String interpolation with hash in ruby

Question

My aim is to replace keys in a string with values in hash. I am doing it like this:

"hello %{name}, today is %{day}" % {name: "Tim", day: "Monday"}

If a key in the string in missing in the hash:

"hello %{name}, today is %{day}" % {name: "Tim", city: "Lahore"}

then it will throw an error.

KeyError: key{day} not found

Expected result should be:

"hello Tim, today is %{day}" or "hello Tim, today is "

Can someone guide me in a direction to replace only the matching keys without throwing any errors?

What's your expected result in the second case, i.e. if a key is missing? — Stefan
– Stefan, Commented Aug 23, 2017 at 9:38
Thanks for looking into it. Expected response can be "hello Tim, today is %{day}" or "hello Tim, today is ". I think second one will be preferred — BIlal Khan
– BIlal Khan, Commented Aug 23, 2017 at 9:41

Stefan · Accepted Answer · 2017-12-15 21:11:33Z

26

Starting with Ruby 2.3, % honors default values set via default=:

hash = {name: 'Tim', city: 'Lahore'}
hash.default = ''

'hello %{name}, today is %{day}' % hash
#=> "hello Tim, today is "

or dynamic defaults set via default_proc=:

hash = {name: 'Tim', city: 'Lahore'}
hash.default_proc = proc { |h, k| "%{#{k}}" }

'hello %{name}, today is %{day}' % hash
#=> "hello Tim, today is %{day}"

Note that only the missing key i.e. :day is passed to the proc. It is therefore unaware of whether you use %{day} or %<day>s in your format string which might result in a different output:

'hello %{name}, today is %<day>s' % hash
#=> "hello Tim, today is %{day}"

edited Dec 15, 2017 at 21:11

answered Aug 23, 2017 at 9:40

Stefan

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4 Comments

abhishek Over a year ago

I am using ruby 2.1.3 and this is not working. Is there any other way to do the same?

Stefan Over a year ago

@Abhishek you could use gsub to replace %{...} with the corresponding value.

Michael K Madison Jan 22 at 23:12

This is cool but I've been noticing in Ruby 3.3.6 in the rails console it only seems to work for string literals, like it won't work on a string stored in a variable and then string_var % some_hash

Stefan Jan 23 at 10:03

@MichaelKMadison must be something else, String#% is just a method. It doesn't matter whether the receiver is given as a literal or a local variable.

Jérôme-Victor Toulouse · Accepted Answer · 2017-08-23 09:52:01Z

4

you can set a default hash value :

h = {name: "Tim", city: "Lahore"}
h.default = "No key"
p "hello %{name}, today is %{day}" % h #=>"hello Tim, today is No key"

answered Aug 23, 2017 at 9:52

Jérôme-Victor Toulouse

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Comments

Faisal Raza · Accepted Answer · 2017-09-15 12:46:43Z

2

I have the hash keys with spaces and it worked after converting the keys to symbols.

Hash with String keys (It returned an error on interpolation):

hash = {"First Name" => "Allama", "Last Name" => "Iqbal"}

Converting the hash keys to Symbols worked for me:

hash = {:"First Name" => "Allama", :"Last Name" => "Iqbal"}
hash.default = ''

'The %{First Name} %{Last Name} was a great poet.' % hash

// The Allama Iqbal was a great poet.

edited Sep 15, 2017 at 12:46

answered Aug 30, 2017 at 19:19

Faisal Raza

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Comments

genkilabs · Accepted Answer · 2023-10-05 16:11:21Z

I know this is old, but I ended up making a utility for my needs. Here is my Gist https://gist.github.com/Genkilabs/71b479e1b1300a581a85e047db1cbb43

  #@param String with %{key} interpolation fields
  #@param Hash or Rails Model
  #@returns String original with any keys replaced if they are in the hash, others ignored
  def self.interpolate str, obj
    #For the hash, we need to re-key the hash to be sure, and add default to skip missing interpolations
    safe_hash = (defined?(obj.attributes) ? obj.attributes : obj.to_h).symbolize_keys
    safe_hash.default_proc = proc{|h, k| "%{#{k}}" }
    #On the string side, safeguard any place ther is a symbol inside an interpolated field
    str.gsub('%{:', '%{') % safe_hash
  end

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