Soliciting some help with inject method

Edmond_Kachale · 2 February 2011 14:56

Rubysters,

I think I am not wrong if I can solicit some knowledge.

Can any one help modify this code?

I have:
array = [['a', 1], ['b', 2], ['a',2], ['c',4], ['d', 5], ['b',4]]

I want to create a hash using inject method:

hash = {'a' => [1, 2], 'b' => [2, 4], 'c' => [4], 'd' => [5]}

The following code gives: {"a"=>[2], "b"=>[4], "c"=>[4], "d"=>[5]}

hash = array.inject({}) do |result, array|
new_hash = {}
new_hash[array.first] = [array.last]
result.merge!(new_hash) # I have tried result.update(new_hash) but it fails
too
result
end

Here is a console friendly version of the same:
hash = array.inject({}){|result, array| new_hash = {};
new_hash[array.first] = [array.last]; result.merge!(new_hash); result;}

Thanks in advance,

···

---
Edmond
Software Developer | Baobab Health Trust (http://www.baobabhealth.org/\) |
Malawi

Cell: +265 999 465 137 | +265 881 234 717

*I wish you a Merry Christmas and a Prosperous New Year 2011!!**
*

Jeremy_Bopp · 2 February 2011 15:04

array.inject({}) do |result, (key, value)|
(result[key] ||= ) << value
result
end

-Jeremy

···

On 2/2/2011 8:56 AM, Edmond Kachale wrote:

Rubysters,

I think I am not wrong if I can solicit some knowledge.

Can any one help modify this code?

I have:
array = [['a', 1], ['b', 2], ['a',2], ['c',4], ['d', 5], ['b',4]]

I want to create a hash using inject method:

hash = {'a' => [1, 2], 'b' => [2, 4], 'c' => [4], 'd' => [5]}

Andrew_Wagner · 2 February 2011 15:17

Here's one simple way to do it:

hash = array.inject({}) do |result, array|
  key,val = array
  if result[key].nil?
    result[key] = [val]
  else
    result[key].push val
  end
  result
end

···

On Wed, Feb 2, 2011 at 9:56 AM, Edmond Kachale < edmond.kachale@baobabhealth.org> wrote:

Rubysters,

I think I am not wrong if I can solicit some knowledge.

Can any one help modify this code?

I have:
array = [['a', 1], ['b', 2], ['a',2], ['c',4], ['d', 5], ['b',4]]

I want to create a hash using inject method:
>> hash = {'a' => [1, 2], 'b' => [2, 4], 'c' => [4], 'd' => [5]}

The following code gives: {"a"=>[2], "b"=>[4], "c"=>[4], "d"=>[5]}

hash = array.inject({}) do |result, array|
new_hash = {}
new_hash[array.first] = [array.last]
result.merge!(new_hash) # I have tried result.update(new_hash) but it
fails
too
result
end

Here is a console friendly version of the same:
hash = array.inject({}){|result, array| new_hash = {};
new_hash[array.first] = [array.last]; result.merge!(new_hash); result;}

Thanks in advance,

---
Edmond
Software Developer | Baobab Health Trust (http://www.baobabhealth.org/\) |
Malawi

Cell: +265 999 465 137 | +265 881 234 717

*I wish you a Merry Christmas and a Prosperous New Year 2011!!**
*

Adam_Prescott1 · 2 February 2011 15:17

You can do this without an inject, with a simple iteration and a bit of ||=

a = [['a', 1], ['b', 2], ['a',2], ['c',4], ['d', 5], ['b',4]]

=> [["a", 1], ["b", 2], ["a", 2], ["c", 4], ["d", 5], ["b", 4]]

h = {}

=> {}

a.each { |k, v| h[k] ||= ; h[k] << v }

=> [["a", 1], ["b", 2], ["a", 2], ["c", 4], ["d", 5], ["b", 4]]

h

=> {"a"=>[1, 2], "b"=>[2, 4], "c"=>[4], "d"=>[5]}

···

On Wed, Feb 2, 2011 at 2:56 PM, Edmond Kachale < edmond.kachale@baobabhealth.org> wrote:

Rubysters,

I think I am not wrong if I can solicit some knowledge.

