|Perl: the Markov chain saw|
Re^3: The error says the value is uninitialized, but it works anywayby Marshall (Abbot)
|on Aug 20, 2019 at 00:07 UTC||Need Help??|
I don't rate your prof's assignments very well so far.....
A first Perl class would not normally have any assignment that required using indices of an array, $array[$i] (although list slice which selects a subset of an array would be there, @subset= (@array)[1,5].
Dealing with user input is a basic skill that should be taught in the first class. Your prof should have given you a prototype framework of how to do this.... Under "standard" command line input rules, any leading or trailing spaces don't matter.
Below I show how to prompt the user for a number that must contain a decimal point.
You wrote: "force it to be a floating point number not a string (for math equations)".
Perl is NOT Python!
In simple terms, every Perl variable starts out as a string. Each variable has kind of dual "type", a string value and a numeric value. Unlike Python, you don't have to worry about converting or worrying much about strings vs numeric types - Perl takes care of that for you.
In the above code, I wrote:
That does indeed do what it says it does. Here Perl is using the string version of $float. I did this so that the calling routine will see the "+" sign, which is what the user perhaps entered. Now consider this:
The plus sign will be missing when printed. Try it!
In general, you don't have to worry about "string" vs "number", Perl will do the right thing without you having to worry about it because there is a duality of values for each variable.
Update: Normally, I would allow an integer input without a decimal to be valid for a "float". Change the regex to eliminate that requirement. Also there is not a Perl "type" for an integer or a float, Perl will figure that difference out for you. There are good things about this and perhaps "bad" things. Usually the programmer doesn't have to worry about the fine details "under the covers".
Oh, just as another comment, your Prof is asking you to do things that haven't been taught in the class yet. A very obvious solution to your assignment is:
Note that both grep and map imply "foreach" loops. They are "hidden", but they are there. A much more verbose version of this will execute in similar time. The assignment is poor because the techniques for the obvious solution for a Perl'er weren't covered in class yet. I also think that it could be that this splice stuff is actually slower!! Splice changes the size of an array and this is a relatively expensive operation. This is not commonly done (except for pop or push) which deal the the beginning or end of an array - not the middle. Making a complete new array with a subset of string values is probably much faster. Underneath Perl is C. An array of strings is to my knowledge an array of pointers to strings. Making a new subset array doesn't involve copying the strings themselves, just their pointers. Changing the size of the array of pointers to strings involves potentially copying a lot of pointers.
Weird thing: I read somewhere in the Perl docs about a "lazy array delete". The element disappears from @array when used in a list context, but the indices of the unaffected elements of @array doesn't change. That sounds "dangerous" albeit much faster than a "splice".
This is more "wordy", but about the same as the shorter version in terms of execution time:
In Perl the various sigils, @,%,$ have their own namespaces. This is not true in many other languages (C,Python). Be aware of that and in general do not use the exact same name for different Perl types. Above I showed how confusing this could be! Just because it is allowed doesn't mean you should do it!
You wrote: "I could only find a way to take user input and make it an integer" Study the above. If a variable is an valid integer string, you can use it as a numeric value! No conversion is required! A $variable can be pretty much be used interchangeably. as a number (int or float) or string.