So working through the above exercise and found this solution on GitHub.
def count_between arr, lower, upper
return 0 if arr.length == 0 || lower > upper
return arr.length if lower == upper
range = (lower..upper).to_a
arr.select { |value| range.include?(value) }.length
end
I understand what the first three lines mean and why they return the values they do. What I'd like to understand are the following lines of code.
Line 4 (below) is defining "range" as a variable and uses the lower...upper as the range variables (just discovered you don't need to put an integer value in a range. What does '.to_a' mean, can't seem to find it in the ruby docs, and what does it do?
range = (lower..upper).to_a
Line 5 (below) is using an Array#select method and its saying select this value if the value is included in this range and then give me the Array#length of all selected values, but I don't quite understand A. what |value| is doing and what it means. B. range.include?(value) means is this value included in this range I am assuming.
arr.select { |value| range.include?(value) }.length
Actually, I'd simplify to this:
def count_between arr, lower, upper
return 0 if lower > upper
arr.count{|v| (lower..upper).include?(v)}
end
to_a is documented here; it returns an Array containing each element in the Range. However, there's no reason to call to_a on the Range before calling include?.
There's also no reason to special-case the empty array.
Returning the length of the array when lower equals upper makes no sense.
value is the name given to the value the block is called with. I think a simple v is better for such a trivial case.
select calls the block for each value in arr and returns a new Array containing the elements for which the block returns true, so the length of that new Array is the number of matching values. However, count exists, and makes more sense to use, since the count is all we care about.
Update: As #steenslag points out in the comments, Comparable#between? can be used instead of creating a Range on which to call include?, and this eliminates the need to ensure that lower is less than or equal to upper:
def count_between arr, lower, upper
arr.count{|v| v.between?(lower, upper)}
end
to_a means convert to array
irb(main):001:0> (1..5).to_a
=> [1, 2, 3, 4, 5]
select method passes each element to the block and Returns a new array containing all elements of ary for which the given block returns a true value.. In your case it simply checks if the value is contained in the range array. range is an array not a range.
## if arr is [1,5] for eg:
irb(main):005:0> [1,5].select {|value| range.include?(value)}
=> [1, 5]
irb(main):006:0> [1,5].select {|value| range.include?(value)}.length
=> 2
so the elements of arr are contained in the |value| variable inside the block.
It's a block.
As the documentation says: select "Returns a new array containing all elements of ary for which the given block returns a true value."
So for each object in arr it is passed to the block in which you provide whatever code you want to that returns true or false, and the select statement uses this result to add the value to the the array that it returns. And after that, length is called on the array.
So you have an array, you filter the array to contain only the numbers that are in the range, and then you take the length - effectively counting the number of elements.
Related
I've tried different ways and this is probably the closest that I got to it. I am trying to write a method that takes in an array of strings and returns it containing the strings that are at least 5 characters long and end with "y".
I'm a beginner and this is my second problem I've come across with, and I've tried multiple if statements and using a while loop, however I could not get to it and now this is where I am at. Thank you!
def phrases(arr1, arr2)
arr1 = ["funny", "tidy", "fish", "foogiliously"]
arr2 = ["happily", "lovely", "hello", "multivitaminly"]
if (arr1.length > 5 && arr1.length == "y")
return arr1
elsif (arr2.length > 5 && arr2.length == "y")
return arr2
end
end
puts phrases(["funny", "tidy", "fish", "foogiliously"])
puts phrases(["happily", "lovely", "hello", "multivitaminly"])
If I'm understanding your question correctly, you want to return a subset of the passed in array matching your conditions (length ≥ 5 and last character = 'y'). In that case:
def phrases(words)
words.grep(/.{4}y\z/)
end
What that regex does:
.{4} means 4 of any character
y is the letter y
\z is the end of the string, so we don't match in the middle of a long word
The docs for Enumerable#select are here (an Array is an Enumerable).
Output:
> phrases(["funny", "tidy", "fish", "foogiliously"])
=> ["funny", "foogiliously"]
> phrases(["happily", "lovely", "hello", "multivitaminly"])
=> ["happily", "lovely", "multivitaminly"]
If you only want word characters, rather than any character, you'd use this regex instead: /\A.{4,}y\z/. In that case, \A means the start of the string, and \w{4,} means at least 4 word characters.
