Related
My apologies for the potentially stupid question, I'm an absolute beginner to Ruby and code in general.
I have set up a hash with some predetermined values. I want to ask the user for input, if that input matches an existing key, I want the corresponding value to be updated (+ 1, in this case). Then I want to print all the current up-to-date values.
hash = {"apple": 6, "banana": 2, "carrot": 3}
order = gets.chomp.downcase
hash.each do |key, value|
if key.to_s == order
value += 1
puts "Your order includes: #{value} #{key}."
end
end
My problem is that I only know how to print a single key value pair.
E.g. if the user inputs "apple", I'd like the output to say "Your order includes: 7 apple, 2 banana, 3 carrot."
hash = {apple: 6, banana: 2, carrot: 3}
order = gets.chomp.downcase.to_sym
hash[order] = hash.fetch(order, 0) + 1
puts "Your order includes: " + hash.map { |k, v| "#{v} #{k}" }.join(", ")
Some notes:
your hash initialization hash = {"apple": 6, "banana": 2, "carrot": 3}. the keys of your hash seem strings, but if you use that syntax with the colon, they become symbols. So, you have two choice. this syntax:
hash = {"apple" => 6, "banana" => 2, "carrot" => 3}
or you can use symbols as I did and convert the user input in a symbol
what's really cool about hash is that you don't need to iterate through the elements to find what you're looking for. There's a mapping between keys and values, so it's easy find and update a value
in the third row, I'm dealing with the fact that the key could not be in the hash, I used fetch to have 0 in that case. then, I increment and I assign back to that key
The question does not specify if you want to mutate the initial hash, so I suppose you do. Then the following will do.
hash = Hash.new(0).merge(apple: 6, banana: 2, carrot: 3)
hash[gets.chomp.downcase.to_sym] += 1
puts "Your order includes: " <<
hash.map { |k, v| [v, k].join(' ') }.join(', ')
or:
puts hash.reduce("Your order includes: ") { |acc, (k, v)|
acc << "#{v} #{k}, "
}[0..-3]
Consider to initialize the hash providing a default value (Hash#default)
basket = {'apple' => 6, 'banana' => 2, 'carrot' => 3}
basket.default = 0 # <-- set default 0 for start counting new keys
Define a method to present the data:
def show(basket)
puts "Your order includes:"
basket.each{ |key, value| puts "#{value}: #{key}" }
end
Capture user input in a loop (explanation in comments):
loop do
puts "Place your order:"
order = gets.downcase.chomp # <-- format the input
break if order == '0' # <-- breaks the input loop if this contition is matched
next unless basket.has_key? order # <-- skip to next loop no matches with hash keys or remove this line if you want to count also non initialised keys
basket[order] += 1 # <-- increment by one the key
show(basket) # <-- call the metod to show the basket
end
show(basket)
This might be a silly question, but I'm struggling with outputting the positions of an array of hashes I have.
If I have an array of hashes, we'll call some_array, that looks like this:
some_array =
[{:id=>7, :people=>["Bob B", "Jimmy J"], :product=>"product1"},
{:id=>2, :people=>["Sally S"], :product=>"product1"},
{:id=>5, :people=>["Hank H", "Stan C"], :product=>"product2"},
{:id=>3, :people=>["Sue T"], :product=>"product2"},
{:id=>4, :people=>["Anne W"], :product=>"product3"}]
I then iterate though some_array like so:
some_array.select{|p| p[:product] == "product2"]}.each do |product|
product[:people].join("<br>")
product[:product]
Which outputs like:
Hank K product 2
Stan C
Sue T product 2
How would I go about outputting the index/position of each hash in the array?
Would I be able to do something along the lines of:
some_array.select{|p| p[:product] == "product2"]}.each do |product|
product.index
product[:people].join("<br>")
product[:product]
And get:
2 Hank K product2
Stan C
3 Sue T product2
Thank you!
