I recently ran into this issue:
I've always used inject like so (i knew that (0) part is optional and can be omitted)
array = [13,23,13]
#=> [13, 23, 13]
array.inject(0) { |sum,i| sum+i }
#=> 49
By chance i found out that you can use:
array.inject(:+)
#=> 49
array.inject(:-)
#=> -23
array.inject(:*)
#=> 3887
array.inject(:/)
#=> 0
Googling on the issue i found a nice article on inject, but no mentioning there about what i've tried....
Can anyone explain to me or give some info about these inject commands that I've just used?
From the doc on Enumerable#inject:
... If you specify a symbol instead, then each element in the collection will be passed to the named method of memo. In either case, the result becomes the new value for memo. At the end of the iteration, the final value of memo is the return value for the method.
If you do not explicitly specify an initial value for memo, then uses the first element of collection is used as the initial value of memo.
So, if you specify a symbol, it treats it as a method name and invokes this method on every element of the enumerable, replacing memo as stated above. Now, the math operators (+-*/) are just methods, nothing else. These lines produce identical result:
13 + 23 # => 36
13.+(23) # => 36
13.send(:+, 23) # => 36
When you pass a symbol to inject or reduce it uses the third form to dynamically apply that operator to elements:
[1,2,3].inject(:+) # => 6
This shorthand can be used with methods other than operators as well:
[{"a"=>1}, {"b"=>2}].inject(:merge) # => {"a"=>1, "b"=>2}
Related
I am working with a third party device which has some implementation of Lua, and communicates in BACnet. The documentation is pretty janky, not providing any sort of help for any more advanced programming ideas. It's simply, "This is how you set variables...". So, I am trying to just figure it out, and hoping you all can help.
I need to set a long list of variables to certain values. I have a userdata 'ME', with a bunch of variables named MVXX (e.g. - MV21, MV98, MV56, etc).
(This is all kind of background for BACnet.) Variables in BACnet all have 17 'priorities', i.e., every BACnet variable is actually a sort of list of 17 values, with priority 16 being the default. So, typically, if I were to say ME.MV12 = 23, that would set MV12's priority-16 to the desired value of 23.
However, I need to set priority 17. I can do this in the provided Lua implementation, by saying ME.MV12_PV[17] = 23. I can set any of the priorities I want by indexing that PV. (Corollaries - what is PV? What is the underscore? How do I get to these objects? Or are they just interpreted from Lua to some function in C on the backend?)
All this being said, I need to make that variable name dynamic, so that i can set whichever value I need to set, based on some other code. I have made several attempts.
This tells me the object(MV12_PV[17]) does not exist:
x = 12
ME["MV" .. x .. "_PV[17]"] = 23
But this works fine, setting priority 16 to 23:
x = 12
ME["MV" .. x] = 23
I was trying to attempt some sort of what I think is called an evaluation, or eval. But, this just prints out function followed by some random 8 digit number:
x = 12
test = assert(loadstring("MV" .. x .. "_PV[17] = 23"))
print(test)
Any help? Apologies if I am unclear - tbh, I am so far behind the 8-ball I am pretty much grabbing at straws.
Underscores can be part of Lua identifiers (variable and function names). They are just part of the variable name (like letters are) and aren't a special Lua operator like [ and ] are.
In the expression ME.MV12_PV[17] we have ME being an object with a bunch of fields, ME.MV12_PV being an array stored in the "MV12_PV" field of that object and ME.MV12_PV[17] is the 17th slot in that array.
If you want to access fields dynamically, the thing to know is that accessing a field with dot notation in Lua is equivalent to using bracket notation and passing in the field name as a string:
-- The following are all equivalent:
x.foo
x["foo"]
local fieldname = "foo"
x[fieldname]
So in your case you might want to try doing something like this:
local n = 12
ME["MV"..n.."_PV"][17] = 23
BACnet "Commmandable" Objects (e.g. Binary Output, Analog Output, and o[tionally Binary Value, Analog Value and a handful of others) actually have 16 priorities (1-16). The "17th" you are referring to may be the "Relinquish Default", a value that is used if all 16 priorities are set to NULL or "Relinquished".
Perhaps your system will allow you to write to a BACnet Property called "Relinquish Default".
In my web page there are 5 values given in the text field(like $10, $20, $30, $40 and $50) and I am trying to sum the values using ruby and selenium WebDriver.
Here is my code:
def get_sum_of_all_elements()
#logger.info("Searching element #{value1, value2, value3, value4, value5}");
allelements = #driver.find_elements(:id = "lbl_val_")
#logger.info("Total Elements Found with locator #{locator} are : #{allelements.size}");
if allelements.start_with?("$")
allelements = "((allelements))".tr('$', '') #removing '$' sign from values
iSum =0
allelements.each do|i|
iSum += i
end
end
end
I am expecting to see output as 150. Do I need to store values in an array?
Any help would be appreciated.
