I'm looking for a better way to merge variables into a string, in Ruby.
For example if the string is something like:
"The animal action the second_animal"
And I have variables for animal, action and second_animal, what is the prefered way to put those variables in to the string?
The idiomatic way is to write something like this:
"The #{animal} #{action} the #{second_animal}"
Note the double quotes (") surrounding the string: this is the trigger for Ruby to use its built-in placeholder substitution. You cannot replace them with single quotes (') or the string will be kept as is.
You can use sprintf-like formatting to inject values into the string. For that the string must include placeholders. Put your arguments into an array and use on of these ways: (For more info look at the documentation for Kernel::sprintf.)
fmt = 'The %s %s the %s'
res = fmt % [animal, action, other_animal] # using %-operator
res = sprintf(fmt, animal, action, other_animal) # call Kernel.sprintf
You can even explicitly specify the argument number and shuffle them around:
'The %3$s %2$s the %1$s' % ['cat', 'eats', 'mouse']
Or specify the argument using hash keys:
'The %{animal} %{action} the %{second_animal}' %
{ :animal => 'cat', :action=> 'eats', :second_animal => 'mouse'}
Note that you must provide a value for all arguments to the % operator. For instance, you cannot avoid defining animal.
I would use the #{} constructor, as stated by the other answers.
I also want to point out there is a real subtlety here to watch out for here:
2.0.0p247 :001 > first_name = 'jim'
=> "jim"
2.0.0p247 :002 > second_name = 'bob'
=> "bob"
2.0.0p247 :003 > full_name = '#{first_name} #{second_name}'
=> "\#{first_name} \#{second_name}" # not what we expected, expected "jim bob"
2.0.0p247 :004 > full_name = "#{first_name} #{second_name}"
=> "jim bob" #correct, what we expected
While strings can be created with single quotes (as demonstrated by the first_name and last_name variables, the #{} constructor can only be used in strings with double quotes.
["The", animal, action, "the", second_animal].join(" ")
is another way to do it.
This is called string interpolation, and you do it like this:
"The #{animal} #{action} the #{second_animal}"
Important: it will only work when string is inside double quotes (" ").
Example of code that will not work as you expect:
'The #{animal} #{action} the #{second_animal}'
The standard ERB templating system may work for your scenario.
def merge_into_string(animal, second_animal, action)
template = 'The <%=animal%> <%=action%> the <%=second_animal%>'
ERB.new(template).result(binding)
end
merge_into_string('tiger', 'deer', 'eats')
=> "The tiger eats the deer"
merge_into_string('bird', 'worm', 'finds')
=> "The bird finds the worm"
You can use it with your local variables, like this:
@animal = "Dog"
@action = "licks"
@second_animal = "Bird"
"The #{@animal} #{@action} the #{@second_animal}"
the output would be: "The Dog licks the Bird"
