What does “string literal in condition” mean?

问题:

Whenever I try to run the program, an error pops up saying "string literal in condition (on line 10)". What am I doing wrong?

puts "Welcome to the best calculator there is. Would you like to (a) calculate the area of a geometric shape or (b) calculate the equation of a parabola? Please enter an 'a' or a 'b' to get started."
response = gets.chomp

if response == "a" or "A"

       puts "ok."

elsif response == "b" or "B"

       puts "awesome."

else

       puts "I'm sorry. I did not get that. Please try again."

end

回答1:

You have to specify the full condition on both sides of the or.

if response == "a" or response == "A"

The two sides of the or are not connected; Ruby makes no assumptions about what's on the right based on what's on the left. If the right side is the bare string "A", well, anything other than false or nil is considered "true", so the whole expression evaluates as "true" all the time. But Ruby notices that it's a string and not actually a boolean value, suspects you might not have specified what you meant to, and so issues the warning in the question.

You can also use a case expression to make it simpler to do multiple tests against a single value; if you supply a list of multiple possibilities in a single when, they are effectively ored together:

case response
  when "a","A"
    puts "ok"
  when "b","B"
    puts "awesome."
  else
    puts "I'm sorry. I did not get that.  Please try again."
end

For the specific situation of ignoring alphabetic case, you could also just convert to either upper or lower before testing:

case response.upcase 
  when "A"
    puts "ok"
  when "B"
    puts "awesome."
  else
    puts "I'm sorry, I did not get that.  Please try again."
 end

回答2:

It is not wrong syntactically; it is wrong in the sense that it is useless. The expression response == "a" or "A" is interpreted as (response == "a") or "A", which is always truthful because of "A", so it is useless to put that in a condition.

回答3:

if response == "a" or "A" is equivalent to if (response == "a") or "A". And "A" is a string literal, that's why the ruby interpreter complains.

回答4:

That's not an error, it's a warning.

You have the condition

response == "a" or "A"

Well, response == "a" is either true or false. If it is true, the condition reduces to

true or "A"

which is

true

If it is false, the condition reduces to

false or "A"

which is

"A"

which is truthy, since everything except false and nil is truthy.

So, regardless of the contents of response, the condition will always be true.

And that's what the warning is warning you about: string literals are always truthy, it just doesn't make sense to use them in a condition.