RogerBW's Blog

The Weekly Challenge 190: Decoding Capital 12 November 2022

I’ve been doing the Weekly Challenges. The latest involved checking capitalisation and decoding an ambiguous string. (Note that this is open until 13 November 2022.)

Task 1: Capital Detection

You are given a string with alphabetic characters only: A..Z and a..z.

Write a script to find out if the usage of Capital is appropriate if it satisfies at least one of the following rules:

1) Only first letter is capital and all others are small.
2) Every letter is small.
3) Every letter is capital.

Although in Perl I reach for them anyway, I think this is very clearly a job for regular expressions. Each formulation can easily be coded into a regexp:

  1. [A-Z][a-z]+
  2. [a-z]+
  3. [A-Z]+

then it's just a matter of anchoring them (i.e. the whole input string must match the pattern) and testing.

sub capitaldetection($s) {
    if ($s =~ /^([A-Z]+|[a-z]+|[A-Z][a-z]+)$/) {
        return 1;
    } else {
        return 0;

Lua patterns are regexp-like but don't have alternation, so there I check each option separately. And PostScript of course doesn't have a regexp library at all, so… I start with a crude upper-case tester (I don't care about characters which aren't in A-Z or a-z):

/uppercase {
    (_) 0 get lt {
    } {
    } ifelse
} bind def

and then looking at the first two characters of the string tells me what cases the rest may be.

/capitaldetection {
    4 dict begin
    /s exch def
    /first s 0 get uppercase def
    /second s 1 get uppercase def
    first not second and {
    } {
        2 1 s length 1 sub {
            s exch get uppercase /this exch def
            first second and this not and { % ABc
                pop false exit
            } if
            second not this and { % AbC or abC
                pop false exit
            } if
        } for
    } ifelse
} bind def

Task 2: Decoded List

You are given an encoded string consisting of a sequence of numeric characters: 0..9, $s.

Write a script to find all the valid different decodings in sorted order.

Encoding is simply done by mapping A,B,C,D,... to 1,2,3,4,... etc.

So for example "11" can be (1, 1) = "AA", or (11) = "K".

I used my standard breadth-first search pattern, stuffing valid match patterns into hash keys to avoid possible duplication (not sure this can actually happen but it was an excuse for making arrays into hash keys), and then decoded the results. Most of the complexity is in the string-slicing (and in Rust I ended up using a Vec<char> for simplicity).


from collections import deque

def decodedlist(s):
  stack = deque()

A stack entry consists of a list of strings: the last entry in the list is the remaining un-parsed part.

  out = set()
  while True:
    ent = stack.popleft()
    tail = ent.pop()

If there's no more unparsed content, we have a full parse; stick it on the output list.

    if len(tail) == 0:

If we get here there's at least one un-parsed character. "0" isn't a valid character index, but any other number is.

      if tail[0] != "0":
        q = ent.copy()

If there are two or more un-parsed characters, there may be a decoding: "26" is valid, but "27" is not. I could do this with regexps but instead I decode it and take it as a number to check for validity.

      if len(tail) >= 2:
        v = int(tail[0:2])
        if v >= 1 and v <= 26:
          q = ent.copy()
    if len(stack) == 0:

Now we build the output list: alphazero is the character offset (which in ASCII will be 64, but this code can in theory work in non-ASCII environments as long as the alphabet is sequential).

  k = []
  alphazero = ord("A") - 1

Decode each character, join them into a string, and put the result in the list to be returned.

  for x in out:
    k.append("".join(chr(int(cs)+alphazero) for cs in x))
  return k

Full code on github.

Comments on this post are now closed. If you have particular grounds for adding a late comment, comment on a more recent post quoting the URL of this one.

Tags 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s 2010s 3d printing action advent of code aeronautics aikakirja anecdote animation anime army astronomy audio audio tech aviation base commerce battletech beer boardgaming book of the week bookmonth chain of command children chris chronicle church of no redeeming virtues cold war comedy computing contemporary cornish smuggler cosmic encounter coup covid-19 crime crystal cthulhu eternal cycling dead of winter doctor who documentary drama driving drone ecchi economics en garde espionage essen 2015 essen 2016 essen 2017 essen 2018 essen 2019 essen 2022 essen 2023 existential risk falklands war fandom fanfic fantasy feminism film firefly first world war flash point flight simulation food garmin drive gazebo genesys geocaching geodata gin gkp gurps gurps 101 gus harpoon historical history horror hugo 2014 hugo 2015 hugo 2016 hugo 2017 hugo 2018 hugo 2019 hugo 2020 hugo 2021 hugo 2022 hugo 2023 hugo 2024 hugo-nebula reread in brief avoid instrumented life javascript julian simpson julie enfield kickstarter kotlin learn to play leaving earth linux liquor lovecraftiana lua mecha men with beards mpd museum music mystery naval noir non-fiction one for the brow opera parody paul temple perl perl weekly challenge photography podcast politics postscript powers prediction privacy project woolsack pyracantha python quantum rail raku ranting raspberry pi reading reading boardgames social real life restaurant reviews romance rpg a day rpgs ruby rust scala science fiction scythe second world war security shipwreck simutrans smartphone south atlantic war squaddies stationery steampunk stuarts suburbia superheroes suspense television the resistance the weekly challenge thirsty meeples thriller tin soldier torg toys trailers travel type 26 type 31 type 45 vietnam war war wargaming weather wives and sweethearts writing about writing x-wing young adult
Special All book reviews, All film reviews
Produced by aikakirja v0.1