I was just digging ruby and want to know why I am not able to re-assign a value to an array element using enumerable method (first or last).
I know I can assign values with index but I want to know more about this. I didn't find much explanation in here and here.
array1 = [1,2,3,4,5]
array1.first # returns 1
array1.first = 7
NoMethodError: undefined method `first=' for [1, 2, 3, 4, 5]:Array
Ruby 2.0
Thanks
There are two ways this question can be read:
first one?The reason is that in order to be able to use foo.bar = :baz, foo's class has to implement the bar= method.
class X
def initialize(y)
@y = y
end
def y
@y
end
end
x = X.new(3)
x.y # => 3
x.y = 7 # => NoMethodError: undefined method `y=' for #<X:0x007fb272726d80 @y=3>
class X
def initialize(y)
@y = y
end
def y
@y
end
def y=(new_y)
@y = new_y
end
end
x = X.new(3)
x.y # => 3
x.y = 7
x.y # => 7
The core library does implement Array#[]=, but does not Array#first=.
Array#first=?The first method is actually part of Enumberable and if one class implemented first=, it would look inconsistent if there was no implementation of Enumerable#first=.
And this can't happen because for some enumerables such method doesn't make sense (Enumerator). It will also require more of you to get the enumerable functionality. As of right now, for a class to become enumerable, it just needs to implement an each method and include the Enumerable module.
Array#first= but not Enumerable#first=, considering that we have Array#[]= but not Enumerable#[]=?
Array#[]= is not inconsistent, because there is no Enumerable#[]. Therefore, indexing is an extension in Array which does not originate from Enumerable and there is no expectation other enumerables to also be indexable. That is what I meant. The method is... was referring to Enumerable#first.The array method .first is a attribute reader method, it does not have writing permission. Check out the Ruby Documentation for array . You need to reassign a value at a specific index in the array just follow what @Andrey wrote
Returns the first element, or the first n elements, of the array. If the array is empty, the first form returns nil, and the second form returns an empty array. See also #last for the opposite effect.
Actually you can create first(12) , imagine how awkward the method first would be if you can set first=(12,arr) , the method become ambiguous (if the array doesn't have 12 elements, should it add the elements or should it edit the x first elements) because first(12) of an array of 5 elements returns the 5 elements. writing such a method like that will violate the single responsibility principle.
x.first(12) = foo is invalid syntax :)
Array#[]=, but there is noArray#first=. Are you asking why they didn't put one?Array#first=. It's as simple as that. IsArray#first=needed? No, because one can use[]=. IsArray#first(which we have) needed? No, because one can use Array#[] (which, incidentally, is as different fromArray[]=as night is from day). So why do we haveArray#firstbut notArray#first=? Because those who argued for the addition of the latter carried the day.