Can any one help modify this code?

I have:
array = [['a', 1], ['b', 2], ['a',2], ['c',4], ['d', 5], ['b',4]]

I want to create a hash using inject method:
>> hash = {'a' => [1, 2], 'b' => [2, 4], 'c' => [4], 'd' => [5]}

The following code gives: {"a"=>[2], "b"=>[4], "c"=>[4], "d"=>[5]}

hash = array.inject({}) do |result, array|
new_hash = {}
new_hash[array.first] = [array.last]
result.merge!(new_hash) # I have tried result.update(new_hash) but it
fails
too
result
end

Here is a console friendly version of the same:
hash = array.inject({}){|result, array| new_hash = {};
new_hash[array.first] = [array.last]; result.merge!(new_hash); result;}

Thanks in advance,

---
Edmond
Software Developer | Baobab Health Trust (http://www.baobabhealth.org/\) |
Malawi

Cell: +265 999 465 137 | +265 881 234 717

*I wish you a Merry Christmas and a Prosperous New Year 2011!!**
*

Jesus_Gabriel_y_Gala · 2 February 2011 15:37

Yet another way without inject:

irb(main):001:0> array = [['a', 1], ['b', 2], ['a',2], ['c',4], ['d',
5], ['b',4]]
=> [["a", 1], ["b", 2], ["a", 2], ["c", 4], ["d", 5], ["b", 4]]
irb(main):002:0> result = Hash.new {|h,k| h[k] = }
=> {}
irb(main):003:0> array.each {|key,value| result[key] << value}
=> [["a", 1], ["b", 2], ["a", 2], ["c", 4], ["d", 5], ["b", 4]]
irb(main):004:0> result
=> {"a"=>[1, 2], "b"=>[2, 4], "c"=>[4], "d"=>[5]}

Jesus.

···

On Wed, Feb 2, 2011 at 3:56 PM, Edmond Kachale <edmond.kachale@baobabhealth.org> wrote:

Rubysters,

I think I am not wrong if I can solicit some knowledge.

Can any one help modify this code?

I have:
array = [['a', 1], ['b', 2], ['a',2], ['c',4], ['d', 5], ['b',4]]

I want to create a hash using inject method:

hash = {'a' => [1, 2], 'b' => [2, 4], 'c' => [4], 'd' => [5]}

The following code gives: {"a"=>[2], "b"=>[4], "c"=>[4], "d"=>[5]}

hash = array.inject({}) do |result, array|
new_hash = {}
new_hash[array.first] = [array.last]
result.merge!(new_hash) # I have tried result.update(new_hash) but it fails
too
result
end

Here is a console friendly version of the same:
hash = array.inject({}){|result, array| new_hash = {};
new_hash[array.first] = [array.last]; result.merge!(new_hash); result;}

Thanks in advance,

Edmond_Kachale · 2 February 2011 15:48

Thanks Adam. I just reduced your lines to make it geeky!!

hash = array.inject({}) do |result, (key, value)|
(result[key].nil?) ? (result[key] = [value]) : (result[key].push(value))
result
end

···

2011/2/2 Andrew Wagner <wagner.andrew@gmail.com>

Here's one simple way to do it:

hash = array.inject({}) do |result, array|
  key,val = array
if result[key].nil?
   result[key] = [val]
else
   result[key].push val
end
result
end

---
Edmond
Software Developer | Baobab Health Trust (http://www.baobabhealth.org/\) |
Malawi

Cell: +265 999 465 137 | +265 881 234 717*
**
An old dog does not hunt because of speed, but his endurance of the heart*

Edmond_Kachale · 2 February 2011 15:52

Sorry, I meant Andrew.