You can use each_with_index and format to your use case:
some_array.each_with_index do |product, index|
if product[:product] == "product2"
p index
p product
end
end
In Ruby, you can chain methods on Enumerable, which allows you to call with_index before you select to get the original index of the element:
some_array.each_with_index.select do |element, _|
element[:product] == "product2"
end.each do |product, index|
p [index, product[:people].join("<br />"), product[:product]]
end
# outputs:
# [2, "Hank H<br />Stan C", "product2"]
# [3, "Sue T", "product2"]
While you can call select.with_index, and it may be tempting to do so, the index won't carry over into the each, because select returns the elements that matched and doesn't care about the input. When you call each_with_index (or each.with_index), though, you now have a new Enumerable which is each element in your array with its index in that array, and select ends up returning these new array elements:
some_array.each.with_index.select { |element, _| element[:product] == "product2" }
# => [[{:id=>5, :people=>["Hank H", "Stan C"], :product=>"product2"}, 2],
[{:id=>3, :people=>["Sue T"], :product=>"product2"}, 3]]
fmt_str_first = "%-4d%-10s%10s"
fmt_str_rest = "#{' '*4}%-10s"
some_array.each_with_index do |h,i|
next unless h[:product] == "product2"
first, *rest = h[:people]
puts fmt_str_first % [i, first, "product2"]
rest.each { |name| puts fmt_str_rest % name }
puts
end
2 Hank H product2
Stan C
3 Sue T product2
See Kernel#sprintf. Note that %-10s in the format string means that the corresponding entry, a string (s), is to be left-adjusted (-) in a field of width 10. %10s would cause the entry to be right-adjusted.
you can just use each_with_index and skip the item you don't need
some_array.each_with_index do |product, index|
next if product[:product] != "product2"
index
product[:people].join("<br>")
product[:product]
end
I have built a version of mastermind that checks a user's input and provides feedback based on how close the user's guess was to the winning sequence. If you're not familiar with the game, you get feedback indicating how many of your characters were guessed correctly at the same index and how many characters guessed are in the sequence, but at the wrong index. If there are duplicates in the guess, then you would not count the extra values unless they correspond to the same number of duplicates in the secret code.
Example: If the sequence is ["G","G","G","Y"] and the user guesses ["G", "Y","G","G"] then you'd want to return 2 for items at the same index and 2 for items at different indexes that are included in the secret sequence.
Another example: If the sequence is ["X","R","Y","T"] and the user guesses ["T","T","Y","Y"] then you'd return 1 for items at the same index 1 for the character guessed that is in the sequence but at the wrong index.
Anyway, to me this is not a simple problem to solve. Here's the code I used to get it to work, but it's not elegant. There must be a better way. I was hoping someone can tell me what I'm missing here?? New to Ruby...
def index_checker(input_array, sequence_array)
count = 0
leftover_input = []
leftover_sequence = []
input.each_with_index do |char, idx|
if char == sequence[idx]
count += 1
else
leftover_input << char
leftover_sequence << sequence[idx]
end
end
diff_index_checker(leftover_input, leftover_sequence, count)
end
def diff_index_checker(input, sequence, count)
count2 = 0
already_counted = []
input.each do |char|
if sequence.include?(char) && !already_counted.include?(char)
count2 += 1
already_counted << char
end
end
[count, count2]
end
Here's a clean Ruby solution, written in idiomatic Ruby object-oriented style:
class Mastermind
def initialize(input_array, sequence_array)
#input_array = input_array
#sequence_array = sequence_array
end
def matches
[index_matches, other_matches]
end
def results
[index_matches.size, other_matches.size]
end
private
attr_reader :input_array, :sequence_array
def index_matches
input_array.select.with_index { |e, i| e == sequence_array[i] }
end
def other_matches
non_exact_input & non_exact_sequence
end
def non_exact_input
array_difference(input_array, index_matches)
end
def non_exact_sequence
array_difference(sequence_array, index_matches)
end
# This method is based on https://stackoverflow.com/a/3852809/5961578
def array_difference(array_1, array_2)
counts = array_2.inject(Hash.new(0)) { |h, v| h[v] += 1; h }
array_1.reject { |e| counts[e] -= 1 unless counts[e].zero? }
end
end
You would use this class as follows:
>> input_array = ["G","G","G","Y"]
>> sequence_array = ["G", "Y","G","G"]
>> guess = Mastermind.new(input_array, sequence_array)
>> guess.results
#> [2, 2]
>> guess.matches
#> [["G", "G"], ["G", "Y"]]
Here's how it works. First everything goes into a class called Mastermind. We create a constructor for the class (which in Ruby is a method called initialize) and we have it accept two arguments: input array (the user guess), and sequence array (the answer).
