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Python question

Thor1590Thor1590 Registered User regular
edited February 2009 in Help / Advice Forum
I'm getting an error in the code our prof sent us to work on, and it isn't something we're supposed to have to touch:


def error(message) :
print message

It says the error is after "print message," and that's there's a problem with the syntax. If I remove this particular definition, it says there's a syntax error at the end of the next line, leading me to believe the problem might be with the code before it:
if isinstance(tree, str) : # a VAR ?
ans = tree
else : # list of form, [op, ...] where op in ("~","^","v")
if tree[0] == "~" :
ans = "~" + toString(tree[1])
elif tree[0] in ("v", "^") :
ans = "(" + toString(tree[1]) + " " + tree[0] + " " + toString(tree[2]) + ")"
else :
error("bad optree structure")
ans = "?"
return ans

but, at the moment, I'm totally lost. Any hints?

Thor1590 on

Posts

  • BarrakkethBarrakketh Registered User regular
    edited February 2009
    Including the exact error message and indenting that code (and/or including all of the code if it's not extremely long) would be helpful. That being said there is some strange stuff in that code but it shouldn't cause any problems. Just...weird for being in Python.

    Barrakketh on
    Rollers are red, chargers are blue....omae wa mou shindeiru
  • Thor1590Thor1590 Registered User regular
    edited February 2009
    Barrakketh wrote: »
    Including the exact error message and indenting that code (and/or including all of the code if it's not extremely long) would be helpful. That being said there is some strange stuff in that code but it shouldn't cause any problems. Just...weird for being in Python.

    It is indented, but me putting it in the spoiler decided to ruin that. No error code, just a pop-up box that says "invalid syntax"
    edit:
    Here's the whole program:
    """"Trans.py translates a proposition written with AND,OR,NOT so that
    all occurrences of AND are replaced by OR,NOT.

    Input: a string whose grammatical structure is defined as follows:
    EXPR ::= VAR | ( EXPR OP EXPR ) | ~ EXPR
    OP is "^" or "v"
    VAR is an alphabetic letter

    Output: an operator tree built according to this structure:
    optree ::= VAR | ["v", optree, optree ] | ["~", optree]

    whose truth-table meaning is identical to the meaning
    of the Input.

    Algorithm:
    1. read the Input string and scan it, that is, split it into
    a list of its words. Call this the ``wordlist''.
    (Example: "(P ^ ~Q)" is scanned into
    wordlist = ["(", "P", "^", "~", "Q", ")"]
    )
    2. One by one, move the strings in the wordlist into variable
    nextword and call parseEXPR, which organizes the strings
    into its operator tree according to the grammar rule for EXPR.
    (Example: the previous wordlist is parsed into this operator
    tree:
    optree = ["^", "P", ["~", "Q"]]
    )
    3. Next, replace each occurrence of ["^", t1, t2] in optree
    by ["~", ["v", ["~", t1], ["~", t2]]]
    (Example:
    optree2 = ["~", ["v", ["~", "P"], ["~", ["~", "Q"]]]]
    )
    4. Finally, erase all double negations: ["~", ["~", t]]
    is replaced by t
    (Example:
    optree3 = ["~", ["v", ["~", "P"], "Q"]]

    5. The final result prints as "~(~P v Q)"
    """

    """
    SAMPLE EXECUTION:

    $ python Trans.py
    Type proposition to translate: ( p ^ ( q ^ ~r ))
    parsed input: ]]
    which denotes (p ^ (q ^ ~r))

    ands removed: , , ]]]]]]
    which denotes ~(~p v ~~(~q v ~~r))

    double negations removed: , , 'r']]]
    which denotes ~(~p v (~q v r))
    """


    ##### SCANNING AND PARSING (this is typically a separate module): ################

    EOF = "!" # a word that marks the end of the input words
    nextword = "" # holds the next unread input word
    wordlist = [] # holds remaining input words
    # global invariant: nextword + wordlist == all remaining unread input words


    def scan(text) :
    """scan splits apart the symbols/words in text into a list of words.

