Hash and frequency of a value - ruby-on-rails

Question

I have the following hash in Ruby :
{
0 => {
:method=> "POST",
:path=> "/api/customer/191023",
:host=> "host.8",
:duration=> "1221"
},
1 => {
:method=> "GET",
:path=> "/api/customer/191023",
:host=> "host.8",
:duration=> "99"
},
2 => {
:method=> "POST",
:path=> "/api/customer/191023",
:host=> "host.10",
:duration=> "142"
},
3 => {
:method=> "POST",
:path=> "/api/customer/191023",
:host=> "host.8",
:duration=> "243"
}
4 => {
:method=> "POST",
:path=> "/api/customer/191023",
:host=> "host.10",
:duration=> "132"
}
}
I would like to do a simple search within these hashes to find the host with the highest frequency. For example, in the previous example, I should get host.8.
Thank you for your help,
M.

Answer 1

To find host value with highest frequency do:
hs = hash.values.group_by { |h| h[:host] =~ /host\.(\d+)/ && $1.to_i || 0 }.to_a
hs.reduce([-1,0]) { |sum,v| v[1].size > sum[1] && [ v[0], v[1].size ] || sum }.first
Description: [-1,0] is the default value for set for #reduce method, where -1 is a number (like in host.number), and 0 is a count of the number. So, when reduce encounters the number with size more than of passed sum, it replaces with the new value on next iteration.

Answer 2

Here's one way to do that.
Code
def max_host(hash)
hash.each_with_object(Hash.new(0)) { |(_,v),h| h[v[:host]] += 1 }
.max_by { |_,v| v }
.first
end
Example
Let's take the simplified example below. Note that I've changed, for example, :host = \"host.10\" to :host = "host.10", as the former is not a correct syntax. You could write the string as '\"host.10\" (=> "\\\"host.10\\\""), but I assume you simply want "host.10". The code is the same for both.
hash = {
0 => {
:method=>"POST",
:host =>"host.8"
},
1 => {
:method=>"GET",
:host =>"host.10"
},
2 => {
:method=>"POST",
:host =>"host.10"
}
}
max_host(hash)
#=> "host.10"
Explanation
For the example hash above,
enum = hash.each_with_object(Hash.new(0))
#=> #<Enumerator: {
# 0=>{:method=>"POST", :host=>"host.8"},
# 1=>{:method=>"GET", :host=>"host.10"},
# 2=>{:method=>"POST", :host=>"host.10"}}:each_with_object({})>
The enumerator will invoke the method Hash#each to pass each element of the enumerator into the block. We can see what those elements are by converting the enumerator to an array:
enum.to_a
#=> [[[0, {:method=>"POST", :host=>"host.8"}], {}],
# [[1, {:method=>"GET", :host=>"host.10"}], {}],
# [[2, {:method=>"POST", :host=>"host.10"}], {}]]
The empty hash shown in the first element is the initial value of the hash created by
Hash.new(0)
This creates a hash h with a default value of zero. By doing it this way, if h does not have a key k, h[k] will return the default value (0), but (important!) this does not change the hash.
The first value passed into the block is
[[0, {:method=>"POST", :host=>"host.8"}], {}]
This is then decomposed (or "disambiguated") into individual objects that are assigned to three block variables:
k => 0
v => {:method=>"POST", :host=>"host.8"}
h => Hash.new(0)
We then execute:
h[v[:host]] += 1
which is
h["host.8"] += 1
which is shorthand for
h["host.8"] = h["host.8"] + 1
[Aside: you may have noticed that in the code I show the block variables as |(_,v),h|, whereas above I refer to them above as |(k,v),h|. I could have used the latter, but since k is not reference in the block, I've chosen to replace it with a "placeholder" _. This ensures k won't be referenced and also tells any readers that I'm not using what would be the first block variable.]
As h does not have a key "host.8", h["host.8"] to the right of = returns the default value:
h["host.8"] = 0 + 1
#=> 1
so now
h #=> {"host.8"=>1}
The second element passed into the block is
[[1, {:method=>"GET", :host=>"host.10"}], {"host.8"=>1}]
so the block variables become:
v => {:method=>"GET", :host=>"host.10"}
h => {"host.8"=>1}
Notice that the hash h has been updated. We execute
h[v[:host]] += 1
#=> h["host.10"] += 1
#=> h["host.10"] = h["host.10"] + 1
#=> h["host.10"] = 0 + 1
#=> 1
so now
h #=> {"host.8"=>1, "host.10"=>1}
Lastly, the block variables are assigned the values
v = {:method=>"POST", :host=>"host.10"}
h => {"host.8"=>1, "host.10"=>1}
so
h[v[:host]] += 1
#=> h["host.10"] += 1
#=> h["host.10"] = h["host.10"] + 1
#=> h["host.10"] = 1 + 1
#=> 2
h #=> {"host.8"=>1, "host.10"=>2}
and the value of h is returned by the method.