There a couple of things you should modify in your code to make it work:
Fix how arguments are passed to find_elements; it should be id: "lbl_val_".
find_elements returns an array of WebDriver::Element objects, so you must check the value for each object.
The string "Searching element #{value1, value2, value3, value4, value5}" is not valid since you are trying to interpolate the value of 5 variables chained with a comma. You either need to interplate only the variable (keeping commas as strings) or use square brackets ([]) to interpolate an array.
Now your code should look something like this1:
def get_sum_of_all_elements
#logger.info("Searching element #{[value1, value2, value3, value4, value5]}")
allelements = #driver.find_elements(id: "lbl_val_")
#logger.info("Total Elements Found with locator #{locator} are : #{allelements.size}");
if allelements.all? { |elem| elem.value.start_with?("$") }
elements = allelements.map { |elem| elem.value.tr('$', '').to_i }
elements.reduce(:+)
end
end
A few things to note:
Parenthesis (()) were removed in method definition, ruby doesn't need them when no arguments are passed.
There is no longer need to assign the final value to a variable (e.g iSum) since ruby will return the result of last evaluated code.
If any value doesn't start with "$", it will return false. You could change this by adding a default value after if block.
Semicolons (;) were removed, you don't need them in ruby (unless you want to chain multiple statements in a singe line).
One more thing, the variables value1, value2, value3, value4, value5 and locator doesn't seem to be set anywhere in your method; you must set them within your method (or pass them as arguments) or you will get an error.
1 This considers the same logic that you seemed to be looking for in your code, that is, sum all values only if all of them start with "$".
It's hard to say exactly what you are trying to do, but this might help. I assume you have an array of string values with dollar signs:
>> allelements = ["$10", "$20", "$30", "$40", "$50"]
=> ["$10", "$20", "$30", "$40", "$50"]
We can make a new array stripping out all non-numeric characters and transforming the string values to integers:
>> integers = allelements.map { |e| e.gsub(/[^\d]/, '').to_i }
=> [10, 20, 30, 40, 50]
Now use inject to sum the values:
>> integers.inject(:+)
=> 150
I wrote a method that takes six names then generates an array of seven random numbers using four 6-sided dice. The lowest value of the four 6-sided dice is dropped, then the remainder is summed to create the value. The value is then added to an array.
Once seven numbers have been generated, the array is then ordered from highest to lowest and the lowest value is dropped. Then the array of names and the array of values are zipped together to create a hash.
This method ensures that the first name in the array of names receives the highest value, and the last name receives the lowest.
This is the result of calling the method:
{:strength=>1, :dexterity=>1, :constitution=>0, :intelligence=>0, :wisdom=>0, :charisma=>1}
As you can see, all the values I receive are either "1" or "0". I have no idea how this is happening.
Here is the code:
module PriorityStatGenerator
def self.roll_stats(first_stat, second_stat, third_stat, fourth_stat, fifth_stat, sixth_stat)
stats_priority = [first_stat, second_stat, third_stat, fourth_stat, fifth_stat, sixth_stat].map(&:to_sym)
roll_array = self.roll
return Hash[stats_priority.zip(roll_array)]
end
private
def self.roll
roll_array = []
7.times {
roll_array << Array.new(4).map{ 1 + rand(6) }.sort.drop(1).sum
}
roll_array.reverse.delete_at(6)
end
end
This is how I'm calling the method while I'm testing:
render plain: PriorityStatGenerator.roll_stats(params[:prioritize][:first_stat], params[:prioritize][:second_stat], params[:prioritize][:third_stat], params[:prioritize][:fourth_stat], params[:prioritize][:fifth_stat], params[:prioritize][:sixth_stat])
I added require 'priority_stat_generator' where I'm calling the method, so it is properly calling it.
Can someone help me make it return proper values between 1 and 18?
Here's a refactoring to simplify things and use an actually random number generator, as rand is notoriously terrible:
require 'securerandom'
module PriorityStatGenerator
def self.roll_stats(*stats)
Hash[
stats.map(&:to_sym).zip(self.roll(stats.length).reverse)
]
end
private
def self.roll(n = 7)
(n + 1).times.map do
4.times.map { 1 + SecureRandom.random_number(6) }.sort.drop(1).inject(:+)
end.sort.last(n)
end
end
This makes use of inject(:+) so it works in plain Ruby, no ActiveSupport required.
The use of *stats makes the roll_stats function way more flexible. Your version has a very rigid number of parameters, which is confusing and often obnoxious to use. Treating the arguments as an array avoids a lot of the binding on the expectation that there's six of them.
As a note it's not clear why you're making N+1 roles and then discarding the last. That's the same as generating N and discarding none. Maybe you meant to sort them and take the N best?
Update: Added sort and reverse to properly map in terms of priority.
You need to learn to use IRB or PRY to test snippets of your code, or better, learn to use a debugger. They give you insight into what your code is doing.
