creating and passing dynamic local variables to partials - ruby-on-rails

Question

I have a form in a partial that renders three times (for the same three fields). However, the placeholder value for each field should ideally be different. Therefore, I'm trying to pass in a local variable that will change for each rendering of the partial.
In the controller, I created an array like this
#placeholder = %w[ firstplaceholder, secondplaceholder, thirdplaceholder]
I then tried to loop through the array while rendering the partial
<%= f.fields_for :answers do |builder| %>
<% #placeholder.each do |k, arrayindex| %>
arrayindex = #placeholder.index(k)
<%= render :partial => 'answer_fields', :locals => { :f => builder, :myplaceholder => #placeholder[arrayindex] } %>
<% end %>
<% end %>
Passing the arrayindex to the #placeholder generates this error
no implicit conversion from nil to integer
So I'm guessing that arrayindex variable is 'nil'. Why?
Inside the partial, in situation where there are error messages and form fields are repopulated, there won't be placeholders (the #placeholder instance variable only gets created in the controller if there's no errors), so I first check if the #placeholder instance variable is present, and try to set the placeholder to the index created in the loop, however, because of the error passing the arrayindex in, it's not working. There may be other problems :)
<% if #placeholder ? myplaceholder = #placeholder[arrayindex] : myplaceholder = "answer choice" %>
<%= f.text_field :content, :class => 'span3', :rows => 1, :placeholder => myplaceholder %>
<%= f.label :correctanswer, "Correct?" %>
<%= f.check_box :correctanswer, :class => 'span1' %>
<% end %>
Update
In the controller, I'm building three answer fields for the question model. The partial therefore is getting rendered three times automatically because of this. If I loop through the #placeholder array wrapped around the partial, it's rendering the partial that many more times 3 * 3.
3.times {#question.answers.build}

Answer 1

First, note that you don't need the comma separators here:
#placeholder = %w[ firstplaceholder secondplaceholder thirdplaceholder]
And you don't need to use the index into the array (if I understand what you're trying to do). So it should be as simple as:
<%= f.fields_for :answers do |builder| %>
<% #placeholder.each do |k| %>
<%= render :partial => 'answer_fields', :locals => { :f => builder, :myplaceholder => k } %>
<% end %>
<% end %>

Answer 2

In the controller, along with the array of placeholder values, I did
#i = 0
The forms_for partial was being rendered 4 times because I created 4 answers.
4.times { #question.answers.build}
Therefore, after each rendering of the partial, I incremented the #i instance variable and passed the incremented value into the #placeholder array #placeholder[#i] and inside the partial used the appropriate element of the array
<%= f.fields_for :kanswers do |builder| %>
<%= render :partial => 'kanswer_fields', :locals => { :f => builder, :k => #placeholder[#i]} %>
<% #i += 1 %>
<% end %>