If, when given an array and inclusion criterion, one wishes to construct an array that contains those elements of the first array that satisfy the inclusion criterion, one generally uses the method Array#select or Array#reject, whichever is more more convenient.
Suppose arr is a variable that holds the given array and include_element? is a method that takes one argument, an element of arr, and returns true or false, depending on whether the inclusion criterion is satisified for that element. For example, say the array comprises the integers 1 through 6 and the inclusion criterion is that the number is even (2, 4 and 6). We could write:
arr = [1,2,3,4,5,6]
def include_element?(e)
e.even?
end
include_element?(2)
#=> true
include_element?(3)
#=> false
arr.select { |e| include_element?(e) }
#=> [2, 4, 6]
The method include_element? is so short we probably would substitute it out and just write:
arr.select { |e| e.even? }
Array#select passes each element of its receiver, arr, to select's block, assigns the block variable e to that value and evaluates the expression in the block (which could be many lines, of course). Here that expresssion is just e.even?, which returns true or false. (See Integer#even? and Integer#odd?.)
If that expression evaluates as a truthy value, the element e is to be included in the array that is returned; if it evaluates as a falsy value, e is not to be included. Falsy values (logical false) are nil and false; truthy values (logical true) are all other Ruby objects, which of course includes true.
Notice that we could instead write:
arr.reject { |e| e.odd? }
Sometimes the inclusion criterion consists of a compound expression. For example, suppose the inclusion criterion were to keep elements of arr that are both even numbers and are at least 4. We would write:
arr.select { |e| e.even? && e >= 4 }
#=> [4, 6]
With other criteria we might write:
arr.select { |e| e.even? || e >= 4 }
#=> [2, 4, 5, 6]
or
arr.select { |e| e < 2 || (e > 3 && e < 6) }
#=> [1, 4, 5]
&& (logical 'and') and || (logical 'or') are operators (search "operator expressions"). As explained at the link, most Ruby operators are actually methods, but these two are among a few that are not.
Your problem now reduces to the following:
arr.select { |str| <length of str is at least 5> && <last character of str is 'y'> }
You should be able to supply code for the <...> bits.
You are trying to write a function that should work on a single array at a time I think. Also, you are taking in an array, and retaining only those elements that satisfy your conditions: at least 5 characters long, and ends with y. This is a filtering operation. Read about the methods available for ruby's Array class here
def phrases(array)
...
filtered_array
end
Now the condition you are using is this arr1.length > 5 && arr1.length == "y".
The first half should check if the string length is greater than 5, not the array length itself. The second half is an indexing operation, and your code for that is incorrect. basically you are checking if the last character in the string is y.
Usually strings are indexed in this manner: string[index]. In your case you can use string[-1]=='y' or string[string.length - 1]=='y'. This because arrays and strings are zero indexed in ruby. The first element has index of 0, the second has an index of 1, and the last one, therefore, will have an index of length-1. If you use negative indexes then the array is indexed from the end, so string[-1] is a quick way to get to the last element.
Considering this, the function will take the following structure:
def phrases(array)
filtered_array = [] # an empty array
loop through the input array
for each element check for the condition element.length > 5 && element[-1]=='y'
if true: push the element into the filtered_array
once the loop is done, return the filtered array
end
Read about ruby arrays, the methods push, filter and select in the above linked documentation to get a better idea. I'd also recommend the codeacademy ruby tutorial.
Edit: Both halves of the condition are incorrect. I had overlooked a mistake in my earlier answer. arr1.length refers to the length of the array. You want to check the length of each string in the array. So in your for loop you should check the length of the loop variable, if that is greater than 5.
You may want to spend some time reading about the methods in the core library, especially String#end_with? and Enumerable#select. You could then write a method that'd contain something like this:
['abc', 'qwerty', 'asdfghjk', 'y'].select{|s| s.length >= 5}.select{|s| s.end_with? 'y'}
#=> ["qwerty"]
I am using this inject method to make a running total of values into an array. I am trying to figure out why I am getting an error.
def running_totals(myarray)
results = []
myarray.inject([]) do |sum,n|
results << sum + n
end
results
end
p running_totals([1,2,3,4,5])
I am getting the error
in `+': no implicit conversion of Fixnum into Array (TypeError)
When breaking this method down, isn't this the same as adding two integers and adding that into an array? I'm a bit confused here. Thanks for the help.