In IRB:
[7,6,5,4,3,2,1].delete_at(6)
1
In other words, delete_at(6) is doing what it's supposed to, but that's not what you want. Instead, perhaps slicing the array will behave more like you expect:
>> [7,6,5,4,3,2,1][0..-2]
[
[0] 7,
[1] 6,
[2] 5,
[3] 4,
[4] 3,
[5] 2
]
Also, in your code, it's not necessary to return a value when that operation is the last logical step in a method. Ruby will return the last value seen:
Hash[stats_priority.zip(roll_array)]
As amadan said, I can't see how you are getting the results you are, but their is a definite bug in your code.
The last line in self.roll is the return value.
roll_array.reverse.delete_at(6)
Which is going to return the value that was deleted. You need to add a new lines to return the roll_array instead of the delete_at value. You are also not sorting your array prior to removing that last item which will give you the wrong values as well.
def self.roll
roll_array = []
7.times {
roll_array << Array.new(4).map{ 1 + rand(6) }.sort.drop(1).sum
}
roll_array.sort.drop(1)
roll_array
end
I'm struggling in understanding when to use the ampersand in passing symbols to functions representing a method. For example, If I wanted to calculate the sum of the range 1..10, I could do the following:
(1..10).inject(:+)
This originally lead me to believe that if you wanted to pass a symbol to define a method to "Magically" be used in the function, you would pass the function name as a symbol. But then I see something like this in rails:
total = Product.find(product_list).sum(&:price)
If I understand correctly, &:price is the same as calling :price.to_proc. I don't understand how the above works.
In actual parameter list of a method call, &object is built-in syntax, which will
convert object to a Proc using object.to_proc
pass the Proc as the block parameter of the method
Symbol#to_proc converts the symbol (eg. :the_symbol) to proc {|obj| obj.send(:the_symbol)}. And you can use this syntax whenever object responds to to_proc method and returns a Proc.
abc = "aha~"
class << abc
def to_proc
proc {|obj| obj.to_i * 2 }
end
end
p ["1", "2", "3"].map(&abc)
#=> [2, 4, 6]
(1..10).inject(:+) shows that inject accepts a symbol as parameter. How the symbol is used is method specific behavior. In inject's special case, it has the same effect as (1..10).inject{|a, b| a.send(:+, b)}. It's just a simple, normal parameter, the effect depends on the implementation of the method that accept the symbol as parameter.
Note that sum in ActiveSupport accepts a block with single parameter, which has the effect "map values in the original sequence to new ones and calculate the sum of them", so
total = Product.find(product_list).sum(&:price)
is equivalent as
total = Product.find(product_list).sum(&proc{|p| p.send(:price)})
# or you may write
total = Product.find(product_list).sum{|p| p.price }
which has the same return value as the following but won't produce intermediate temp array:
total = Product.find(product_list).map{|p| p.price}.sum
I was looking through some Rails source code and came across
# File vendor/rails/activesupport/lib/active_support/vendor/builder-2.1.2/builder/css.rb, line 129
129: def target!
130: #target * ''
131: end
What does the * '' do? Is that multiplication by an empty string...? And why would you do that.
This is a bizarre syntax. These are equivalent:
>> [1, 2, 3] * 'joiner'
=> "1joiner2joiner3"
>> [1, 2, 3].join 'joiner'
=> "1joiner2joiner3"
so in this case it joins all the entries of #target into one string, with nothing between the entries.
Note: if you do something like [1, 2, 3] * 3 (using an int instead of a str), you'll get three concatenated copies of the array instead.
It does the same thing as:
["foo","bar","baz"].join
http://ruby-doc.org/core/classes/Array.html#M002210
Per Z.E.D.'s suggestion, you would use it if you want to confuse people and make your code more error prone.
Really cryptic code indeed.
After checking the source code, I realized that #target is actually an Array instance, I know you can do stuff like this
[5] * 5 # => [5,5,5,5,5]
I don't know where Array#* is defined (maybe in ActiveSupport), but what I can tell you is that, this is the behaviour when it gets multiplied by a String
[1,2,3] * 'comma' # => "1comma2comma3"
[1,2,3] * '' # => '123'
So I can infer it is concatanating all the elements of the array without any separators.
Array#* with a String argument is equivalent to Array#join.
Two comments:
Having a ! end a method name implies that it's a mutating operation, which this example doesn't seem to be.
As others have stated it's indeed cryptic. I would go for #target.to_s or #target.join
The only reason I'd see someone wanting to use * to join an array of strings is if they want to avoid the word join, which is also used on threads.
With the following:
strings = ["John", "Smith"]
threads = [Thread.new{puts "hai"}, Thread.new{ puts "world!"}]
strings.join(" ") # Legit
threads.each {|thread| thread.join} # Legit
you could make the mistake of doing
threads.join # Not legit, but won't raise an error
If you replaced strings.join with strings.*, then you'd have fewer joins in your code. Now you could do a grep for them, and check that each one is being done to a thread, not to an array. In fact, you could choose to make Array#join throw an exception.