Considering this Ruby code:
format_string = "%{greeting}, %{name}"
puts format_string %{greeting: "Hi", name: "John"} # <-- returns "Hi, John"
puts format_string # <-- returns "%{greeting}, %{name}"
puts format_string %{greeting: "Hi"} # <-- returns key{name} not found (KeyError)"
The following code produces an error because it is missing a key.
puts format_string %{greeting: "Hi"} # returns "key{name} not found (KeyError)"
While the code below returns the exact value of the format_string variable, even though it missing both keys. I would expect the value to be equal to nil, or result in an error like the code above.
puts format_string # returns "%{greeting: "Hi", name: "John"}"
I'm looking for an explanation of how and why the exact value is returned, when no keys or variables are set to the string. I have read through the ruby docs and multiple web pages relating to ruby formatting, yet I have not been able to find an answer.
Your:
puts format_string
prints format_string because that's what puts does. The formatting (and complaining about missing keys) that you're seeing is done by the String#% method; puts doesn't interpret the %{...} things in the string, the % operator does that. Perhaps breaking it into two steps (and being more consistent with your whitespace) will clarify things for you:
formatted_string = format_string % {greeting: "Hi", name: "John"}
# -------------------------------^ Just an operator like any other, %{ has no special meaning outside the string you use the % operator on.
puts formatted_string
The % in format_string % hash is just an operator like any other, %{ has no special meaning outside the string you use the % operator on so removing the space between the operator (%) and the opening brace ({) of the hash operand is misleading.