What does “wrong number of arguments (1 for 0)” mean in Ruby?

When you define a function, you also define what info (arguments) that function needs to work. If it is designed to work without any additional info, and you pass it some, you are going to get that error.

Example:
Takes no arguments:

def dog
end

Takes arguments:

def cat(name)
end

When you call these, you need to call them with the arguments you defined.

dog                  #works fine
cat("Fluffy")        #works fine


dog("Fido")          #Returns ArgumentError (1 for 0)
cat                  #Returns ArgumentError (0 for 1)

Check out the Ruby Koans to learn all this.