An accumulator carries a value across iterations: each pass calculates a result, replaces the accumulator with it, and makes that value available to the next pass. In `inject`, an initial value can be supplied; otherwise, the first collection element initializes the accumulator. The last result becomes the method’s return value. section 10.5.6
>> [1,2,3,4].inject(0) {|acc,n| acc + n }
=> 10
Adding each element to the carried total shows the accumulator changing on every iteration. section 10.5.6
An accumulator can also collect results rather than build a single value. A `map` implementation stores each block’s return value in an accumulator array and returns that array. Unlike `each`, which discards block return values, `map` uses them; a block that calls `puts` therefore contributes `nil` for each iteration. section 6.3.7 section 10.6.1
class Array
# Put the definition of my_each here
def my_map
acc = []
my_each {|e| acc << yield(e) }
acc
end
end
This implementation builds `map` on `my_each`, appending each block result before returning the accumulated array. section 6.3.7
