How to make a variable based on array length and member value

Ruby
1

Hi

I have an array full_array[] of an arbitrary number of transaction
objects.
within the transaction is a Date member…

From the array I have created a new array years[].uniq! based on the
Date.year members

what id like to do is take the new arrays members and use them as names
for variables…

Say the array holds members for 2007, 2008, 2009
i`d like to iterate through the full_array and map all members from 2008
in an array.

my options are creating an array based on years.length where each member
holds all transactions from a specific year.

or iterate through the members of years and create an array based on the
value or index of years…

I can`t seem to think of the logic to accomplish this…

Any ideas?

Eelco

2

Catsquotl wrote:

names for variables…

I can`t seem to think of the logic to accomplish this…

Any ideas?

Build up a suite of tests and the code in parallel, like this.

  1. Think of a very simple case that isn’t yet covered by your code.
    Start with a very simple cases, and proceed to edge cases and
    pathological cases.

  2. Write a new test to run your code and test the result, and run it. It
    will fail, because you HAVE NOT YET WRITTEN THE CODE. (If it doesn’t
    fail, you have made a mistake somewhere).

  3. Alter your code to pass the new test and all of the old ones. Only
    when this is the case, may you proceed to stage 4.

  4. Improve you code in some small way - Remove duplicate code, repair a
    bad choice of variable names etc.

  5. After each improvement, Rerun all the tests. If any fail, you
    introduced an error. Fix before proceeding.

  6. When the all pass and your code is nice and clean, save code (and
    tests) to the version control system.

  7. If the code is not yet complete, go to step 1.

The key is to go in really tiny baby steps at stages 1 and 4, and to
push stage 4 until you have really nice code you would be proud to show
anyone. Note - you are testing for anything that might break/be broken,
not aiming for 100% code coverage.

You will find that defining the tests will clarify your thinking about
what is required. Stage 4 means the code remains clean and easy to
modify. The test suite will stop you introducing errors into your code
later.

When you are done, you have nice code, that you can demonstrate is
correct.

Regards

Ian

3

Catsquotl wrote:

I have an array full_array[] of an arbitrary number of transaction
objects.
within the transaction is a Date member…

From the array I have created a new array years[].uniq! based on the
Date.year members

what id like to do is take the new arrays members and use them as names
for variables…

Say the array holds members for 2007, 2008, 2009
i`d like to iterate through the full_array and map all members from 2008
in an array.

Sounds like what you want is a Hash, not an Array. Something like:

year_to_members = {}
members.each do |mem|
y = mem.date.year
year_to_members[y] ||= []
year_to_members[y] << mem
end

p year_to_members[2009]

this prints an array containing elememnts where date.year == 2009

As others will probably point out, this pattern can be simplified a bit.
But hopefully it’s clear enough as a starting point.

The fundamental point is, you don’t want to create “names of variables”
for each year - even though that is technically possible, as others may
also point out. The right thing to do is to build a Hash, which is an
object which associates a key (a year number) with a value (an array of
transaction objects).

HTH,

Brian.

4

2009/6/29 Brian C. [email protected]:

Catsquotl wrote:

Say the array holds members for 2007, 2008, 2009
i`d like to iterate through the full_array and map all members from 2008
in an array.

Sounds like what you want is a Hash, not an Array. Something like:

As others will probably point out, this pattern can be simplified a bit.

Since you asked… :slight_smile:

year_to_members = Hash.new {|h,k| h[k] = []}

members.each do |mem|
year_to_members[mem.date.year] << mem
end

The fundamental point is, you don’t want to create “names of variables”
for each year - even though that is technically possible, as others may
also point out. The right thing to do is to build a Hash, which is an
object which associates a key (a year number) with a value (an array of
transaction objects).

100% agree. It is definitively a bad idea to create variables with
the year in their name.

Kind regards

robert

5

David A. Black schreef:

Thanks for the reply…(Robert, Brian also)
As a matter of fact I installed 1.9 last week awaiting the arrival of
the well-grounded rubyist which should be sometime tomorrow

Eelco

6

Hi –

On Sun, 28 Jun 2009, Catsquotl wrote:

I can`t seem to think of the logic to accomplish this…
You could use group_by (if you’ve got Ruby 1.9 or a 1.9-flavored 1.8):

full_array.group_by {|transaction| transaction.date.year }

That will give you a hash whose keys are the years and whose values
are arrays of transaction objects, one such array per year-key.

David

7

Ian H. schreef:

  1. Alter your code to pass the new test and all of the old ones. Only

later.

When you are done, you have nice code, that you can demonstrate is correct.

Regards

Ian
Hi Ian,

As I am a relative beginner do you know any sites/articles/tutorials on
test driven development?
All I could find was sites on the benefits of tdd but not a tutorial
within the ruby language… found a few in Java

I have manually tested all classes I write until now which is a bit
tedious and time consuming…

I had a quick look at the test/unit Rdocs but am not quite certain on
how to use them

Eelco