Understanding grep within Enumerable module - ruby-on-rails

Question

I greatly appreciate any assistance on my first stack-overflow post!
I was generally confused why I am receiving and empty array every time I try to run my code.
I am creating a method that will filter for a gender of "M" and return its elements.
I am sure there are a multitude of ways to successfully run this code but I was interested in using grep and its functionality. It being a shortcut and all I thought it would be good to learn. Thank you once again.
students = [
{name: 'John', grade: 8, gender: 'M'},
{name: 'Sarah', grade: 12, gender: 'F'},
{name: 'Bob', grade: 16, gender: 'M'},
{name: 'Johnny', grade: 2, gender: 'M'},
{name: 'Ethan', grade: 4, gender: 'M'},
{name: 'Paula', grade: 8, gender: 'F'},
{name: 'Donald', grade: 5, gender: 'M'},
{name: 'Jennifer', grade: 13, gender: 'F'},
{name: 'Courtney', grade: 15, gender: 'F'},
{name: 'Jane', grade: 9, gender: 'F'}
]
def is_male(gender)
gender.grep(/M/) { |gend| gend[:gender] }
end
p is_male(students)

Answer 1

From the docs of Enumerable#grep:
grep(pattern) → array
grep(pattern) { |obj| block } → array
Returns an array of every element in enum for which Pattern === element. If the optional block is supplied, each matching element is passed to it, and the block’s result is stored in the output array.
The important part is this method returns elements that evaluate Pattern === element to true. But /M/ === {name: 'John', grade: 8, gender: 'M'} does not return true, same for all other elements in your array.
Therefore your result set is empty in the first place.
The block – { |gend| gend[:gender] } in your example – is only evaluated when and after there was a pattern match. The block changes the return value of the whole call but not how the pattern matching is done.
Please note the docs for Rexgxp#=== in this context too.

Answer 2

I am creating a method that will filter for a gender of "M" and return its elements.
With the above requirements in mind, your naming seems quite misleading:
def is_male(gender)
# ...
end
The above looks like a method that takes a gender and checks whether it is male. I'd expect something like this:
is_male('M') #=> true
is_male('F') #=> false
And is_male(students) isn't clear either – does it check whether there is a male student in the given array? Or if all of the students are male? Either way, it doesn't sound like filtering.
Let's start by renaming the method and its argument to match your requirements more closely:
def male_students(students)
# ....
end
If you want to use grep you have to provide a pattern which is an object responding to ===. A regular expression won't work, because it operates on strings and our students are hashes (more on that below). You could use a Proc instead which also responds to ===:
def male_students(students)
is_male = ->(student) { student[:gender] == 'M' }
students.grep(is_male)
end
But it's easier to use select:
def male_students(students)
students.select { |student| student[:gender] == 'M' }
end
Another option is to use a custom class for your students because right now, your students are merely hashes:
class Student
attr_accessor :name, :grade, :gender
def initialize(name:, grade:, gender:)
#name = name
#grade = grade
#gender = gender
end
def male?
gender == 'M'
end
def female?
gender == 'F'
end
end
Now, change your array accordingly:
students = [
Student.new(name: 'John', grade: 8, gender: 'M'),
Student.new(name: 'Sarah', grade: 12, gender: 'F'),
Student.new(name: 'Bob', grade: 16, gender: 'M'),
Student.new(name: 'Johnny', grade: 2, gender: 'M'),
Student.new(name: 'Ethan', grade: 4, gender: 'M'),
Student.new(name: 'Paula', grade: 8, gender: 'F'),
Student.new(name: 'Donald', grade: 5, gender: 'M'),
Student.new(name: 'Jennifer', grade: 13, gender: 'F'),
Student.new(name: 'Courtney', grade: 15, gender: 'F'),
Student.new(name: 'Jane', grade: 9, gender: 'F'),
]
And you can use this pretty minimal syntax:
students.select(&:male?)
#=>
# [
# #<Student:0x00007fb18d826ce8 #name="John", #grade=8, #gender="M">,
# #<Student:0x00007fb18d826b58 #name="Bob", #grade=16, #gender="M">,
# #<Student:0x00007fb18d826a90 #name="Johnny", #grade=2, #gender="M">,
# #<Student:0x00007fb18d8269c8 #name="Ethan", #grade=4, #gender="M">,
# #<Student:0x00007fb18d826838 #name="Donald", #grade=5, #gender="M">
# ]
Or any of the other useful methods in Array / Enumerable:
students.any?(&:male?) #=> true
students.all?(&:male?) #=> false
students.count(&:male?) #=> 5