Can somebody give some examples of its use and state in words how it is
"injecting" something into something?

thanks,
jp

P.S.
If anybody wants to take a stab at "collect", that would be welcome
also.

On May 20, 2006, at 10:42 PM, Jeff Pritchard wrote:

Well one possible explanation is that it's injecting the contents of  
the array and the result of the last call to the block back into the  
block, sort of an auto-transfusion.

The canonical inject example is the sum of the elements of an array:
irb(main):001:0> [1, 7, 2].inject(0) { |sum_so_far, current_item|  
sum_so_far + current_item }
=> 10

As far as collect goes, you're collecting the results of the block  
applied to each element in the enumerable. (I personally prefer map  
to collect as far as terminology goes).

The basic usage (just to have an example):

You have an array:  [1,2,3,4,5]
You call:  new_array = array.collect {|element| element * 10 }
The result (new_array) is: [10,20,30,40,50]

Now, in terms of the word "collect" itself:

Imagine that the elements of the first array are sitting on a bus.
The conductor comes through the bus and *collects* the fare from each
element.  The fare is: yourself times 10.  The cumulative result is a
new array, consisting of the old array's elements but each multiplied
by 10.

So collect is an iterative process, where a new value is "collected"
from each old value, courtesy of being passed to the code block.  The
new array is thus a one-to-one mapping of the old array, with the
values transformed.  (So collect is also called "map" :-)

David

This example injects the result of the block applied to each element of
the collection into a single value.

I actually prefer the Haskell term, "fold" -- it repeatedly folds the
function of the value and an element of the collection back into the value.

ys = xs.map {|x| x * x }

It takes a block (a mapping from X to Y) and applies it to a collection
of Xs to map them to Ys.

As for Inject, nobody has come up with a wording that makes any sense to
me yet.  Does Inject have any aliases/synonyms?

thanks,
jp

P.S.
If I were to choose a synonym for the "inject" method, I might come up
with "collect".  vis. "iterate over these items and collect the results
in one answer."  It seems to me that this method does the same thing
that the big Latin "E" symbol (Epsilon) does in Mathematics.  Is there a
name for that symbol within the math community?  (I mean - besides
Epsilon).  Calculus and Physics are "twenty some odd years ago" for me.

P.P.S.
Is it considered bad form to create your own aliases for things like
this?  For instance, if I were to create an alias for Enumerated#inject
like "gather", and use that all over my programs, would this be
considered bad form, or just hunky-dory use of the language?

total_cost = [[pie,2.00],[coffee,1.00]].combine { |cost, [food, price]|
cost += price }

jp

On May 21, 2006, at 1:05 AM, Jeff Pritchard wrote:

On May 21, 2006, at 1:17 AM, Jeff Pritchard wrote:

each_with_state

(reduce (function +) (list 1 2 3 4 5 6))

(1..6).inject(0) {|c,v| c + v}

Also: yes, the "E" in math is a Sigma or summation. I wouldn't say
they're quite the same since summnation always involves summing whereas
inject/reduce are more abstract and multipurpose.

Ex:
(reduce (function *) (list 1 2 3 4 5 6))
(1..6).ineject(1) {|c,v| c * v}
These both act more like a factorial than a sum.

   addresses = contacts.collect{|contact| contact.address}

Here we collect the addresses of the contacts.

David

I have it marked in my "Ruby notes", as "Don't use".
I do this so that when I come across it again, I won't spend 2 hours
trying to make sense out of it's purpose.

# this "inject" method is "combining" the results from the block
operation into a single answer
[1,2,3,4,5].inject(0){|sum,num|sum+num}

This topic makes me wonder, and perhaps one of the old-timers here can
answer this.  If I understand it correctly, Matz basically "invented"
ruby.  Matz is, as far as I know, Japanese.  Was Ruby first written with
Japanese class and method names and later translated to English?  Or did
it start out in English?

thanks,
jp

On May 21, 2006, at 10:42 AM, Jeff Pritchard wrote:

sum = 0
enum.each {|x| sum += x}
sum

enum.inject(0) {|sum, x| sum + x}

Kind regards

robert

* discourage use of symbol variables (e.g. $/, etc.) in manual
* discourage use of Perlish features by giving warnings.

:-)

David

klass = fully_qualified_name.split('::').inject(Module) { |_module,
_symbol|
  _module.const_get(_symbol)

In this example name "inject" totally corresponds to what it is doing
here. To me at least.

Sincerely,
Gennady.

   [1,2,3].inject { |a,b| a+b }  => 1+2+3

-- Jim Weirich

  (1..5).inject { |injected_value, this_item| injected_value + this_item

Also

[1,2,3].each {|a| b += a}

is easier to read ...for me.