In the first iteration sum will be an array (as you specified an array as the default when calling inject([])) and you try to add a number to it. in the results << sum + n statement
Instead, set the initial value to 0, then add, then add the result to the array, then make sure you let sum get passed into the next iteration of inject.
def running_totals(myarray)
results = []
myarray.inject(0) do |sum,n| # First iteration sum will be 0.
sum += n # Add value to sum.
results << sum # Push to the result array.
sum # Make sure sum is passed to next iteration.
end
results
end
p running_totals([1,2,3,4,5]) #=> [1, 3, 6, 10, 15]
The result of results << sum + n is an array results and it's this that's replacing the sum value and so the next iteration you're trying to add a fixnum n into an array sum ... plus it doesn't help that you're initializing the value of sum to be an array.
Make sure that the last executed statement in your inject block is what you want the accumulated value to be.
def running_totals(myarray)
results = []
results << myarray.inject do |sum, n|
results << sum
sum + n
end
results
end
p running_totals([1,2,3,4,5])
=> [1, 3, 6, 10, 15]
Note that I moved the result of the inject into results array as well, so that the final value is also included, otherwise you'd only have the four values and would be missing the final (15) value.
The return value of the inject block is passed as the first argument the next time the block is called, so those have to match. In your code, you're passing an array as an intital value, and then returning an array; so far, so good. But inside the code block you treat that array parameter (sum) as a number, which won't work. Try this:
def running_totals(myarray)
myarray.inject([]) do |results,n|
results << n + (results.last || 0)
end
end
The [] passed as an argument to inject becomes the first value of results; the first array element (1 in your example) becomes the first value of n. Since results is empty, results.last is nil and the result of (results.last || 0) is 0, which we add to n to get 1, which we push onto results and then return that newly-modified array value from the block.
The second time into the block, results is the array we just returned from the first pass, [1], and n is 2. This time results.last is 1 instead of nil, so we add 1 to 2 to get 3 and push that onto the array, returning [1,3].
The third time into the block, results is [1,3], and n is 3, so it returns [1,3,6]. And so on.
According to ri, you have to return result of the computation from inject's block.
From: enum.c (C Method):
Owner: Enumerable
Visibility: public
Signature: inject(*arg1)
Number of lines: 31
Combines all elements of enum by applying a binary
operation, specified by a block or a symbol that names a
method or operator.
If you specify a block, then for each element in enum
the block is passed an accumulator value (memo) and the element.
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.
Examples:
# Sum some numbers
(5..10).reduce(:+) #=> 45
# Same using a block and inject
(5..10).inject {|sum, n| sum + n } #=> 45
# Multiply some numbers
(5..10).reduce(1, :*) #=> 151200
# Same using a block
(5..10).inject(1) {|product, n| product * n } #=> 151200
# find the longest word
longest = %w{ cat sheep bear }.inject do |memo,word|
memo.length > word.length ? memo : word
end
longest
So your sample would work if you return computation result for each iteration, something like this:
def running_totals(myarray)
results = []
myarray.inject do |sum,n|
results << sum + n
results.last # return computation result back to Array's inject
end
results
end
Hope it helps.
def random_select(array, n)
result = []
n.times do
# I do not fully understand how this line below works or why. Thank you
result.push array[rand(array.length)]
end
result
end
You are probably confused by this part:
n.times do
result.push(array[rand(array.length)])
end
n.times says it should loop n times.
result.push says to basically "push" or "put" something in the array. For example:
a = []
a.push(1)
p a #=> [1]
In array[rand(array.length)] , rand(array.length) will produce a random number as an index for the array. Why? rand(n) produces a number from 0 to n-1. rand(5) will produce either 0,1,2,3 or 4, for example.
Arrays use 0-based indexing, so if you have an array, say a = ['x', 'y', 'z'], to access 'x' you do a[0], to access y you do a[1] and so on. If you want to access a random element from a, you do a[rand(array.length)], because a.length in this case is 3, and rand(3) will produce a number that is either 0, 1 or 2. 0 is the smallest index and 2 is the largest index of our example array.
