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TASK #1 - Mirror Dates
You are given a date (yyyy/mm/dd).
Assuming, the given date is your date of birth. Write a script to find
the mirror dates of the given date.
Assuming today is 2021/09/22.
Example 1:
Input: 2021/09/18
Output: 2021/09/14, 2021/09/26
On the date you were born, someone who was your current age, would have
been born on 2021/09/14. Someone born today will be your current age
on 2021/09/26.
Example 2:
Input: 1975/10/10
Output: 1929/10/27, 2067/09/05
On the date you were born, someone who was your current age, would have
been born on 1929/10/27. Someone born today will be your current age
on 2067/09/05.
Example 3:
Input: 1967/02/14
Output: 1912/07/08, 2076/04/30
On the date you were born, someone who was your current age, would have
been born on 1912/07/08. Someone born today will be your current age
on 2076/04/30.
MY NOTES: Sounds like a pretty easy date manipulation exercise: dob - delta,
today + delta where delta = today - date. The hardest part is working out
which Date manipulation module to use, as Perl has so many.
TASK #2 - Hash Join
Write a script to implement Hash Join algorithm as suggested by wikipedia.
1. For each tuple r in the build input R
1.1 Add r to the in-memory hash table
1.2 If the size of the hash table equals the maximum in-memory size:
1.2.1 Scan the probe input S, and add matching join tuples to the output relation
1.2.2 Reset the hash table, and continue scanning the build input R
2. Do a final scan of the probe input S and add the resulting join tuples to the output relation
Example
Input:
@player_ages = (
[20, "Alex" ],
[28, "Joe" ],
[38, "Mike" ],
[18, "Alex" ],
[25, "David" ],
[18, "Simon" ],
);
@player_names = (
["Alex", "Stewart"],
["Joe", "Root" ],
["Mike", "Gatting"],
["Joe", "Blog" ],
["Alex", "Jones" ],
["Simon","Duane" ],
);
Output:
Based on index = 1 of @players_age and index = 0 of @players_name.
20, "Alex", "Stewart"
20, "Alex", "Jones"
18, "Alex", "Stewart"
18, "Alex", "Jones"
28, "Joe", "Root"
28, "Joe", "Blog"
38, "Mike", "Gatting"
18, "Simon", "Duane"
MY NOTES: Ok, I think I understand, but I'm going to ignore the
whole "out of memory" part as that's too complicated.
Also, I can't see what logical order the example output is ordered by,
as far as I can see, the described algorithm leads to the order that I
produce - not the order the above example output shows; so I'm going to
ignore that too. After all, in a relation, order doesn't matter, right?
So for the example I build %name2ages containing:
"Alex" => [20, 18],
"Joe" => [28],
"Mike" => [38],
"David" => [25],
"Simon" => [18].
Then use %name2ages while iterating over @player_names.
There's also the question of how to provide the relations,
in ch-2.pl I hard-coding them as arrays of pairs as shown above,
but see also ch-2a.pl which generalises them as files, read by Text::CSV
and containing fieldnames in row 1.
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