    pre: text is a string holding a proposition
    post: answer is a wordlist of the words and symbols within text
    (spaces and newlines are removed)
    returns: answer
    """
    OPERATORS = ("(", ")", "^", "v", "~") # must be one-symbol operators
    SPACES = (" ", "\n")
    SEPARATORS = OPERATORS + SPACES

    next = "" # the next symbol/word to be added to the answer list
    answer = []

    for letter in text:
    # invariant: answer + next + letter equals all the words/symbols
    # read so far from text with SPACES removed

    # see if next is complete and should be appended to answer:
    if letter in SEPARATORS and next != "" :
    answer.append(next)
    next = ""

    if letter in OPERATORS :
    answer.append(letter)
    elif letter in SPACES :
    pass # discard space
    else : # build a word or numeral
    next = next + letter

    if next != "" :
    answer.append(next)

    return answer


    def getNextword() :
    """moves the front word in wordlist to nextword.
    If wordlist is empty, sets nextword = EOF
    """
    global nextword, wordlist
    if wordlist == [] :
    nextword = EOF
    else :
    nextword = wordlist[0]
    wordlist = wordlist[1:]


    def parseEXPR() :
    """builds an EXPR operator tree from the words in nextword + wordlist,
    where EXPR ::= VAR | ( EXPR OP EXPR ) | ~ EXPR
    OP is "^" or "v"
    VAR is an alphabetic letter

    pre: global var nextword holds the next unread word
    post: ans is an operator tree holding the expression that
    was read starting from nextword.
    returns: ans
    """
    if nextword.isalpha() : # a VAR
    ans = nextword
    getNextword()
    elif nextword == "(" : # ( EXPR op EXPR ) ?
    getNextword()
    tree1 = parseEXPR()
    op = nextword
    if op == "^" or op == "v" :
    getNextword()
    tree2 = parseEXPR()
    if nextword == ")" :
    ans = [op, tree1, tree2]
    getNextword()
    else :
    error("missing )")
    else :
    error("missing operator")
    elif nextword == "~" :
    getNextword()
    tree = parseEXPR()
    ans = ["~", tree]
    else :
    error("illegal symbol to start an expression")

    return ans


    ###### TOSTRING (FORMATTING) FUNCTION: #############################

    def toString(tree):
    """reformats tree as a linear string

    pre: tree is an optree
    post: ans is a string that holds the words within tree
    returns: ans
    """

    if isinstance(tree, str) : # a VAR ?
    ans = tree
    else : # list of form, [op, ...] where op in ("~","^","v")
    if tree[0] == "~" :
    ans = "~" + toString(tree[1])
    elif tree[0] in ("v", "^") :
    ans = "(" + toString(tree[1]) + " " + tree[0] + " " + toString(tree[2]) + ")"
    else :
    error("bad optree structure")
    ans = "?"
    return ans


    def error(message) :
    print message



    ###### THE TRANSLATION FUNCTIONS: ########################

    def replaceAnd(tree) :
    """replaces all occurrences of ["^", t1, t2 ] within tree
    by ["~', ["v", ["~", t1], ["~", t2]] ]

    pre: tree is an operator tree
    post: ans is a new tree that looks like tree with the replacements
    returns: ans
    """
    # write this function's body:
    print "replaceAnd not yet implemented"
    if isinstance(tree, str)
    ans = tree
    else(tree[0] == "^")
    ans = ["~", ["v", ["~", replaceAnd[tree1]], ["~", replaceAnd[tree2]]]]
    return ans


    def removeNeg(tree):
    """replaces all occurrences of ["~", ["~", t]] by t within tree

    pre: tree is an operator tree
    post: ans is an operator tree that looks like tree but with
    all double negations removed
    returns: ans
    """
    # write this function's body:
    print "removeNeg not yet implemented"
    ans = tree
    return ans




    #### MAIN starts here: ######################################

    # this code can be placed in a main function; as it stands, it is
    # executed immediately when this module is loaded:


    inputtext = raw_input("Type proposition to translate: ")
    wordlist = scan(inputtext) # holds the words from the inputtext

    print "wordlist =", wordlist
    print
    getNextword() # moves the first word of wordlist into nextword

    optree = parseEXPR()
    print "parsed input:", optree
    print "which denotes", toString(optree)
    print

    optree2 = replaceAnd(optree)
    print "ands removed: ", optree2
    print "which denotes", toString(optree2)
    print

    optree3 = removeNeg(optree2)
    print "double negations removed:", optree3
    print "which denotes", toString(optree3)

    Edit edit: That's indented, I swear to god, but c+p didn't leave it that way.

