while behaviour

do_not_work.rb

require "securerandom"

def get_secure_random
  puts "Func called"
  SecureRandom.hex(16)
end

res = get_secure_random while res[0] != "a"


----- Result:

cuca.rb:11:in `<main>': undefined method `[]' for nil:NilClass (NoMethodError)


So, in the while evaluation here the `res` variable is nil, and the `get_secure_random` function is never called. 

works.rb

require "securerandom"

def get_secure_random
  puts "Func called"
  SecureRandom.hex(16)
end

begin
  res = get_secure_random
end while res[0] != "a"


----- Result:

Func called # Repeat N times
"ad34b41ab0af4b2cdff238b3536fa5ef"