Why for i in (1...10) clobbers your i

TL;DR

for does not create a new scope. The loop variable i is the same local variable as the i = 42 above it, reassigned on every iteration. The range 1...10 excludes 10, so the last assignment is i = 9, and that value survives the loop.

Step by step

i = 42

for i in (1...10)
end

i # => 9

1. i = 42 creates the local variable i.
2. for i in (1...10) iterates over 1 through 9 (three dots exclude the upper bound), assigning each value to that same i.
3. After the loop, i holds the last assigned value, 9.

With an inclusive range the answer changes accordingly:

i = 42

for i in (1..10)
end

i # => 10

for vs each

each with a block is the construct that scopes its variable. A block parameter is local to the block, so the outer i is untouched:

i = 42

(1...10).each do |i|
end

i # => 42

The absence of a scope in for cuts both ways: variables created inside the body also leak out, which is occasionally convenient and usually a source of bugs:

for x in [1, 2, 3]
  y = x * 2
end

x # => 3
y # => 6

With each, both x and y would be gone after the block and referencing them would raise NameError. This scoping behavior is the main reason idiomatic Ruby prefers each over for.