What is the difference of "i, v" and "_, v" in for loops?

The character itself doesn’t have much significance, it just simply represents an ignorance figure—a blank space. Even though it still is supplied with the identical index data, we still use it a form to conventionally signify that the parameter is insignificant to actions we’re performing inside the iteration function.

Here is a example to help put the information above into perspective:

local arbitraryTable = {1,5,true,"Hello!", nil}
for index, value in pairs(arbitraryTable) do
    print(index, value)
end

--[[ Output:
1    1
2    5
3    true
4    Hello
5    nil
]]

In that code, we wanted to use the current index (where were are in the table) and it’s affiliated element. But let’s say we didn’t want to use that index, we signify that like so:

local arbitraryTable = {1,5,true,"Hello!", nil}
for _, value in pairs(arbitraryTable) do
    print(value) --[[ We mark that we don’t want i, 
    and as you see it isn’t used at all.]]
    --[[ print(_) If you run this, you’ll see _ still 
    contains the information anyway]]
end

--[[ Output:
1
5
true
Hello
nil
]]

This can be done vice-versa, as it really doesn’t matter how you use _. Like I said, it’s not significant to the language at all, so technically wherever we don’t want to use a space, we fill it with “nothing” by using _. You can write for i,_ in pairs(table) do for all that matters.

As for for i =, this is identical to the behaviour as i,v, it simply provides us with a variable local to the function scope that contains information that is determined by the function itself. In this case, i will represent a # based on the iteration principles given: start, stop, skip:

for i = 1,5 do --// *start* at 1, go to 5 *stop at*
    print(i)
end

--[[ Output:
1
2
3
4
5
]]

for i = 0,10,2 do --// Skip by 2
    print(i)
end

--[[ Output:
0
2
4
6
8
10
]]