We set each of these arguments to an instance variable, which is indicated by its beginning with #. Then we use attr_reader to create getter methods for #input_array and #sequence_array, which allows us to get the values by calling input_array and sequence_array from any instance method within the class.
We then define two public methods: matches (which returns an array of exact matches and an array of other matches (the ones that match but at the wrong index), and results (which returns a count of each of these two arrays).
Now, within the private portion of our class, we can define the guts of the logic. Each method has a specific job, and each is named to (hopefully) help a reader understand what it is doing.
index_matches returns a subset of the input_array whose elements match the sequence_array exactly.
other_matches returns a subset of the input_array whose elements do not match the sequence_array exactly, but do match at the wrong index.
other_matches relies on non_exact_input and non_exact_sequence, each of which is computed using the array_difference method, which I copied from another SO answer. (There is no convenient Ruby method that allows us to subtract one array from another without deleting duplicates).
Code
def matches(hidden, guess)
indices_wo_match = hidden.each_index.reject { |i| hidden[i] == guess[i] }
hidden_counts = counting_hash(hidden.values_at *indices_wo_match)
guess_counts = counting_hash(guess.values_at *indices_wo_match)
[hidden.size - indices_wo_match.size, guess_counts.reduce(0) { |tot, (k, cnt)|
tot + [hidden_counts[k], cnt].min }]
end
def counting_hash(arr)
arr.each_with_object(Hash.new(0)) { |s, h| h[s] += 1 }
end
Examples
matches ["G","G","G","Y"], ["G", "Y","G","G"]
#=> [2, 2]
matches ["X","R","Y","T"] , ["T","T","Y","Y"]
#=> [1, 1]
Explanation
The steps are as follows.
hidden = ["G","G","G","Y"]
guess = ["G", "Y","G","G"]
Save the indices i for which hidden[i] != guess[i].
indices_wo_match = hidden.each_index.reject { |i| hidden[i] == guess[i] }
#=> [1, 3]
Note that the number of indices for which the values are equal is as follows.
hidden.size - indices_wo_match.size
#=> 2
Now compute the numbers of remaining elements of guess that pair with one of the remaining values of hidden by having the same value. Begin by counting the numbers of instances of each unique element of hidden and then do the same for guess.
hidden_counts = counting_hash(hidden.values_at *indices_wo_match)
#=> {"G"=>1, "Y"=>1}
guess_counts = counting_hash(guess.values_at *indices_wo_match)
#=> {"Y"=>1, "G"=>1}
To understand how counting_hash works, see Hash::new, especially the explanation of the effect of providing a default value as an argument of new. In brief, if a hash is defined h = Hash.new(3), then if h does not have a key k, h[k] returns the default value, here 3 (the hash is not changed).
Now compute the numbers of matches of elements of guess that were not equal to the value of hidden at the same index and which pair with an element of hidden that have the same value.
val_matches = guess_counts.reduce(0) do |tot, (k, cnt)|
tot + [hidden_counts[k], cnt].min
end
#=> 2
Lastly, return the values of interest.
[hidden.size - indices_wo_match.size, val_matches]
#=> [2, 2]
In the code presented above I have substituted out the variable val_matches.
With Ruby 2.4+ one can use Enumerable#sum to replace
guess_counts.reduce(0) { |tot, (k, cnt)| tot + [hidden_counts[k], cnt].min }
with
guess_counts.sum { |k, cnt| [hidden_counts[k], cnt].min }
def judge(secret, guess)
full = secret.zip(guess).count { |s, g| s == g }
semi = secret.uniq.sum { |s| [secret.count(s), guess.count(s)].min } - full
[full, semi]
end
Demo:
> judge(["G","G","G","Y"], ["G","Y","G","G"])
=> [2, 2]
> judge(["X","R","Y","T"], ["T","T","Y","Y"])
=> [1, 1]
A shorter alternative, though I find it less clear:
full = secret.zip(guess).count(&:uniq!)
