Hash Sorting

Can not understand how the block after sort works! Need help. thanks.

h = { “a” => 20, “b” => 30, “c” => 10 }
puts h.sort #=> [[“a”, 20], [“b”, 30], [“c”, 10]]
puts h.sort {|a,b| a[0]<=>b[0]} # as above
puts h.sort {|a,b| a[1]<=>b[1]} #=> [[“c”, 10], [“a”, 20], [“b”, 30]]

I:\RubyNV>zzz-tut6-hash.rb
a
20
b
30
c
10

a
20
b
30
c
10

c
10
a
20
b
30

I:\RubyNV>

Alle giovedì 29 novembre 2007, Nathan V. ha scritto:

Can not understand how the block after sort works! Need help. thanks.

h = { “a” => 20, “b” => 30, “c” => 10 }
puts h.sort #=> [[“a”, 20], [“b”, 30], [“c”, 10]]
puts h.sort {|a,b| a[0]<=>b[0]} # as above
puts h.sort {|a,b| a[1]<=>b[1]} #=> [[“c”, 10], [“a”, 20], [“b”, 30]]

What Hash#sort does is:

  • first: convert the hash into a nested array:
    [[‘a’, 20], [‘b’, 30], [‘c’, 10]]
  • second: sort that array according to the block.

In the first call to sort, you don’t pass a block to it, so the sorting
will
be performed by calling the <=> of the contents of the array (i.e, on
the
key - value arrays). Array#<=> compares the contents of the two arrays
in
order, returning +1 or -1 as soon as one of the items is different from
the
other. For example, [‘a’, 20]<=>[‘c’, 10] returns -1 because ‘a’
precedes ‘c’. If two entries had the same first element (here this is
impossible since they come from a hash), the second element would be
compared, an so on.

Your second call to send gives the same result, because you’re
explicitly
telling sort to compare only the first element of the array. As
explained
above, even in the blockless case sort never needs to check the second
element, so the results are the same.

In the third case, you tell sort to compare the second element of each
pair
(i.e, the value in the original hash). This way, the pair [‘c’, 10]
becomes
the first, because 10 is the lesser of the three values; [‘a’, 20] is
the
second because 20 is the middle value and [‘b’, 30] is the last because
30 is
the greater value.

I hope this helps

Stefano

On Nov 29, 2007 11:09 AM, Nathan V. [email protected] wrote:

Can not understand how the block after sort works! Need help. thanks.

h = { “a” => 20, “b” => 30, “c” => 10 }
puts h.sort #=> [[“a”, 20], [“b”, 30], [“c”, 10]]
puts h.sort {|a,b| a[0]<=>b[0]} # as above
puts h.sort {|a,b| a[1]<=>b[1]} #=> [[“c”, 10], [“a”, 20], [“b”, 30]]

As can be read in the documentation:

Converts hsh to a nested array of [ key, value ] arrays and sorts it,
using Array#sort.

This means that sort will convert the array to this (in some order):

[[“a”, 20], [“b”, 30], [“c”, 10]]

and then call array sort on it. Checking the documentation on
Array#sort:

Returns a new array created by sorting self. Comparisons for the sort
will be done using the <=> operator or using an optional code block.
The block implements a comparison between a and b, returning -1, 0, or
+1. See also Enumerable#sort_by.

a = [ “d”, “a”, “e”, “c”, “b” ]
a.sort #=> [“a”, “b”, “c”, “d”, “e”]
a.sort {|x,y| y <=> x } #=> [“e”, “d”, “c”, “b”, “a”]

What this means is that in the block form, whenever the sort algorithm
needs to compare two elements of the array, it will yield to the block
passing both elements (remember that each element is itself an array
of [key,value] generated by the hash) and expecting a -1, 0 or 1
result, depending on which one you consider to be less than the other
or if they are equal.

In the case of the hash example above:

h.sort {|a,b| a[0]<=>b[0]}

the block will receive, for example:
a = [“a”, 20]
b = [“b”, 30]

or

a = [“a”, 20]
b = [“c”, 10]

or whatever pairs the sorting algorithm needs to compare. Then inside
the block you need to provide the comparison of the two elements. The
first example is doing a comparison of the keys, because the first
element of the pair is the hash key, so:

a[0] <=> b[0]

calls the spaceship operator on the keys of the hash. While the second
one a[1] <=> b[1] compares the values of the hash.

Hope this makes any sense.

Jesus.

Thanks Stefano, Jesus for your time and explanation. Makes sense now.

Nathan V. wrote:

Thanks Stefano, Jesus for your time and explanation. Makes sense now.

Hi,

I’m also trying to sort a hash, but don’t succeed for some reason.

