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Hackerrank Lazy Evaluation Solution

Hackerrank Lazy Evaluation Solution

Beeze Aal
Beeze Aal

Lazy evaluation is an evaluation strategy that delays the assessment of an expression until its value is needed.

Ruby  introduced a lazy enumeration feature. Lazy evaluation increases performance by avoiding needless calculations, and it has the ability to create potentially infinite data structures.

Example:

power_array = -> (power, array_size) do 
    1.upto(Float::INFINITY).lazy.map { |x| x**power }.first(array_size) 
end

puts power_array.(2 , 4)    #[1, 4, 9, 16]
puts power_array.(2 , 10)   #[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
puts power_array.(3, 5)     #[1, 8, 27, 64, 125]

In this example, lazy avoids needless calculations to compute power_array.
If we remove lazy from the above code, then our code would try to compute all  ranging from  to Float::INFINITY.
To avoid timeouts and memory allocation exceptions, we use lazy. Now, our code will only compute up to first(array_size).


Task

Your task is to print an array of the first  palindromic prime numbers.
For example, the first  palindromic prime numbers are .

Input Format.

A single line of input containing the integer .

Constraints

You are not given how big  is.

Output Format.

Print an array of the first  palindromic primes.

Sample Input.

5

Sample Output.

[2, 3, 5, 7, 11]

Solution in ruby

Approach 1.

# Enter your code here. Read input from STDIN. Print output to STDOUT
require 'prime'
n = gets.to_i
p Prime.each.lazy.select {|i| i.to_s == i.to_s.reverse}.first(n)

Approach 2.


# Enter your code here. Read input from STDIN. Print output to STDOUT
require 'prime'

def yield_var(var_num)
    sum = (2..Float::INFINITY).lazy.select{ 
        |x| x.to_s.reverse.to_i == x && Prime.prime?(x) }.first(var_num.to_i)
    return sum
end

num = gets.chomp
print yield_var(num)

Approach 3.

# Enter your code here. Read input from STDIN. Print output to STDOUT
def prime?(num)
  num >= 2 && (2..Math.sqrt(num)).none? { |div| (num % div).zero? } ? true : false
end

def palindrome?(num)
  num.to_s == num.to_s.reverse
end

p 2.upto(Float::INFINITY).lazy.select { |num| palindrome?(num) && prime?(num) }
   .first(gets.to_i)