w1d2 revisions Flashcards
When we want to select a certain number of items from an array, what method should we use?
Array#select
What does #dup do? When is it useful?
It is an inherited method from the Object class, and returns a copy of the object. It is useful when we do not want to modify the original object and/or ‘self’
What is the shorthand for reassigning values to multiple objects? How can we use it to swap the values of two variables?
var1, var2 = new_var1, new_var2
-‘var1’ now has the value of new_var1, var2 now has the value of new_var1
We can swap the values of variables like so:
var1, var2 = var2, var1
What would be the code to quickly get an array of lines from a file?
lines = File.readlines(‘filename’).map(&:chomp)
What is the following syntax short for?
arr.map(&:upcase)
arr.map do |elem|
elem.upcase
end
T/F: Enumerator#each returns the original object that has been modified by the code block.
T
As soon as we find ourselves adding a ‘!’ to a condition in an ‘if’ or ‘while’, what should we use instead?
‘unless’ or ‘until’, respectively. They are much cleaner.
What would be the code to quickly retrieve the first element of each element of a multidimensional array?
starts = arr.map(&:first)
USE MAP TO RETURN A NEW ARRAY AND EACH TO EITHER RETURN THE ORIGINAL OBJECT OR THE ORIGINAL OBJECT AFTER IT HAS BEEN MUTATED
What is a shorthand to sample a value from a range?
rand(start..end)
Explain the ‘’ operator
It returns -1 if the left-hand operand is less than the right hand operator, 1 if the opposite, and 0 if they are equal.
What does File::extname return?
The file extension
How can File::basename return ONLY the filename and not the extension?
File.basename(‘filepath’, “.*”)
When should we use Enumerator#any?
When we could use #each, but we only want to check if something occurs at least once:
def has_conflict?(new_course)
self.courses.any? do |enrolled_course|
new_course.conflicts_with?(enrolled_course)
end
end
How can we raise a generic error?
raise “error message” if condition
How can we raise a specific class of error?
raise ErrorClass.new(“error message”) if condition
What is a case statement and when should we use it?
Basically a switch statement; declare something and check if it evaluates to each of the when statements:
case (statement) when value actions when another_value actions else default_actions end
What is the method to round numbers correctly (i.e. not always down)
Fixnum#fdiv
When should we use the ‘=~’ operator?
When we are checking to see if a string contains a certain regex.
Explain the following code:
Array.new(3) {Array.new(3)}
Creates a multidimensional array of 3 by 3
T/F: Putting ‘.join’ after the ‘end’ keyword will not do anything.
F; it serves to concatenate outputs.
What is the difference between:
a, b = [1, 2, 3]
a, *b = [1, 2, 3]
In the first example, a = 1 and b = 2
In the second example, a = 1 and b = [2, 3]; the ‘*’ operator here means to catch all remaining elements left over after assignment
Why bother with passing |x, y| in the following example:
[arr1, arr2].map do |arrs|
arrs.map{|x, y| newarr[x][y]
end
B/C there are two elements in the array passed to the first ‘map’, we need to account for the elements within the second map.
Assuming rows, cols and diagonals are multidimensional arrays, what does the following code do?
def winner
(rows + cols + diagonals).each do |triple|
return :x if triple == [:x, :x, :x]
return :o if triple == [:o, :o, :o]
end
It puts the arrays together into one new array with 3 sub arrays. The each statement grabs the nth element of each sub array and passes them as a new array to ‘triple’
