Re: [Tutor] Roman to digital (pseudocode)
Hi All,
> - A dictionary will help you look up values, but not rules. It does
> not retain its order and order is essential. Instead, create a tuple
> of the roman numerals in ascending order (roman). Create a paired
> tuple with the base 10 value (baseten).
> Now get an element from the string you want to test -- use the index
> value in roman to test the rules -- you will know what is higher,
> lower, and the same
> Then look for things like the following:
> - does the following element have a lower index in roman? if yes, you
> know the value (match indexes from the tuples) -- step forward
> - what if the follower is the same? then check the one after, if it is
> the same (and the following digit is lower) then you have a value. --
> step over the last matching digit
> - if the following value is higher, it must be by only one unit, if so
> you have a value, but also a new rule: the following digit must be
> lower than the last digit of the pair.
> and so on.
> not pseudocode I know, and I am certain there are better ways to do
> this, but maybe something here helps.
#
roman_codec = {'M':1000, 'D':500, 'C':100, 'L':50, 'X':10, 'V':5, 'I':1}
original = raw_input('Enter a roman number:')
roman = original.upper()
roman = list(roman) # I would like to use pop instead roman = roman[1:]
roman.reverse() # pop picks from the end, so prepare for it
decimal = [roman_codec[ch] for ch in roman] # turn the chars into decimal
result = 0
while len(decimal):
act = decimal.pop()
if len(decimal) and act < max(decimal): act = -act # if there is a
char with higher score it is minus :)
result += act
print original, '=', result
Python is nearly pseudocode.
This is the reason we like it so much.
Best regards,
Janos Juhasz
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Re: [Tutor] Roman to digital (pseudocode)
Bob Gailer wrote: > Deep breath and big step back. > > The problem is not just how to code this in Python, but how to parse a > language (in this case Roman Numbers). > > I have studied formal language theory but am not an expert. So here's my > take on an algorithm that would save a lot of stress. > > Create a dictionary with > keys for each roman letter and each "pair" (iv, ix, xl, xc, etc) > values are the corresponding decimal values. > Starting at the left of the roman_input and repeating until the end of > the roman_input > Take the next pair of roman_input. > If it is in the dictionary > add the decimal value and step to the next letter. > Else > take the leftmost letter of that pair > If it is in the dictionary > add the decimal value and step to the next letter. >Else > complain about bad letter Or make a list of all the pairs and values, followed by all the single letters and values, and just find the first match from the list against the start of the test string. Neither of these is really a parser, though; they allow badly formed numbers like IICCXM. Kent ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Roman to digital (pseudocode)
Alan Gilfoy wrote: > This, I heard, is more difficult than digital-to-Roman, since you have > to "read" the subtractive cases, with a smaller numeral placed before > a larger numeral, without simply adding all the numerals' values up > > I'm going to use a raw_input prompt to ask the user which Roman > numeral he/she wants to convert. How do I "screen" for inputs that > have characters besides "I", "V", "X", "L", "C", "D", or "M"? You could have a string containing all the allowed characters and make sure each input character is in the allowed list. > > Second Python question: > > I know there's a way to "find out" the name of the first item in a list > (ListName[0]), but is there a way to find out the first character of a string? Lists and strings are both sequences and many of the same operations work on both. (All of the immutable sequence operations work on both.) So MyString[0] is the first character in a string. > > Also, is there a way to "ask" Python what characters are before and > after the character in the string that you're asking about? > > For example, usign the sample string "MCMXVII" (1917): > > How would you ask Python: > "What's the 3rd character in this string?" (returns "M") s[2] > "What's before that character?" (returns "C") s[1] or maybe s[2-1] if the 2 is actually in a variable. Kent ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Roman to digital (pseudocode)
I am new to python and programming, so not sure if I should weigh in here. Nonetheless . . . Creating a dictionary seems fair, but I wonder about using pairs rather than single letters. Roman numerals are constructed from strict rules other than the allowable letter set -- here is the relevant wikipedia bit: Rules regarding Roman numerals often state that a symbol representing 10x may not precede any symbol larger than 10x+1. For example, C cannot be preceded by I or V, only by X (or, of course, by a symbol representing a value equal to or larger than C). Thus, one should represent the number "ninety-nine" as XCIX, not as the "shortcut" IC. However, these rules are not universally followed. Although the shortcut may be easier for people, the universal notation may be easier to define programmatically. I am wondering what would be the best way to create the rules - A dictionary will help you look up values, but not rules. It does not retain its order and order is essential. Instead, create a tuple of the roman numerals in ascending order (roman). Create a paired tuple with the base 10 value (baseten). Now get an element from the string you want