I prefer my other answer for its simplicity, but this one would be faster if someone wanted to use this for arrays larger than Mastermind's.
def judge(secret, guess)
full = secret.zip(guess).count { |s, g| s == g }
pool = secret.group_by(&:itself)
[full, guess.count { |g| pool[g]&.pop } - full]
end
Demo:
> judge(["G","G","G","Y"], ["G","Y","G","G"])
=> [2, 2]
> judge(["X","R","Y","T"], ["T","T","Y","Y"])
=> [1, 1]
I have a hash that has some keys as an array like so:
foo = {[45, 121]=>:some_field}
How can I select :some_field where a foo key contains 45?
And secondary to that, if it finds a match, how do I retrieve the other elements in the same key?
Although you can do this, it kind of defeats the purpose of using a hash since you will have to do a linear scan through the entire thing. It would be a lot better to have multiple hash keys for the same value since you can use the hash as an index then.
Example:
found = foo.find { |k, v| k.include?(n) }
found and found[1]
Keep in mind the performance of this will be terrible if you have large numbers of entries in the key and a large number of items in the hash since it will have to test against all keys and all values individually.
foo = {[45, 121]=>:some_field}
foo.detect{ |k,v| k.include? 45 }
#=> [[45, 121], :some_field]
foo.detect{ |k,v| k.include? 45 }.last
#=> :some_field
I would suggest to reverse your hash if it's not one element only:
foo = {[45, 121]=>:some_field, [1, 45, 7] => :some_other_field}
bar = {}
foo.each do |k, v|
k.each do |x|
if bar.has_key?(x)
bar[x] << [[k, v]]
else
bar[x] = [[k, v]]
end
end
end
p bar[45]
I have a Ruby array containing some string values. I need to:
Find all elements that match some predicate
Run the matching elements through a transformation
Return the results as an array
Right now my solution looks like this:
def example
matchingLines = #lines.select{ |line| ... }
results = matchingLines.map{ |line| ... }
return results.uniq.sort
end
Is there an Array or Enumerable method that combines select and map into a single logical statement?
I usually use map and compact together along with my selection criteria as a postfix if. compact gets rid of the nils.
jruby-1.5.0 > [1,1,1,2,3,4].map{|n| n*3 if n==1}
=> [3, 3, 3, nil, nil, nil]
jruby-1.5.0 > [1,1,1,2,3,4].map{|n| n*3 if n==1}.compact
=> [3, 3, 3]
Ruby 2.7+
There is now!
Ruby 2.7 is introducing filter_map for this exact purpose. It's idiomatic and performant, and I'd expect it to become the norm very soon.
For example:
numbers = [1, 2, 5, 8, 10, 13]
enum.filter_map { |i| i * 2 if i.even? }
# => [4, 16, 20]
Here's a good read on the subject.
Hope that's useful to someone!
You can use reduce for this, which requires only one pass:
[1,1,1,2,3,4].reduce([]) { |a, n| a.push(n*3) if n==1; a }
=> [3, 3, 3]
In other words, initialize the state to be what you want (in our case, an empty list to fill: []), then always make sure to return this value with modifications for each element in the original list (in our case, the modified element pushed to the list).
This is the most efficient since it only loops over the list with one pass (map + select or compact requires two passes).
In your case:
def example
results = #lines.reduce([]) do |lines, line|
lines.push( ...(line) ) if ...
lines
end
return results.uniq.sort
end
Another different way of approaching this is using the new (relative to this question) Enumerator::Lazy:
def example
#lines.lazy
.select { |line| line.property == requirement }
.map { |line| transforming_method(line) }
.uniq
.sort
end
The .lazy method returns a lazy enumerator. Calling .select or .map on a lazy enumerator returns another lazy enumerator. Only once you call .uniq does it actually force the enumerator and return an array. So what effectively happens is your .select and .map calls are combined into one - you only iterate over #lines once to do both .select and .map.
My instinct is that Adam's reduce method will be a little faster, but I think this is far more readable.
