facing issues about array in rails - ruby-on-rails

Question

I am implementing a program in rails where there is a form and after submitting the form it will check if there is any record with duplicate value for a specific field in database. My database table is students. So my corresponding model name is Student. I am writing this code (what I have just discussed) in my controller.
But I am facing the following error. I am using some arrays for internal operations. When I wrote that particular function in ruby only(not rails) then it was working fine. Moreover I am also facing error due to the use of "length".
My error is:
NoMethodError in StudentsController#create
undefined method `[]' for nil:NilClass
My controller code is:
class StudentsController < ApplicationController
def new
#student=Student.new
#students=Student.all
end
def create
#student=Student.new(u_params)
ret_val=string_check
if ret_val==1
#student.save
redirect_to new_student_path , :notice => "Inserted!!!"
else
redirect_to new_student_path , :notice => "Match,Not inserted!!!"
end
end
def u_params
params.require(:student).permit(:id ,:firstname, :lastname)
end
def u_params_second
params.require(:student).permit(:firstname)
end
def string_check
count =0;
#temp1=:firstname
temp1=params[:firstname]
supplied_val=temp1
puts "Entered in string_check method"
for i in 46..100
temp2=Student.find_by(id:i)
table_val=temp2.firstname
size1=supplied_val.to_s.length
size2=table_val.to_s.length
arr1=Array.new
arr2=Array.new
# arr1[i] ||= []
# arr2[i] ||= []
for i in 0..size1
arr1.push(supplied_val[i])
end
for i in 0..size2
arr2.push(table_val[i])
end
for i in 0..size1
if arr1[i]=="#" || arr1[i]=="#" || arr1[i]=="{" || arr1[i]=="}" || arr1[i]=="(" || arr1[i]==")" || arr1[i]=="[" || arr1[i]=="]" || arr1[i]=="." || arr1[i]==";" || arr1[i]=="," || arr1[i]=="%" || arr1[i]=="&" || arr1[i]=="*" || arr1[i]=="!" || arr1[i]=="?" || arr1[i]=="$" || arr1[i]=="^" || arr1[i]==":" || arr1[i]=="-" || arr1[i]=="/"
count=count+1
# puts count
arr1[i]=""
end
end
# puts arr1
puts arr1.join
final1=arr1.join
for i in 0..size2
if arr2[i]=="#" || arr2[i]=="#" || arr2[i]=="{" || arr2[i]=="}" || arr2[i]=="(" || arr2[i]==")" || arr2[i]=="[" || arr2[i]=="]" || arr2[i]=="." || arr2[i]==";" || arr2[i]=="," || arr2[i]=="%" || arr2[i]=="&" || arr2[i]=="*" || arr2[i]=="!" || arr2[i]=="?" || arr2[i]=="$" || arr2[i]=="^" || arr2[i]==":" || arr2[i]=="-" || arr2[i]=="/"
count=count+1
# puts count
arr2[i]=""
end
end
# puts arr2
puts arr2.join
final2=arr2.join
if final1==final2
flag=0
else
flag=1
end
return flag
end
end
end
The routes.rb file is:
Rails.application.routes.draw do
resources :students
end

Answer 1

My error is: NoMethodError in StudentsController#create
undefined method `[]' for nil:NilClass
It simply means that you are trying to access something as an array that is actually a nil object, and not an array.
To get rid of this error, you can a technique called short-circuit in Ruby.
Let's say your following piece of code is producing the said error:
arr1[i]
You can use an if condition like this:
if arr1
arr1[i]
end
Or use short-circuit technique like this:
arr1 && arr1[i]

Answer 2

If you sure that the relevant code snippet was working for ruby and it's not for rails, the problem is most likely due to variable i used at inner and outer loops both. In any case, this needs to be fixed first or else it will result in unexpected behaviour only.
Outer Loops:
for i in 46..100
Inner Loops:
for i in 0..size1
for i in 0..size2
...
Keep i for outer loop and change the inner loop iterator to j
Hope it Helps : )

Answer 3

Adding to the answers of #harish and #arslan, there may be a case where, temp2=Student.find_by(id:i) may fail because there may not be a student with that id.
So, temp2 may return nil at that time.
for i in 0..size2
arr2.push(table_val[i]) // this may get failed
end
Then arr2.push will not work because table_val[i] is nil, so there are chances of undefined method [] for nil class.