My hash is build like this:

hash[‘key1’] = array_of_database_records_type_1
hash[‘key2’] = array_of_database_records_type_2
hash[‘key3’] = array_of_database_records_type_3
hash[‘key4’] = array_of_database_records_type_4


and so on. The hash’s data values are arrays of rails active record
objects,
receifed from a find(:all) call, although their type shouldn’t be of
importance when sorting by key, as I do:

sorted_hash_array = hash.sort

The array I receive is not sorted by keys at all, in fact its the same
array I get when I call hash.to_a without any sorting.

Sitting on this ‘simple’ problem for a few hours now, having no idea
what’s the problem.

Any ideas?

Nico

James T. wrote:

On 28 Jan 2008, at 17:45, Nico Ritsche wrote:

Any ideas?

hash.to_a.sort

Tried that, the result is the same, no sorting is happening at all.
Same as if I just do hash.to_a.

On 28 Jan 2008, at 17:45, Nico Ritsche wrote:

Any ideas?

hash.to_a.sort

Nico Ritsche wrote:

James T. wrote:

On 28 Jan 2008, at 17:45, Nico Ritsche wrote:

Any ideas?

hash.to_a.sort

Tried that, the result is the same, no sorting is happening at all.
Same as if I just do hash.to_a.

James is correct, the array will be sorted but the hash will remain
“unsorted”. Hashes are random access unsorted collections… you can’t
“sort” a hash…

you can however as James suggested, move the elements to a hash and then
sort the result any way you want…

irb(main):001:0> h = {1=>2,3=>4,5=>6}
=> {5=>6, 1=>2, 3=>4}
irb(main):002:0> h.to_a.sort
=> [[1, 2], [3, 4], [5, 6]]
irb(main):003:0> h.to_a.sort_by {|a,b| b}
=> [[1, 2], [3, 4], [5, 6]]
irb(main):004:0>

Ilan B. wrote:

you can however as James suggested, move the elements to a hash and then
sort the result any way you want…

errrr… I mean move the elements to an array… sorry for the confusion

ilan

Ilan B. wrote:

Ilan B. wrote:

you can however as James suggested, move the elements to a hash and then
sort the result any way you want…

errrr… I mean move the elements to an array… sorry for the confusion

ilan

Well, yea, but that is what I did, with no success. The hash.sort
function
simply calls .to_a and then sort on the generated array, so it does
exactly what you and James suggested. Unfortunately the array is not
sorted when I iterate over it using

array.each do

end

Is it because iterating like this doesn’t preserve the order of array
elements? I assumed .each do iterates in element order. I will test
this…

Ilan B. wrote:

ilan

irb(main):001:0> h = {1=>2,3=>4,5=>6}
=> {5=>6, 1=>2, 3=>4}
irb(main):002:0> h.to_a.sort
=> [[1, 2], [3, 4], [5, 6]]
irb(main):003:0> h.to_a.sort_by {|a,b| b}
=> [[1, 2], [3, 4], [5, 6]]
irb(main):004:0>

h.to_a.sort_by {|a,b| b} #sorts by value; the OP wants to sort by key.
h.to_a.sort_by {|key,value| key}
#or just
h.to_a.sort_by {|key|key}

Regards,

Siep

Nico Ritsche wrote:

Siep K. wrote:

Ilan B. wrote:

ilan

irb(main):001:0> h = {1=>2,3=>4,5=>6}
=> {5=>6, 1=>2, 3=>4}
irb(main):002:0> h.to_a.sort
=> [[1, 2], [3, 4], [5, 6]]
irb(main):003:0> h.to_a.sort_by {|a,b| b}
=> [[1, 2], [3, 4], [5, 6]]
irb(main):004:0>

h.to_a.sort_by {|a,b| b} #sorts by value; the OP wants to sort by key.
h.to_a.sort_by {|key,value| key}
#or just
h.to_a.sort_by {|key|key}

Regards,

Siep

array.sort sorts by key, I know that. That’s what I’m trying to do, but
it doesnt work in my case for some reason

maybe this excerpt of my code helps to clarify what I’m trying to do
(for those who know rails):

@parent_items = {}

@parent_items[‘teachers’] = Teacher.find(:all)
@parent_items[‘prices’] = Price.find(:all)
@parent_items[‘general_event_types’] = GeneralEventType.find(:all,
:conditions => “language_parent_id = id”)
@parent_items[‘specific_event_types’] = SpecificEventType.find(:all,
:conditions => “language_parent_id = id”)

pi_array_sorted = @parent_items.to_a.sort

and the i like to iterate over the elements of the sorted array, so the
result should look like this:

pi_array_sorted =
{ ‘general_event_types’, [GeneralEventType1, GeneralEventType2, …],
‘prices’, [Price1, Price2, …],
‘specific_event_types’, [SpecificEventType1, SpecificEventType2, …],
‘general_event_types’, [Teacher1, Teacher2, …] }

So the keys should be in alphabetical order, but they arent, at least
not when I iterate over the array using

pi_array_sorted.each do |a|

end

Nico Ritsche wrote:

result should look like this:

pi_array_sorted =
{ ‘general_event_types’, [GeneralEventType1, GeneralEventType2, …],
‘prices’, [Price1, Price2, …],
‘specific_event_types’, [SpecificEventType1, SpecificEventType2, …],
‘general_event_types’, [Teacher1, Teacher2, …] }

So the keys should be in alphabetical order, but they arent, (…)

Well, they are, if the last ‘general_event_types’ is substituted with
‘teachers’, what it probably should be.
I guess you want the values sorted.