    Thor1590 on
  • BarrakkethBarrakketh Registered User regular
    edited February 2009
    Thor1590 wrote: »
    Barrakketh wrote: »
    Including the exact error message and indenting that code (and/or including all of the code if it's not extremely long) would be helpful. That being said there is some strange stuff in that code but it shouldn't cause any problems. Just...weird for being in Python.

    It is indented, but me putting it in the spoiler decided to ruin that. No error code, just a pop-up box that says "invalid syntax"

    When you're pasting code put it in code tags. You can wrap that in a spoiler tag (which is usually what we do for length). For instance, your snippet from the first post:
    #!/usr/bin/env python

if isinstance(tree, str) : # a VAR ?
  ans = tree
else : # list of form, [op, ...] where op in ("~","^","v")
  if tree[0] == "~" :
    ans = "~" + toString(tree[1])
  elif tree[0] in ("v", "^") :
    ans = "(" + toString(tree[1]) + " " + tree[0] + " " + toString(tree[2]) + ")"
  else :
    error("bad optree structure")
ans = "?"

    Obviously that won't run since tree isn't defined, but you get the point. Since Python cares about indentation it makes running samples a pain when it isn't preserved.

    Barrakketh on
    Rollers are red, chargers are blue....omae wa mou shindeiru
  • Thor1590Thor1590 Registered User regular
    edited February 2009
    Edit: Whoops, fucked up the first time. Here we go.
    """"Trans.py   translates a proposition written with AND,OR,NOT so that
    all occurrences of AND are replaced by OR,NOT.

    Input: a string whose grammatical structure is defined as follows:
            EXPR ::=  VAR  |  ( EXPR OP EXPR )  | ~ EXPR
                 OP is  "^"  or  "v"
                 VAR is an alphabetic letter

    Output: an operator tree built according to this structure:
            optree ::=  VAR  |  ["v", optree, optree ]  | ["~", optree]

            whose truth-table meaning is identical to the meaning
            of the Input.

    Algorithm:
      1. read the Input string and scan it, that is, split it into
         a list of its words.  Call this the  ``wordlist''.
         (Example:   "(P ^ ~Q)" is scanned into
                     wordlist = ["(", "P", "^",  "~", "Q", ")"]  
         )
      2. One by one, move the strings in the wordlist into variable
         nextword  and  call  parseEXPR,  which organizes the strings
         into its operator tree according to the grammar rule for EXPR.
         (Example:  the previous  wordlist  is parsed into this operator
          tree:
                   optree =  ["^", "P", ["~", "Q"]]
         )
     3. Next, replace each occurrence of  ["^", t1, t2]  in optree
        by  ["~", ["v", ["~", t1], ["~", t2]]]
        (Example:
                  optree2 = ["~", ["v", ["~", "P"], ["~", ["~", "Q"]]]]
        )
     4. Finally, erase all double negations:  ["~", ["~", t]]
        is replaced by  t
        (Example:
                  optree3 = ["~", ["v", ["~", "P"], "Q"]]

     5.  The final result prints as  "~(~P v Q)"
"""

"""
SAMPLE EXECUTION:

$ python Trans.py
Type proposition to translate: ( p ^ ( q ^ ~r ))
parsed input: ['^', 'p', ['^', 'q', ['~', 'r']]]
which denotes (p ^ (q ^ ~r))

ands removed:  ['~', ['v', ['~', 'p'], ['~', ['~', ['v', ['~', 'q'], ['~', ['~', 'r']]]]]]]
which denotes ~(~p v ~~(~q v ~~r))

double negations removed: ['~', ['v', ['~', 'p'], ['v', ['~', 'q'], 'r']]]
which denotes ~(~p v (~q v r))
"""


