Let's say I have following block of code:
arr = ['a','b','c']
arr.map {|item| item <<'1'} #=> ['a1','b1','c1']
arr #=> ['a1','b1','c1']
Why does Array#map change the array? It should only create a new one. When I'm using + in the block instead of <<, it works as expected. Does Array#each change the array itself, or does it only iterate over it and return itself?
My question is: Why does
mapchange array? it should only create new.
map doesn't change the Array. But << changes the Strings in the Array.
See the documentation for String#<<:
str << obj → strAppend—Concatenates the given object to
str.
Although it isn't mentioned explicitly, the code example clearly shows that << mutates its receiver:
a = "hello "
a << "world" #=> "hello world"
a.concat(33) #=> "hello world!"
It's strange, because when I'm using
+operator in the block insted of<<it works as expected.
+ doesn't change the Strings in the Array.
See the documentation for String#+:
str + other_str → new_strConcatenation—Returns a new
Stringcontainingother_strconcatenated tostr.
Note how it says "new String" and also the return value is given as new_str.
And my second question: Does
Array#eachchange array itself or it only iterate over array and return itself?
Array#each does not change the Array. But of course, the block passed to Array#each may or may not change individual elements of the Array:
arr = %w[a b c]
arr.map(&:object_id) #=> an array of three large numbers
arr.each {|item| item <<'1' } #=> ['a1', 'b1', 'c1']
arr.map(&:object_id) #=> an array of the same three large numbers
As you can see, Array#each did not change the Array: it is still the same Array with the same three elements.
Using map or each makes difference on the outer array (map will return a new array, each will return the original array), but it will not make difference on what strings the array contains; in either case, the strings contained in the array will be the original strings modified.