I don’t now Rails, but hey, it’s ruby. You can probably do

@parent_items[‘teachers’] = Teacher.find(:all).sort

in your second and following lines. Or:

pi_array_sorted = @parent_items.sort.by{|key, value|value.sort!;key}

to have your subarrays sorted and the keys as well in one go.

Regards,

Siep

Siep K. wrote:

Ilan B. wrote:

ilan

irb(main):001:0> h = {1=>2,3=>4,5=>6}
=> {5=>6, 1=>2, 3=>4}
irb(main):002:0> h.to_a.sort
=> [[1, 2], [3, 4], [5, 6]]
irb(main):003:0> h.to_a.sort_by {|a,b| b}
=> [[1, 2], [3, 4], [5, 6]]
irb(main):004:0>

h.to_a.sort_by {|a,b| b} #sorts by value; the OP wants to sort by key.
h.to_a.sort_by {|key,value| key}
#or just
h.to_a.sort_by {|key|key}

Regards,

Siep

array.sort sorts by key, I know that. That’s what I’m trying to do, but
it doesnt work in my case for some reason

Siep K. wrote:

Nico Ritsche wrote:

result should look like this:

pi_array_sorted =
{ ‘general_event_types’, [GeneralEventType1, GeneralEventType2, …],
‘prices’, [Price1, Price2, …],
‘specific_event_types’, [SpecificEventType1, SpecificEventType2, …],
‘general_event_types’, [Teacher1, Teacher2, …] }

So the keys should be in alphabetical order, but they arent, (…)

Well, they are, if the last ‘general_event_types’ is substituted with
‘teachers’, what it probably should be.
I guess you want the values sorted.

I don’t now Rails, but hey, it’s ruby. You can probably do

@parent_items[‘teachers’] = Teacher.find(:all).sort

in your second and following lines. Or:

pi_array_sorted = @parent_items.sort.by{|key, value|value.sort!;key}

to have your subarrays sorted and the keys as well in one go.

Regards,

Siep

Hi Siep, yeah, that was a typo, the last general_event_types should be
‘teachers’.

But nevertheless, this is NOT what I get, its what I like to get!
And I really only want to sort by key, as I explained erlier…

Thanks,
Nico

Rodrigo K. wrote:

pi_array_sorted =
{ ‘general_event_types’, �[GeneralEventType1, GeneralEventType2, …],
� ‘prices’, � � � � � � � [Price1, Price2, …],
� ‘specific_event_types’, [SpecificEventType1, SpecificEventType2, …],
� ‘general_event_types’, �[Teacher1, Teacher2, …] }

So the keys should be in alphabetical order, but they arent, at least
not when I iterate over the array using

This code above doesn’t create an array, it creates an Hash, and Hash
doesn’t preserve order.
Do the following:

pi_array_sorted = [
[‘general_event_types’, [GeneralEventType1,
GeneralEventType2, …]],
[‘prices’, [Price1, Price2, …]],
[‘specific_event_types’, [SpecificEventType1,
SpecificEventType2, …]],
[‘general_event_types’, [Teacher1, Teacher2, …]]
]

pi_array_sorted.each do |key,value|

do stuff here

end

Cheers

Hi Rodrigo,

sorry, maybe the code was confusing, I just wanted to visualize the
result array I like to get after sorting. So the “pi_array_sorted =”
shouldn’t be an assignement and I used the wrong brakets. Thanks all
for your comments, but unfortunately concerning
the sorting issue I’m not a single step further.

