Ruby Inject with Index and Brackets

Ruby inject with index and brackets

What is the use of these brackets?

It's a very nice feature of ruby. I call it "destructuring array assignment", but it probably has an official name too.

Here's how it works. Let's say you have an array

arr = [1, 2, 3]

Then you assign this array to a list of names, like this:

a, b, c = arr
a # => 1
b # => 2
c # => 3

You see, the array was "destructured" into its individual elements. Now, to the each_with_index. As you know, it's like a regular each, but also returns an index. inject doesn't care about all this, it takes input elements and passes them to its block as is. If input element is an array (elem/index pair from each_with_index), then we can either take it apart in the block body

sorted.each_with_index.inject(groups) do |group_container, pair|
  element, index = pair

  # or
  # element = pair[0]
  # index = pair[1]

  # rest of your code
end

Or destructure that array right in the block signature. Parentheses there are necessary to give ruby a hint that this is a single parameter that needs to be split in several.

Hope this helps.

Ruby inject with initial being a hash

Your block needs to return the accumulating hash:

['a', 'b'].inject({}) {|m,e| m[e] = e; m }

Instead, it's returning the string 'a' after the first pass, which becomes m in the next pass and you end up calling the string's []= method.

Ruby `each_with_object` with index

Instead of

|arr, hash|

you can do

|(v, i), hash|

Ruby each_with_index offset

Actually, Enumerator#with_index receives offset as an optional parameter:

[:foo, :bar, :baz].to_enum.with_index(1).each do |elem, i|
  puts "#{i}: #{elem}"
end

outputs:

1: foo
2: bar
3: baz

BTW, I think it is there only in 1.9.2.

Does using curly braces go against the Ruby way?

While some people go with "braces for one-liners, do-end for multi-liners", I personally find the following rule the most logical:

• use do-end when your block has side-effects (typically, with each and related methods) and
• use braces when your block is without side-effects (map, inject and alike)

This logic goes well with method chaining issue that Matt wrote about.

One benefit of this approach is that it is going to make you think about side-effects every time you write a block, and they are very important, although sometimes overlooked by coders with no functional programming background.

Another way to put it, without involving side-effects terminology would be:

• use do-end for blocks that perform
• use { and } for blocks that return

Here are couple of articles with more info:

http://onestepback.org/index.cgi/Tech/Ruby/BraceVsDoEnd.rdoc

http://talklikeaduck.denhaven2.com/2007/10/02/ruby-blocks-do-or-brace

Ruby dynamically updating value in nested array

Your iteration is almost there, you just need to stop one step before you run through path so that you can have the array you need to modify rather than the element.

So split the path into the pieces you want:

*p, target = path
# [3], 1

Then use #inject to find the array:

ary = p.inject(example) { |i, a| a[i] }
# [3, 4]

and then do your assignment:

ary[target] = 9

Of course you'll need to add some logic to deal with the unexpected such as path leading you to a non-array element or path not matching the structure of example (consider path = [11, 6, 23] in your example).

You could also use #dig instead of #inject:

ary = example.dig(*p)
ary[target] = 9

# or
example.dig(*p)[target] = 9

That would take care of some of the problematic paths and you'd be left with deciding what to do if ary.nil?.

Ruby Syntax: Bitmask

Basically the index call converts the given roles to integers by determining their position in the array of ROLES. 2 is then raised to the power of this index and inject simply sums the the results.

Put simply

Step 1 is to get roles that are found in ROLES by using & to se which values in the two arrays match.

Step 2, for each matching role compute 2**index. Map returns an array of these values.

Step 3, sum the array to get the final integer.

The idea is to compute a unique integer for each combination of roles.

The getter simply unwinds the process returning the combination of roles that match.

Ruby push a hash into an array with out curly braces

You method expects an hash (not an array of hashes). Therefore you have to use a hash to build your params:

parameters = { :key1 => value1 }
parameters[:key2] = value2

somefunction parameters

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