > mx_len_fol:
            mx_len_fol = k2
    print(f"{{:<{10}}} "
          f"{{:<{mx_len_first + 5}}} "
          f"{{:<{mx_len_fol + 5}}}"
          .format("Non-T", "FIRST", "FOLLOW"))
    for u in diction:
        print(f"{{:<{10}}} "
              f"{{:<{mx_len_first + 5}}} "
              f"{{:<{mx_len_fol + 5}}}"
              .format(u, str(firsts[u]), str(follows[u])))
    # create matrix of row(NT) x [col(T) + 1($)]
    # create list of non-terminals
    ntlist = list(diction.keys())
    terminals = copy.deepcopy(term_userdef)
    # create the initial empty state of ,matrix
    mat = []
    for x in diction:
        row = []
        for y in terminals:
        # of $ append one more col
    # Classifying grammar as LL(1) or not LL(1)
    grammar_is_LL = True
    # rules implementation
    for lhs in diction:
        rhs = diction[lhs]
        for y in rhs:
            res = first(y)
            # epsilon is present,
            # - take union with follow
            if '#' in res:
                if type(res) == str:
                    firstFollow = []
                    fol_op = follows[lhs]
                    if fol_op is str:
                        for u in fol_op:
                    res = firstFollow
                    res = list(res) +\
            # add rules to table
            ttemp = []
            if type(res) is str:
                res = copy.deepcopy(ttemp)
            for c in res:
                xnt = ntlist.index(lhs)
                yt = terminals.index(c)
                if mat[xnt][yt] == '':
                    mat[xnt][yt] = mat[xnt][yt] \
                                   + f"{lhs}->{' '.join(y)}"
                    # if rule already present
                    if f"{lhs}->{y}" in mat[xnt][yt]:
                        grammar_is_LL = False
                        mat[xnt][yt] = mat[xnt][yt] \
                                       + f",{lhs}->{' '.join(y)}"
    # final state of parse table
    print("\nGenerated parsing table:\n")
    frmt = "{:>12}" * len(terminals)
    j = 0
    for y in mat:
        frmt1 = "{:>12}" * len(y)
        print(f"{ntlist[j]} {frmt1.format(*y)}")
        j += 1
    return (mat, grammar_is_LL, terminals)
def validateStringUsingStackBuffer(parsing_table, grammarll1,
                                   table_term_list, input_string,
    print(f"\nValidate String => {input_string}\n")
    # for more than one entries
    # - in one cell of parsing table
    if grammarll1 == False:
        return f"\nInput String = " \
               f"\"{input_string}\"\n" \
               f"Grammar is not LL(1)"
    # implementing stack buffer
    stack = [start_symbol, '$']
    buffer = []
    # reverse input string store in buffer
    input_string = input_string.split()
    buffer = ['$'] + input_string
    print("{:>20} {:>20} {:>20}".
          format("Buffer", "Stack","Action"))
    while True:
        # end loop if all symbols matched
        if stack == ['$'] and buffer == ['$']:
            print("{:>20} {:>20} {:>20}"
                  .format(' '.join(buffer),
                          ' '.join(stack),
            return "\nValid String!"
        elif stack[0] not in term_userdef:
            # take font of buffer (y) and tos (x)
            x = list(diction.keys()).index(stack[0])
            y = table_term_list.index(buffer[-1])
            if parsing_table[x][y] != '':
                # format table entry received
                entry = parsing_table[x][y]
                print("{:>20} {:>20} {:>25}".
                      format(' '.join(buffer),
                             ' '.join(stack),
                             f"T[{stack[0]}][{buffer[-1]}] = {entry}"))
                lhs_rhs = entry.split("->")
                lhs_rhs[1] = lhs_rhs[1].replace('#', '').strip()
                entryrhs = lhs_rhs[1].split()
                stack = entryrhs + stack[1:]
                return f"\nInvalid String! No rule at " \
            # stack top is Terminal
            if stack[0] == buffer[-1]:
                print("{:>20} {:>20} {:>20}"
                      .format(' '.join(buffer),
                              ' '.join(stack),
                buffer = buffer[:-1]
                stack = stack[1:]
                return "\nInvalid String! " \
                       "Unmatched terminal symbols"
# NOTE: To test any of the sample sets, uncomment ->
# 'rules' list, 'nonterm_userdef' list, 'term_userdef' list
# and for any String validation uncomment following line with
# 'sample_input_String' variable.
sample_input_string = None
# sample set 1 (Result: Not LL(1))
# rules=["A -> S B | B",
#        "S -> a | B c | #",
#        "B -> b | d"]
# nonterm_userdef=['A','S','B']
# term_userdef=['a','c','b','d']
# sample_input_string="b c b"
# sample set 2 (Result: LL(1))
# rules=["S -> A | B C",
#        "A -> a | b",
#        "B -> p | #",
#        "C -> c"]
# nonterm_userdef=['A','S','B','C']
# term_userdef=['a','c','b','p']
# sample_input_string="p c"
# sample set 3 (Result: LL(1))
# rules=["S -> A B | C",
#        "A -> a | b | #",
#        "B-> p | #",
#        "C -> c"]
# nonterm_userdef=['A','S','B','C']
# term_userdef=['a','c','b','p']
# sample_input_string="a c b"
# sample set 4 (Result: Not LL(1))
# rules = ["S -> A B C | C",
#          "A -> a | b B | #",
#          "B -> p | #",
#         "C -> c"]
# nonterm_userdef=['A','S','B','C']
# term_userdef=['a','c','b','p']
# sample_input_string="b p p c"
# sample set 5 (With left recursion)
# rules=["A -> B C c | g D B",
#        "B -> b C D E | #",
#        "C -> D a B | c a",
#        "D -> # | d D",
#        "E -> E a f | c"
#       ]
# nonterm_userdef=['A','B','C','D','E']
# term_userdef=["a","b","c","d","f","g"]
# sample_input_string="b a c a c"
# sample set 6
# rules=["E -> T E'",
#        "E' -> + T E' | #",
#        "T -> F T'",
#        "T' -> * F T' | #",
#        "F -> ( E ) | id"
# ]
# nonterm_userdef=['E','E\'','F','T','T\'']
# term_userdef=['id','+','*','(',')']
# sample_input_string="id * * id"
# example string 1
# sample_input_string="( id * id )"
# example string 2
# sample_input_string="( id ) * id + id"
# sample set 7 (left factoring & recursion present)
rules=["S -> A k O",
       "A -> A d | a B | a C",
       "C -> c",
       "B -> b B C | r"]
sample_input_string="a r k O"
# sample set 8 (Multiple char symbols T & NT)
# rules = ["S -> NP VP",
#          "NP -> P | PN | D N",
#          "VP -> V NP",
#          "N -> championship | ball | toss",
#          "V -> is | want | won | played",
#          "P -> me | I | you",
#          "PN -> India | Australia | Steve | John",
#          "D -> the | a | an"]
# nonterm_userdef = ['S', 'NP', 'VP', 'N', 'V', 'P', 'PN', 'D']
# term_userdef = ["championship", "ball", "toss", "is", "want",
#                 "won", "played", "me", "I", "you", "India",
#                 "Australia","Steve", "John", "the", "a", "an"]
# sample_input_string = "India won the championship"
# diction - store rules inputted
# firsts - store computed firsts
diction = {}
firsts = {}
follows = {}
# computes all FIRSTs for all non terminals
# assuming first rule has start_symbol
# start symbol can be modified in below line of code
start_symbol = list(diction.keys())[0]
# computes all FOLLOWs for all occurrences
# generate formatted first and follow table
# then generate parse table
(parsing_table, result, tabTerm) = createParseTable()
# validate string input using stack-buffer concept
if sample_input_string != None:
    validity = validateStringUsingStackBuffer(parsing_table, result,
                                              tabTerm, sample_input_string,
    print("\nNo input String detected")
# Author: Tanmay P. Bisen