##### SCANNING AND PARSING (this is typically a separate module): ################

EOF = "!"      # a word that marks the end of the input words
nextword = ""  # holds the next unread input word
wordlist = []  # holds remaining input words
# global invariant:  nextword + wordlist == all remaining unread input words


def scan(text) :
    """scan splits apart the symbols/words in  text  into a list of words.

       pre:  text is a string holding a proposition
       post: answer is a wordlist of the words and symbols within  text
             (spaces and newlines are removed)
       returns: answer
    """
    OPERATORS = ("(", ")", "^", "v", "~")  # must be one-symbol operators
    SPACES = (" ", "\n")
    SEPARATORS = OPERATORS + SPACES
 
    next = ""  # the next symbol/word to be added to the answer list
    answer = []

    for letter in text:
        # invariant:  answer + next + letter  equals all the words/symbols
        #     read so far from  text  with  SPACES  removed

        # see if  next  is complete and should be appended to answer:
        if letter in SEPARATORS  and  next != "" :
            answer.append(next)
            next = ""

        if letter in OPERATORS :
            answer.append(letter)
        elif letter in SPACES :
            pass  # discard space
        else :    # build a word or numeral
            next = next + letter

    if next != "" :   
        answer.append(next)

    return answer


def getNextword() :
    """moves the front word in  wordlist  to  nextword.
       If wordlist is empty, sets  nextword = EOF
    """
    global nextword, wordlist
    if wordlist == [] :
        nextword = EOF
    else :
        nextword = wordlist[0]
        wordlist = wordlist[1:]


def parseEXPR() :
    """builds an EXPR operator tree from the words in  nextword + wordlist,
          where  EXPR ::=  VAR  |  ( EXPR OP EXPR )  | ~ EXPR
                 OP is  "^"  or  "v"
                 VAR is an alphabetic letter

       pre: global var  nextword  holds the next unread word
       post:  ans  is an operator tree holding the expression that
              was read starting from  nextword.
       returns: ans
    """
    if  nextword.isalpha() :   # a VAR
        ans = nextword        
        getNextword()
    elif nextword == "(" :     # ( EXPR op EXPR ) ?
        getNextword()
        tree1 = parseEXPR()
        op = nextword
        if op == "^"  or  op == "v" :
            getNextword()
            tree2 = parseEXPR()
            if nextword == ")" :
                ans = [op, tree1, tree2]
                getNextword()
            else :
                error("missing )")
        else :
            error("missing operator")
    elif nextword == "~" :
        getNextword()
        tree = parseEXPR()
        ans = ["~", tree]
    else :
        error("illegal symbol to start an expression")

    return ans


###### TOSTRING (FORMATTING) FUNCTION: #############################

def toString(tree):
    """reformats  tree  as a linear string

       pre: tree is an optree
       post: ans is a string that holds the words within tree
       returns: ans
    """
    
    if isinstance(tree, str) :  # a VAR ?
        ans = tree
    else :  # list of form,  [op, ...]  where  op in ("~","^","v")
        if tree[0] == "~" :
            ans = "~" + toString(tree[1])
        elif tree[0] in ("v", "^") :
            ans = "(" + toString(tree[1]) + " " + tree[0] + " " + toString(tree[2]) + ")"
        else :
            error("bad optree structure")
            ans = "?"
    return ans


def error(message) :
    print message
    


###### THE TRANSLATION FUNCTIONS:  ########################

def replaceAnd(tree) :
    """replaces all occurrences of  ["^", t1, t2 ]  within  tree
       by  ["~', ["v", ["~", t1], ["~", t2]] ]

      pre: tree is an operator tree
      post:  ans  is a new tree that looks like  tree  with the replacements
      returns: ans
    """
    # write this function's body:
    print "replaceAnd  not yet implemented"
    if isinstance(tree, str)
        ans = tree
    else(tree[0] == "^")
        ans = ["~", ["v", ["~", replaceAnd[tree1]], ["~", replaceAnd[tree2]]]]
    return ans


def removeNeg(tree):
    """replaces all occurrences of  ["~", ["~", t]]  by  t  within tree

       pre:  tree is an operator tree
       post: ans is an operator tree that looks like tree but with
             all double negations removed
       returns: ans
    """
    # write this function's body:
    print "removeNeg  not yet implemented"
    ans = tree
    return ans




