什么时候使用<％ - ％>而不是<%%>

Question

I've noticed that in some lines of rails views, this is used:

<% # Code... -%> 

instead of:

<% # Code... %>

What is the difference?

Answer 1

    <ul>
    <% @posts.each do |post| -%> 
      <li><%=post.title%></li>
    <% end -%>
    </ul>

There will be no new lines in between the <ul> and first <li> and the last closing </li> and </ul> . If the - was omitted, there would.

Answer 2

The different options for evaluating code in ERB are as follows (they can be accessed in Textmate using Ctrl-Shift-. ):

• <% %> Just evaluate the contents.
• <%= %> Evaluate the contents and puts the result.
• <%= -%> Evaluate the contents and prints the result.
• <%# %> The contents is treated as a comment and not outputted.

Notice the difference between puts and print . Puts always adds a new line at the end of the string whereas print doesnt.

Basically, the -%> says don't output a new line at the end.

Answer 3

Consider this

<div>
    <% if @some_var == some_value %>
    <p>Some message</p>
    <% end %>
</div>

The code above yields to the HTML below if the @some_var is some_value

<div>

    <p>Some message</p>

</div>

If you've put - in the closing tag, then the ERB interpreter would remove the new lines for those with code tag including - and result in the following

<div>       
    <p>Some message</p>        
</div>

This is useful if you need to have a good looking code for HTML. Sometimes you'll find it useful when working sideby side with a designer

Hope this helps.

Answer 4

A little late, but I think it's worth pointing out that you can also do this:

<%- @posts.each do |post| -%>
  <li><%= post.title %></li>
<%- end %>

This strips away any whitespace in front.