{"id":6219,"date":"2018-12-28T04:41:07","date_gmt":"2018-12-28T04:41:07","guid":{"rendered":"https:\/\/www.appservgrid.com\/paw92\/?p=6219"},"modified":"2018-12-28T23:10:33","modified_gmt":"2018-12-28T23:10:33","slug":"more-roman-numerals-and-bash","status":"publish","type":"post","link":"https:\/\/www.appservgrid.com\/paw92\/index.php\/2018\/12\/28\/more-roman-numerals-and-bash\/","title":{"rendered":"More Roman Numerals and Bash"},"content":{"rendered":"

When in Rome: finishing the Roman numeral converter script.<\/em><\/p>\n

In my last article<\/a>, I started digging in to a classic computer science puzzle: converting Roman numerals to
\nArabic numerals. First off, it more accurately should be called Hindu-Arabic, and it’s worth
\nmentioning that it’s believed to have been invented somewhere between the first and fourth
\ncentury\u2014a counting system based on 0..9 values.<\/p>\n

The script I ended up with last time offered the basics of parsing a specified Roman numeral and
\nconverted each value into its decimal equivalent with this simple function:<\/p>\n

mapit() {
\ncase $1 in
\nI|i) value=1 ;;
\nV|v) value=5 ;;
\nX|x) value=10 ;;
\nL|l) value=50 ;;
\nC|c) value=100 ;;
\nD|d) value=500 ;;
\nM|m) value=1000 ;;
\n* ) echo “Error: Value $1 unknown” >&2 ; exit 2 ;;
\nesac
\n}<\/p>\n

Then I demonstrated a slick way to use the underutilized seq command to parse a string character by
\ncharacter, but the sad news is that you won’t be able to use it for the final Roman numeral to
\nArabic numeral converter. Why? Because depending on the situation, the script sometimes
\nwill need to jump two ahead, and not just go left to right linearly, one character at a time.<\/p>\n

Instead, you can build the main loop as a while loop:<\/p>\n

while [ $index -lt $length ] ; do<\/p>\n

our code<\/p>\n

index=$(( $index + 1 ))
\ndone<\/p>\n

There are two basic cases to think about in terms of solving this algorithmic puzzle: the subsequent
\nvalue is greater than the current value, or it isn’t\u2014for example, IX versus II. The first is 9
\n(literally 1 subtracted from 10), and the second is 2. That’s no surprise; you’ll need to know both the
\ncurrent and next values within the script.<\/p>\n

Sharp readers already will recognize that the last character in a sequence is a special case,
\nbecause there won’t be a next value available. I’m going to ignore the special case to start with,
\nand I’ll address it later in the code development. Stay tuned, sharpies!<\/p>\n

Because Bash shell scripts don’t have elegant in-line functions, the code to get the current and
\nnext values won’t be value=mapit(romanchar), but it’ll be a smidge clumsy with its use of the global
\nvariable value:<\/p>\n

mapit $
\ncurrentval=$value<\/p>\n

mapit $
\nnextval=$value<\/p>\n

It’s key to realize that in the situation where the next value isn’t greater than the current value
\n(for example, MC), you can’t automatically conclude that the next value isn’t going to be part of a
\ncomplex two-value sequence anyway. Like this: MCM. You can’t just say M=1000 and C=500, so let’s
\njust convert it to 1500 and process the second M when we get to it. MCM=1900, not 2500!<\/p>\n

The basic logic turns out to be pretty straightforward:<\/p>\n

if [ $nextval -gt $currentval ] ; then
\nsum=$(( $sum + $nextval – $currentval ))
\nelse
\nsum=$(( $sum + currentval ))
\nfi<\/p>\n

Done!<\/p>\n

Or, um, not. The problem with the conditional code above is that in the situation where you’ve
\nreferenced both the current and next value, you need to ensure that the next value isn’t again
\nprocessed the next time through the loop.<\/p>\n