So suppose we call this method:
random_select([6,3,1,4], 2)
Try to see this code from the inside out. When the code reaches this part:
result.push(array[rand(array.length)])
it will first execute array.length which will produce 4. It will then execute rand(array.length) or rand(4) which will get a number between 0 and 3. Then, it will execute array[rand(array.length)] or array(some_random_number_between_0_and_3) which will get you a random element from the array. Finally, result.push(all_of_that_code_inside_that_got_us_a_random_array_element) will put the random element from the array in the method (in our example, it will be either 6, 3, 1 or 4) in the results array. Then it will repeat this same process once again (remember, we told it to go 2 times through the iteration).
The code can be rewritten to be much simpler, using the block-form Array constructor:
def random_select(array, n)
Array.new(n) {array.sample}
end
This creates a new array of size n and fills it with random samples from the array.
Note that the above solution, like your sample code, selects from the entire array each time which allows duplicate selections. If you don't want any duplicate selections, it's even simpler, since it is the default behavior of Array#sample:
def random_select(array, n)
array.sample(n)
end
Here's what I thought:
def appendUnique(a,x)
for i in 0 .. a.size-1 do
if a[i]=x then
a==a
else
a=a+x
end
p(a)
end
end
appendUnique([-1,5,3],4)
Compare each member of a with x, if a equals x, return a, else return a+x. Why doesn't this work? It just replaces all array members with 4s...
I want this: result [-1, 5, 3, 4] from the above since 4 isn't in the array and [-1, 5, 3] from appendUnique([-1,5,3],5).
There are several issues with your code:
in Ruby we usually use each instead of for to iterate collections
a[i] = x is an assignment, you want a[i] == x
a == a just returns true
a + x concatenates two arrays, but x is not an array
I would simply use Array#include? to check if the item is present:
def appendUnique(array, item)
if array.include? item
array
else
array + [item]
end
end
If you want an array with unique elements you can use Set class
It just replaces all array members with 4s...
a[i]=x is an assignment rather than comparison. Running this in a loop, as you do, would set every element of a to x (which is 4).
The rest of the code needs quite a lot of work too. For example: you should only be appending to a after you've run the loop and have established that x isn't in the array.
I have two arrays: fasta_ids & frags_by_density. Both contain the same set of ≈1300 strings.
fasta_ids is ordered numerically e.g. ['frag1', 'frag2', 'frag3'...]
frags_by_density contains the same strings ordered differently e.g. ['frag14', 'frag1000'...]
The way in which frag_by_density is ordered is irrelevant to the question (but for any bioinformaticians, the 'frags' are contigs ordered by snp density).
What I want to do is find the indexes in the frag_by_density array, that contain each of the strings in fasta_ids. I want to end up with a new array of those positions (indexes), which will be in the same order as the fasta_ids array.
For example, if the order of the 'frag' strings was identical in both the fasta_ids and frags_by_density arrays, the output array would be: [0, 1, 2, 3...].
In this example, the value at index 2 of the output array (2), corresponds to the value at index 2 of fasta_ids ('frag3') - so I can deduce from this that the 'frag3' string is at index 2 in frags_by_density.
Below is the code I have come up with, at the moment it gets stuck in what I think is an infinite loop. I have annotated what each part should do:
x = 0 #the value of x will represent the position (index) in the density array
position_each_frag_id_in_d = [] #want to get positions of the values in frag_ids in frags_by_density
iteration = []
fasta_ids.each do |i|
if frags_by_density[x] == i
position_each_frag_id_in_d << x #if the value at position x matches the value at i, add it to the new array
iteration << i
else
until frags_by_density[x] == i #otherwise increment x until they do match, and add the position
x +=1
end
position_each_frag_id_in_d << x
iteration << i
end
x = iteration.length # x should be incremented, however I cannot simply do: x += 1, as x may have been incremented by the until loop
end
puts position_each_frag_id_in_d
This was quite a complex question to put into words. Hopefully there is a much easier solution, or at least someone can modify what I have started.
Update: renamed the array fasta_ids, as it is in the code (sorry if any confusion)
fasta_id = frag_id
Non optimized version. array.index(x) returns index of x in array or nil if not found. compact then removes nil elements from the array.
position_of_frag_id_in_d = frag_ids.map{|x| frag_by_density.index(x)}.compact