But I also value you contribution, Adam and Jesûs. In fact, I think your
methods are faster than the inject's way.

···

Le 2 février 2011 17:46:42 UTC+2, edmond.kachale < edmond.kachale@baobabhealth.org> a écrit :

2011/2/2 Andrew Wagner <wagner.andrew@gmail.com>

Here's one simple way to do it:

hash = array.inject({}) do |result, array|
  key,val = array
if result[key].nil?
   result[key] = [val]
else
   result[key].push val
end
result
end

Thanks Adam. I just reduced your lines to make it geeky!!

hash = array.inject({}) do |result, (key, value)|
  (result[key].nil?) ? (result[key] = [value]) : (result[key].push(value))
  result
end

---
Edmond
Software Developer | Baobab Health Trust (http://www.baobabhealth.org/\) |
Malawi

Cell: +265 999 465 137 | +265 881 234 717*
**
An old dog does not hunt because of speed, but his endurance of the heart.*
*
*

Jesus_Gabriel_y_Gala · 2 February 2011 16:10

But I also value you contribution, Adam and Jesûs. In fact, I think your
methods are faster than the inject's way.

You'd be surprised :-). If I didn't do anything wrong (I usually make
mistakes when benchmarking), my method is the slowest:

require 'benchmark'

TIMES = 100_000
array = [['a', 1], ['b', 2], ['a',2], ['c',4], ['d', 5], ['b',4]]

Benchmark.bm do |x|
  x.report "inject Jeremy" do
    TIMES.times do
      array.inject({}) do |result, (key, value)|
        (result[key] ||= ) << value
        result
      end
    end
  end

  x.report "Adam" do
    TIMES.times do
      result = {}
      array.each { |k, v| result[k] ||= ; result[k] << v }
    end
  end

  x.report "inject Andrew" do
    TIMES.times do
      hash = array.inject({}) do |result, values|
        key,val = values
        if result[key].nil?
          result[key] = [val]
        else
          result[key].push val
        end
        result
      end
    end
  end

  x.report "Jesus" do
    TIMES.times do
      result = Hash.new {|h,k| h[k] = }
      array.each {|key,value| result[key] << value}
    end
  end
end

$ ruby bm_inject.rb
user system total real
inject Jeremy 1.440000 0.300000 1.740000 ( 1.770496)
Adam 1.100000 0.190000 1.290000 ( 1.290073)
inject Andrew 1.810000 0.210000 2.020000 ( 2.064162)
Jesus 1.800000 0.320000 2.120000 ( 2.118433)

I guess it's because you need to call the default proc many times, as
opposed to the other methods, which don't have any proc to call or
block to yield to.

Jesus.

botp1 · 2 February 2011 16:14

On Wed, Feb 2, 2011 at 11:52 PM, Edmond Kachale > But I also value you
contribution,

... are faster than the inject's way.

inject is a lot faster in 1.9
its added adv is that you lessen the use of temp vars and everything
is encap in the method..

eg, using jeremy bopps technique, one would do

a.inject(Hash.new{|hash,key| hash[key]=}) do |hash,(key,value)|
hash[key] << value
hash
end

or add a touch of tap,

a.inject(Hash.new{|h,k| h[k]=}) do |h,(k,value)|
h.tap{|h| h[k] << value}
end

best regards -botp

Topic		Replies	Views
Inject problem ruby-talk	25	181	26 September 2012
Array#inject with hash as initial, unexpected error ruby-talk	2	121	13 March 2006
Populating a hash from an array using inject ruby-talk	11	143	11 September 2009
Appropriate use of "inject()"? ruby-talk	6	65	16 September 2005
Array#inject to create a hash versus Hash[*array.collect{}.flatten] -- Speed, segfault ruby-talk	4	124	11 June 2007

Soliciting some help with inject method

Related topics