S->[['A', 'k', 'O']]
A->[['A', 'd'], ['a', 'B'], ['a', 'C']]
B->[['b', 'B', 'C'], ['r']]

After elimination of left recursion:

S->[['A', 'k', 'O']]
A->[['a', 'B', "A'"], ['a', 'C', "A'"]]
B->[['b', 'B', 'C'], ['r']]
A'->[['d', "A'"], ['#']]

After left factoring:

S->[['A', 'k', 'O']]
A->[['a', "A''"]]
A''->[['B', "A'"], ['C', "A'"]]
B->[['b', 'B', 'C'], ['r']]
A'->[['d', "A'"], ['#']]

Calculated firsts: 
first(S) => {'a'}
first(A) => {'a'}
first(A'') => {'c', 'r', 'b'}
first(C) => {'c'}
first(B) => {'r', 'b'}
first(A') => {'d', '#'}

Calculated follows: 
follow(S) => {'$'}
follow(A) => {'k'}
follow(A'') => {'k'}
follow(C) => {'d', 'c', 'k'}
follow(B) => {'d', 'c', 'k'}
follow(A') => {'k'}

Firsts and Follow Result table

Non-T      FIRST                FOLLOW              
S          {'a'}                {'$'}               
A          {'a'}                {'k'}               
A''        {'c', 'r', 'b'}      {'k'}               
C          {'c'}                {'d', 'c', 'k'}     
B          {'r', 'b'}           {'d', 'c', 'k'}     
A'         {'d', '#'}           {'k'}               

Generated parsing table:

           k           O           d           a           c           b           r           $
S                                         S->A k O                                                
A                                         A->a A''                                                
A''                                                    A''->C A'   A''->B A'   A''->B A'            
C                                                         C->c                                    
B                                                                 B->b B C        B->r            
A'        A'->#                A'->d A'                                                            

Validate String => a r k O

              Buffer                Stack               Action
           $ O k r a                  S $        T[S][a] = S->A k O
           $ O k r a              A k O $        T[A][a] = A->a A''
           $ O k r a          a A'' k O $            Matched:a
             $ O k r            A'' k O $     T[A''][r] = A''->B A'
             $ O k r           B A' k O $            T[B][r] = B->r
             $ O k r           r A' k O $            Matched:r
               $ O k             A' k O $          T[A'][k] = A'->#
               $ O k                k O $            Matched:k
                 $ O                  O $            Matched:O
                   $                    $                Valid

Valid String!


Sample set 7 is used to represent code output, it covers all the aspects of LL(1) parsing. Grammar after left recursion removal and after left factoring is printed. After that, we also have first and follow computation results. Then we generate the parsing table, if there are no multiple entries at any position (Table[NT][T]) in the table, we say grammar is LL(1). Finally, the sample input string is validated using stack buffer validation.

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Last Updated : 06 Apr, 2023
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