The primary consequence of this is that no intermediate array objects are created for each subsequent method call. In a normal #lines.select.map situation, select returns an array which is then modified by map, again returning an array. By comparison, the lazy evaluation only creates an array once. This is useful when your initial collection object is large. It also empowers you to work with infinite enumerators - e.g. random_number_generator.lazy.select(&:odd?).take(10).
If you have a select that can use the case operator (===), grep is a good alternative:
p [1,2,'not_a_number',3].grep(Integer){|x| -x } #=> [-1, -2, -3]
p ['1','2','not_a_number','3'].grep(/\D/, &:upcase) #=> ["NOT_A_NUMBER"]
If we need more complex logic we can create lambdas:
my_favourite_numbers = [1,4,6]
is_a_favourite_number = -> x { my_favourite_numbers.include? x }
make_awesome = -> x { "***#{x}***" }
my_data = [1,2,3,4]
p my_data.grep(is_a_favourite_number, &make_awesome) #=> ["***1***", "***4***"]
I'm not sure there is one. The Enumerable module, which adds select and map, doesn't show one.
You'd be required to pass in two blocks to the select_and_transform method, which would be a bit unintuitive IMHO.
Obviously, you could just chain them together, which is more readable:
transformed_list = lines.select{|line| ...}.map{|line| ... }
Simple Answer:
If you have n records, and you want to select and map based on condition then
records.map { |record| record.attribute if condition }.compact
Here, attribute is whatever you want from the record and condition you can put any check.
compact is to flush the unnecessary nil's which came out of that if condition
No, but you can do it like this:
lines.map { |line| do_some_action if check_some_property }.reject(&:nil?)
Or even better:
lines.inject([]) { |all, line| all << line if check_some_property; all }
I think that this way is more readable, because splits the filter conditions and mapped value while remaining clear that the actions are connected:
results = #lines.select { |line|
line.should_include?
}.map do |line|
line.value_to_map
end
And, in your specific case, eliminate the result variable all together:
def example
#lines.select { |line|
line.should_include?
}.map { |line|
line.value_to_map
}.uniq.sort
end
def example
#lines.select {|line| ... }.map {|line| ... }.uniq.sort
end
In Ruby 1.9 and 1.8.7, you can also chain and wrap iterators by simply not passing a block to them:
enum.select.map {|bla| ... }
But it's not really possible in this case, since the types of the block return values of select and map don't match up. It makes more sense for something like this:
enum.inject.with_index {|(acc, el), idx| ... }
AFAICS, the best you can do is the first example.
Here's a small example:
%w[a b 1 2 c d].map.select {|e| if /[0-9]/ =~ e then false else e.upcase end }
# => ["a", "b", "c", "d"]
%w[a b 1 2 c d].select.map {|e| if /[0-9]/ =~ e then false else e.upcase end }
# => ["A", "B", false, false, "C", "D"]
But what you really want is ["A", "B", "C", "D"].
You should try using my library Rearmed Ruby in which I have added the method Enumerable#select_map. Heres an example:
items = [{version: "1.1"}, {version: nil}, {version: false}]
items.select_map{|x| x[:version]} #=> [{version: "1.1"}]
# or without enumerable monkey patch
Rearmed.select_map(items){|x| x[:version]}
If you want to not create two different arrays, you can use compact! but be careful about it.
array = [1,1,1,2,3,4]
new_array = map{|n| n*3 if n==1}
new_array.compact!
Interestingly, compact! does an in place removal of nil. The return value of compact! is the same array if there were changes but nil if there were no nils.
array = [1,1,1,2,3,4]
new_array = map{|n| n*3 if n==1}.tap { |array| array.compact! }
Would be a one liner.
Your version:
def example
matchingLines = #lines.select{ |line| ... }
results = matchingLines.map{ |line| ... }
return results.uniq.sort
end
My version:
def example
results = {}
#lines.each{ |line| results[line] = true if ... }
return results.keys.sort
end
This will do 1 iteration (except the sort), and has the added bonus of keeping uniqueness (if you don't care about uniq, then just make results an array and results.push(line) if ...
Here is a example. It is not the same as your problem, but may be what you want, or can give a clue to your solution:
def example
lines.each do |x|
new_value = do_transform(x)
if new_value == some_thing
return new_value # here jump out example method directly.
else
next # continue next iterate.
end
end
end