ruby not understanding arguments provided to a method

Question

I come across this piece of ruby code :

def link_to(link_text, url, mode=:path_only)
# You should add "!!" at the beginning if you're directing at the Sinatra url
    if(url_for(url,mode)[0,2] == "!!")
      trimmed_url = url_for(url,mode)[2..-1]
      "<a href=#{trimmed_url}> #{link_text}</a>"
    else
      "<a href=#{url_for(url,mode)}> #{link_text}</a>"
    end  
end  

def url_for url_fragment, mode=:full_url
  case mode
    when :path_only
#cut for brievity. The rest of the function gets rack params and renders full url (or not)

I have no clue what this line of code does : (url_for(url,mode)[0,2] == "!!")

Tip: Method calls are like f(x) while constructs like if are expressed as if x or if (x) if parentheses are used. The space is a small detail, but it makes it read differently. — tadman
– tadman, Commented Jan 21, 2021 at 18:00
Oh, it does work, it's just that at a glance this looks like a method call to a method named if, which it is not. Tools like Rubocop can help "lint" your code and find issues like this. — tadman
– tadman, Commented Jan 21, 2021 at 18:02

tadman · Accepted Answer · 2021-01-21 18:10:39Z

3

This code:

url_for(url,mode)[0,2] == "!!"

Checks that the first (offset 0) two characters (,2) are equivalent to "!!". This is now something you can express as:

url_for(url,mode).start_with?("!!")

Which might make it easier to understand.

The String#[] method has two forms relevant to understanding this:

"hello"[0] # Character index
# => "h"
"hello"[0,1] # Equivalent to above
# => "h"
"hello"[0,2] # Su
# => "he"

edited Jan 21, 2021 at 18:10

answered Jan 21, 2021 at 18:01

tadman

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3 Comments

tadman Over a year ago

The first one is readable if you're used to the notation, but the start_with? form is abundantly clear.

3limin4t0r Over a year ago

"two forms" might be an understatement. There are also the range, string and regex (optionally with capture group) variants.

tadman Over a year ago

@3limin4t0r True, added some clarification here as to the ones being focused on.

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