to test -- use the index value in roman to test the rules -- you will know what is higher, lower, and the same Then look for things like the following: - does the following element have a lower index in roman? if yes, you know the value (match indexes from the tuples) -- step forward - what if the follower is the same? then check the one after, if it is the same (and the following digit is lower) then you have a value. -- step over the last matching digit - if the following value is higher, it must be by only one unit, if so you have a value, but also a new rule: the following digit must be lower than the last digit of the pair. and so on. not pseudocode I know, and I am certain there are better ways to do this, but maybe something here helps. -c On 3/13/07, Bob Gailer <[EMAIL PROTECTED]> wrote: > Deep breath and big step back. > > The problem is not just how to code this in Python, but how to parse a > language (in this case Roman Numbers). > > I have studied formal language theory but am not an expert. So here's my > take on an algorithm that would save a lot of stress. > > Create a dictionary with > keys for each roman letter and each "pair" (iv, ix, xl, xc, etc) > values are the corresponding decimal values. > Starting at the left of the roman_input and repeating until the end of > the roman_input > Take the next pair of roman_input. > If it is in the dictionary > add the decimal value and step to the next letter. > Else > take the leftmost letter of that pair > If it is in the dictionary > add the decimal value and step to the next letter. >Else > complain about bad letter > > -- > Bob Gailer > 510-978-4454 > > ___ > Tutor maillist - Tutor@python.org > http://mail.python.org/mailman/listinfo/tutor > -- - - - - - - - Clay S. Wiedemann voice: 718.362.0375 aim: khlav wii: 3905 4571 6175 2469 twitter: seastokes ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Roman to digital (pseudocode)
Deep breath and big step back. The problem is not just how to code this in Python, but how to parse a language (in this case Roman Numbers). I have studied formal language theory but am not an expert. So here's my take on an algorithm that would save a lot of stress. Create a dictionary with keys for each roman letter and each "pair" (iv, ix, xl, xc, etc) values are the corresponding decimal values. Starting at the left of the roman_input and repeating until the end of the roman_input Take the next pair of roman_input. If it is in the dictionary add the decimal value and step to the next letter. Else take the leftmost letter of that pair If it is in the dictionary add the decimal value and step to the next letter. Else complain about bad letter -- Bob Gailer 510-978-4454 ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Roman to digital (pseudocode)
On 08/03/07, Alan Gilfoy <[EMAIL PROTECTED]> wrote: This, I heard, is more difficult than digital-to-Roman, since you have to "read" the subtractive cases, with a smaller numeral placed before a larger numeral, without simply adding all the numerals' values up I'm going to use a raw_input prompt to ask the user which Roman numeral he/she wants to convert. How do I "screen" for inputs that have characters besides "I", "V", "X", "L", "C", "D", or "M"? Second Python question: I know there's a way to "find out" the name of the first item in a list (ListName[0]), but is there a way to find out the first character of a string? Also, is there a way to "ask" Python what characters are before and after the character in the string that you're asking about? For example, usign the sample string "MCMXVII" (1917): How would you ask Python: "What's the 3rd character in this string?" (returns "M") "What's before that character?" (returns "C") Pseudocode time: If character Y is "M": and the character before character Y is "C", add 900 to digital_result and remove that "C" and that "M" from the string. #How would you do *that*? and the character before character Y is another "M", or if character Y is the first character in the string, add 1000 to digital_result and remove that "M" from the string. you can index a string just like a list: roman_numeral = "MCMXVII" roman_numeral[2] 'M' roman_numeral[2-1] 'C' HTH Adam ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
[Tutor] Roman to digital (pseudocode)
This, I heard, is more difficult than digital-to-Roman, since you have to "read" the subtractive cases, with a smaller numeral placed before a larger numeral, without simply adding all the numerals' values up I'm going to use a raw_input prompt to ask the user which Roman numeral he/she wants to convert. How do I "screen" for inputs that have characters besides "I", "V", "X", "L", "C", "D", or "M"? Second Python question: I know there's a way to "find out" the name of the first item in a list (ListName[0]), but is there a way to find out the first character of a string? Also, is there a way to "ask" Python what characters are before and after the character in the string that you're asking about? For example, usign the sample string "MCMXVII" (1917): How would you ask Python: "What's the 3rd character in this string?" (returns "M") "What's before that character?" (returns "C") Pseudocode time: If character Y is "M": and the character before character Y is "C", add 900 to digital_result and remove that "C" and that "M" from the string. #How would you do *that*? and the character before character Y is another "M", or if character Y is the first character in the string, add 1000 to digital_result and remove that "M" from the string. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