#### MAIN starts here: ######################################

# this code can be placed in a main function;  as it stands, it is
#   executed immediately when this module is loaded:


inputtext = raw_input("Type proposition to translate: ")
wordlist = scan(inputtext)  # holds the words from the inputtext

print "wordlist =", wordlist
print
getNextword()   # moves the first word of  wordlist  into  nextword

optree = parseEXPR()
print "parsed input:", optree
print "which denotes", toString(optree)
print

optree2 = replaceAnd(optree)
print "ands removed: ", optree2
print "which denotes", toString(optree2)
print

optree3 = removeNeg(optree2)
print "double negations removed:", optree3
print "which denotes", toString(optree3)

    Thor1590 on
  • admanbadmanb unionize your workplace Seattle, WARegistered User regular
    edited February 2009
    (edit: ignore this. Barrakketh wins.)

    admanb on
    twitter, github
  • BarrakkethBarrakketh Registered User regular
    edited February 2009
    The replaceAnd function is missing a colon after an if statement and an else should be elif (also missing a colon). The changes I made to get it running are simple:
    --- Trans.orig.py	2009-02-23 23:40:26.000000000 -0500
+++ Trans.py	2009-02-23 23:43:13.000000000 -0500
@@ -189,9 +189,9 @@
     """
     # write this function's body:
     print "replaceAnd  not yet implemented"
-    if isinstance(tree, str)
+    if isinstance(tree, str):
         ans = tree
-    else(tree[0] == "^")
+    elif(tree[0] == "^"):
         ans = ["~", ["v", ["~", replaceAnd[tree1]], ["~", replaceAnd[tree2]]]]
     return ans

    Barrakketh on
    Rollers are red, chargers are blue....omae wa mou shindeiru
  • Thor1590Thor1590 Registered User regular
    edited February 2009
    admanb wrote: »
    Hmmm. The code looks fine, which leads me to believe the indentation is being mis-read. I would cut out everything from the error function on. Make sure it evaluates, then paste those functions back in, one-by-one, and make sure the indentation is consistent with the rest of the file.

    Alright. Just tried this, exact same error.

    I tried removing the error block (edit: and everything below it), and it will compile without that.

    Thor1590 on
  • BarrakkethBarrakketh Registered User regular
    edited February 2009
    Thor1590 wrote: »
    admanb wrote: »
    Hmmm. The code looks fine, which leads me to believe the indentation is being mis-read. I would cut out everything from the error function on. Make sure it evaluates, then paste those functions back in, one-by-one, and make sure the indentation is consistent with the rest of the file.

    Alright. Just tried this, exact same error.

    I tried removing the error block (edit: and everything below it), and it will compile without that.

    Just a bump in case you missed it. Read the post above yours.

    Barrakketh on
    Rollers are red, chargers are blue....omae wa mou shindeiru
  • Thor1590Thor1590 Registered User regular
    edited February 2009
    Barrakketh wrote: »
    Thor1590 wrote: »
    admanb wrote: »
    Hmmm. The code looks fine, which leads me to believe the indentation is being mis-read. I would cut out everything from the error function on. Make sure it evaluates, then paste those functions back in, one-by-one, and make sure the indentation is consistent with the rest of the file.

    Alright. Just tried this, exact same error.

    I tried removing the error block (edit: and everything below it), and it will compile without that.

    Just a bump in case you missed it. Read the post above yours.

    I got it after that, yeah. Fixed it, but same error above it.

    Thor1590 on
  • admanbadmanb unionize your workplace Seattle, WARegistered User regular
    edited February 2009
    This is odd. The code runs fine on my machine as well, with the exception of the errors Barrakketh found. What are you using to edit and interpret?

    admanb on
    twitter, github
  • Thor1590Thor1590 Registered User regular
    edited February 2009
    admanb wrote: »
    This is odd. The code runs fine on my machine as well, with the exception of the errors Barrakketh found. What are you using to edit and interpret?

    IDLE.

    Thor1590 on
  • Thor1590Thor1590 Registered User regular
    edited February 2009
    Ok, I'm going to try to meet with the professor tomorrow. No use throwing myself against this all night. Thanks for your help, guys.

    Thor1590 on
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