Updated description. Let me know if OK so we can close.

That's exactly the same than before (almost). Again, the """problem""" is that you made a choice that might seem wierd. So say it "loudly". Here, you still let it appear as if it was something like a "rule" or more likely some kind of "truth". You might see people again with the same questions/arguments.

Again, be explicit. It's not funny, yes, but nobody will feel spoiled about the rules, this way (the explicit one, I mean). That or... change the algo. ;)

See the changes in the description. Previously it said : ...For example, solve(0,10) = 3, because there are only 3 upside down numbers >= 0 and < 10. They are 0, 1, 8...

Now it says ...For example, solve(0,10) = 3, because there are only 3 upside down numbers >= 0 and < 10. They are 0, 1, 8. No other numbers less than 10 are upside down numbers...

Thx B4B. I will update.

Maybe...:

"Considere the numbers 6969 and 9116. When you rotate them 180 degrees (upside down), these numbers remain the same (effectively, in this kata, we will considere that 1 is made of a simple vertical line)."

And maybe a foot note in addition:

"Note: digits 2 and 5 will not be considered "upside downable" in this kata."

Ok. Give me some wording I can use in the description. Then we can close this question.

ok, but not ok. ;)

You define the rules like you want, that's not the problem here. I just want you understand the somewhat illogical way you chose ("to justify") them here. Maybe you do only a straight line for your "ones", but just look at the font used on CW. It's NOT a straight line since it has 3 tiny things in addition, say a right arm and two feet. ;). And considering one doesn't know how one writes those digits by hand, one has to rely on the common thing to anyone: the font use on CW. And there lays the problem with your approach: with CW font, 1 != 1 reversed, so if you define 1 == 1 reversed nothing forbid to considere that 2 == 2 reversed and the same for 5. Nothing except... you saying it out loud in the description. Again, that's not a problem as long as you justify it with the "right argument", this one being here "you decide that 2 and 5 won't be considered" (and that's ok if it's explained), but not "try it by hand and you'll see..." because that approach doesn't work. ;)

That's just details, of course. But devil is in the details... ;)

In this Kata, we are only dealing with valid numbers. 1 is just a vertical line. Knowing this, then after rotation, all numbers must be valid. if you write 5 on a paper and turn upside down, you don't get 2 or any other number. We can make 1 an exception to address your point. In the description, I included all rotatable numbers and gave examples. I included 1 specifically; see the part that say ...there are only 3 upside down numbers >= 0 and < 10. They are 0, 1, 8.But I'm happy to expand the description if you feel the need for that.

It is possible to include other numbers as well, but this is the subject of another Kata.

there's still a problem: 1 != 1 at 180° (except if you write 1 with a simple vertical line). If you accept that for 1, you should accept it for 2 and 5 too (but not 2 <-> 5. Rather 1,2,5,8 and 0 would all be "reversible" to themselves).

Or do you visualize a different 180° rotation than the one I have in mind?

@kalvy, it's not a translation of numbers.

0 -> 0
1 -> 1
2 -> 5 (?) If you turn 2 upside down, you don't 5. Infact, you don't get a number.
.

Thanks B4B. For this Kata, numbers such as 0, 1, 8 are the same when rotated 180 degrees (thanks myjinxin2015 for suggesting this description) because 1 is still 1, as is 8. However, 6 on its own becomes 9 so it's not the same number. 69 on the other hand becomes 69 when turned upside down. The description states that if we write them down on a paper and turn the paper upside down, the numbers will be the same. Try it and see!. This is true for 0, 1, 8 , 6969, 9116 and many other numbers, such as 1961.

If this helps, we can resolve this question.

but this way, 1 shouldn't be reversible. It depends on the font you use, actually. Same for 6 and 9. Even 8, if one loop is smaller than the other.

You have 2 possibilities: either you change the algo adding 2-5 in the problem or you specify exactly all the reversible characters and how they are reversed.

Make sure the number reads the same when turned upside down.

For some numbers, like 2, they stop becoming numbers when turned upside down. Right?

nope (but that's not normal, yep).