Why is character in string not mutated?

def test_problem(str)
str[3].upcase! # str[3] = str[3].upcase! works
str
end

p test_problem("hello")


My question is why String.upcase! which is mutating method doesn't mutate string in a case above but you need to reassign that character in string?

2 Answers
2

String# returns a new string, as is documented.

String#

a = "foo"
a.object_id
# => 70217975553640

a[0].object_id
# => 70217957574840


A string is not composed of character objects, it is a single object (at least on a superficial level, I'm not sure of the C internals). So there is no way to extract a character and still have it belong to the original string - you need to work with the string as a whole if you want to mutate it.

String#= on the other hand does mutate the string

String#=

You could make your method like so:

def test_problem(str)
str[3] = str[3].upcase # dont use bang methods when assigning
str
end


It does mutate the string. But you ignore it and never look at it. Therefore, you cannot know whether it was mutated or not.

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