Cheers,
Nico

pi_array_sorted =
{ ‘general_event_types’, [GeneralEventType1, GeneralEventType2, …],
‘prices’, [Price1, Price2, …],
‘specific_event_types’, [SpecificEventType1, SpecificEventType2, …],
‘general_event_types’, [Teacher1, Teacher2, …] }

So the keys should be in alphabetical order, but they arent, at least
not when I iterate over the array using

This code above doesn’t create an array, it creates an Hash, and Hash
doesn’t preserve order.
Do the following:

pi_array_sorted = [
[‘general_event_types’, [GeneralEventType1,
GeneralEventType2, …]],
[‘prices’, [Price1, Price2, …]],
[‘specific_event_types’, [SpecificEventType1,
SpecificEventType2, …]],
[‘general_event_types’, [Teacher1, Teacher2, …]]
]

pi_array_sorted.each do |key,value|

do stuff here

end

Cheers

Nico Ritsche wrote:

Rodrigo K. wrote:

pi_array_sorted =
{ ‘general_event_types’, �[GeneralEventType1, GeneralEventType2, …],
� ‘prices’, � � � � � � � [Price1, Price2, …],
� ‘specific_event_types’, [SpecificEventType1, SpecificEventType2, …],
� ‘general_event_types’, �[Teacher1, Teacher2, …] }

Nico

Nico,

We can help but you must strive to explain what you want clearly and
legibly… For the pi_array_sorted, it appears that you want to convert a
hash of arrays into an array of arrays where the second arrays is
sorted.

Here is a solution that sorts the nested arrays and then the initial
array

irb(main):002:0> a = {1=>[3,7,1,7], 2=>[3,7,1,1],3=>[3,2,2,9]}
=> {1=>[3, 7, 1, 7], 2=>[3, 7, 1, 1], 3=>[3, 2, 2, 9]}
irb(main):003:0> a.each {|b,c| c.sort!}
=> {1=>[1, 3, 7, 7], 2=>[1, 1, 3, 7], 3=>[2, 2, 3, 9]}
irb(main):004:0> a.sort
=> [[1, [1, 3, 7, 7]], [2, [1, 1, 3, 7]], [3, [2, 2, 3, 9]]]

hth

If it doesn’t then write the code EXACTLY as you have it… and then the
exact output that you desire and we will code it for you… The code that
you submitted is barely readable and leaves a lot open to
interpretation.

ilan

Ilan B. wrote:

Nico Ritsche wrote:

Rodrigo K. wrote:

pi_array_sorted =
{ ‘general_event_types’, �[GeneralEventType1, GeneralEventType2, …],
� ‘prices’, � � � � � � � [Price1, Price2, …],
� ‘specific_event_types’, [SpecificEventType1, SpecificEventType2, …],
� ‘general_event_types’, �[Teacher1, Teacher2, …] }

Nico

Nico,

We can help but you must strive to explain what you want clearly and
legibly… For the pi_array_sorted, it appears that you want to convert a
hash of arrays into an array of arrays where the second arrays is
sorted.

Here is a solution that sorts the nested arrays and then the initial
array

irb(main):002:0> a = {1=>[3,7,1,7], 2=>[3,7,1,1],3=>[3,2,2,9]}
=> {1=>[3, 7, 1, 7], 2=>[3, 7, 1, 1], 3=>[3, 2, 2, 9]}
irb(main):003:0> a.each {|b,c| c.sort!}
=> {1=>[1, 3, 7, 7], 2=>[1, 1, 3, 7], 3=>[2, 2, 3, 9]}
irb(main):004:0> a.sort
=> [[1, [1, 3, 7, 7]], [2, [1, 1, 3, 7]], [3, [2, 2, 3, 9]]]

hth

If it doesn’t then write the code EXACTLY as you have it… and then the
exact output that you desire and we will code it for you… The code that
you submitted is barely readable and leaves a lot open to
interpretation.

ilan

Hi Ilan,

actually I supplied my exact code already.

So here again, my exact code that doesn’t work:

@parent_items = {}

@parent_items[‘teachers’] = Teacher.find(:all)
@parent_items[‘prices’] = Price.find(:all)
@parent_items[‘general_event_types’] = GeneralEventType.find(:all,
:conditions => “language_parent_id = id”)
@parent_items[‘specific_event_types’] = SpecificEventType.find(:all,
:conditions => “language_parent_id = id”)

pi_array_sorted = @parent_items.to_a.sort

So what I’m doing here is building a hash containing arrays of rails
active record objects. So for example @parent_items[‘teachers’] is an
array of Teacher records. Then I call the hashs’s sort method. After
doing this I expect pi_array_sorted to be a nested array, sorted by key,
as the documentation of Hash.sort suggests (“Converts hsh to a nested
array of [ key, value ] arrays and sorts it, using Array#sort.”). So the
result for the above code should be this nested array:

[ ‘general_event_types’, [GeneralEventType1, GeneralEventType2, …],
‘prices’, [Price1, Price2, …],
‘specific_event_types’, [SpecificEventType1, SpecificEventType2, …],
‘teachers’, [Teacher1, Teacher2, …] ]

But this is not what I get. The keys in the resulting array still have
an arbitrary order, the exact same order as if I don’t call .sort at
all.

Nico

On Jan 30, 2008, at 1:39 AM, Nico Ritsche wrote:

all.
Can you cut, paste, and post the snippit of code you are using to
display your incorrect results?

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