In other words, when the sequence “CM” is converted, the M shouldn’t be converted yet
\nagain the second time through the loop.<\/p>\n

This is precisely why I stepped away from the for loop, so you can have some passes through the loop
\nbe a +1 iteration but others be a +2 iteration as appropriate.<\/p>\n

With that in mind, let’s add the necessary line to the conditional statement:<\/p>\n

if [ $nextval -gt $currentval ] ; then
\nsum=$(( $sum + $nextval – $currentval ))
\nindex=$(( $index + 1 ))
\nelse
\nsum=$(( $sum + currentval ))
\nfi<\/p>\n

Remember that the very bottom of the while loop still has the index value increment +1. The above
\naddition to the conditional statement is basically that when the situation of next > current is
\nencountered, the script will process both values and jump ahead an extra character.<\/p>\n

This means that for any given Roman numeral, the number of times through the loop will be equal to or
\nless than the total number of characters in the sequence.<\/p>\n

Which means the problem is now solved except for the very last value in the sequence. What happens if
\nit isn’t part of a next-current pair? At its most simple, how do you parse “X”?<\/p>\n

That turns out to require a bunch of code to sidestep both the conversion of nextval from the string
\n(which will fail as it’s reaching beyond the length of the string) and any test reference to
\nnextval.<\/p>\n

That suggests a simple solution: wrap the entire if-then-else code block in a conditional that tests
\nfor the last character:<\/p>\n

if [ $index -lt $length ] ; then
\nif-then-else code block
\nelse
\nsum=$(( $sum + $currentval ))
\nfi<\/p>\n

That’s it. By George, I think you’ve got it! Here’s the full while statement, so you can
\nsee how this fits into the overall program logic:<\/p>\n

while [ $index -le $length ] ; do<\/p>\n

mapit $
\ncurrentval=$value<\/p>\n

if [ $index -lt $length ] ; then
\nmapit $
\nnextval=$value<\/p>\n

if [ $nextval -gt $currentval ] ; then
\nsum=$(( $sum + $nextval – $currentval ))
\nindex=$(( $index + 1 ))
\nelse
\nsum=$(( $sum + $currentval ))
\nfi
\nelse
\nsum=$(( $sum + $currentval ))
\nfi<\/p>\n

index=$(( $index + 1 ))<\/p>\n

done<\/p>\n

It turns out not to be particularly complex after all. The key is to recognize that you need to parse the
\nRoman number in a rather clumped manner, not letter by letter.<\/p>\n

Let’s give this script a few quick tests:<\/p>\n

$ sh roman.sh DXXV
\nRoman number DXXV converts to Arabic value 525
\n$ sh roman.sh CMXCIX
\nRoman number CMXCIX converts to Arabic value 999
\n$ sh roman.sh MCMLXXII
\nRoman number MCMLXXII converts to Arabic value 1972<\/p>\n

Mission accomplished.<\/p>\n

In my next article, I plan to look at the obverse of this coding challenge, converting Arabic numerals to
\nRoman numeral sequences. In other words, you enter 99, and it returns XCIX. Why not take a stab at
\ncoding it yourself while you’re waiting?<\/p>\n

Source<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

When in Rome: finishing the Roman numeral converter script. In my last article, I started digging in to a classic computer science puzzle: converting Roman numerals to Arabic numerals. First off, it more accurately should be called Hindu-Arabic, and it’s worth mentioning that it’s believed to have been invented somewhere between the first and fourth … <\/p>\n

Continue reading “More Roman Numerals and Bash”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-6219","post","type-post","status-publish","format-standard","hentry","category-linux"],"_links":{"self":[{"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/posts\/6219","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/comments?post=6219"}],"version-history":[{"count":1,"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/posts\/6219\/revisions"}],"predecessor-version":[{"id":6845,"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/posts\/6219\/revisions\/6845"}],"wp:attachment":[{"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/media?parent=6219"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/categories?post=6219"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.appservgrid.com\/paw92\/index.php\/wp-json\/wp\/v2\/tags?post=6219"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}