nimforum mirror - For loop using iterator `..`

loczaj (orginal) [2021-12-07T13:41:14+01:00] view original

Hello Everyone,

I am quite new to Nim and solve puzzles of Advent of Code 2021 using Nim. 🙂 I enjoy the flexibility, compactness and elegance of the language. Tanks for Nim. 🙏

I ran into a unexpected behavior. The for loop

for i in  1..3: echo i

would iterate over the 3 numbers and print them. However, the loop

for i in  3..1: echo i

does nothing. For me this was completely unexpected (and wasted much of valuable time in the competition 😉 )

Can anyone explain what is concept behind this asymmetry?

What is the right way to iterate from a .. b if it is unknown, whether a<=b or a>b ?

Thanks.

Zoom (orginal) [2021-12-07T14:03:13+01:00] view original

iterator count[T: SomeInteger](x, y: T): T =
  if x < y:
    for n in countup(x, y): yield n
  else:
    for n in countdown(x, y): yield n

This is what I used. This retains the direction.

SolitudeSF (orginal) [2021-12-07T14:03:39+01:00] view original

Can anyone explain what is the concept behind this asymmetry?

in practice, you know the direction of iteration in 99.9% of cases and terminating behaviour (1..sequence.high) is useful far more often.

What is the right way to iterate from a .. b if it is unknown, whether a<=b or a>b ?

if

freeflow (orginal) [2021-12-07T14:32:46+01:00] view original

I feel your pain. Nim is so good you are encouraged to explore without reading the manual, then you get tripped up by a template........ It only took me about 10 mins to find countdown to resulve the same issue.

Araq (orginal) [2021-12-07T14:35:34+01:00] view original

The alternative "design" makes no sense, that's why:

let a = stdin.readLine.parseInt
let b = stdin.readLine.parseInt
for i in a..b:
  echo "so the iteration order depends on the input? Terrible..."

No other programming language that I know of does this either, but my memory of other programming languages is fading quickly as I only use Nim. :P

miran (orginal) [2021-12-07T16:19:10+01:00] view original

Can anyone explain what is the concept behind this asymmetry?

What does for n in range(3, 1): print(n) print in Python?

(Spoiler: Also nothing, just like Nim)

freeflow (orginal) [2021-12-07T17:29:24+01:00] view original

Does no one here take any pride in the fact that nim is so good that newbies just expect a..b to work when a>b. C'mon guys, take a compliment, even when its indirect.

loczaj (orginal) [2021-12-08T12:43:24+01:00] view original

Hi Folks, Thanks for your answers. I already enjoy this community. 🙂

Some years ago I taught youngsters programming and realized that newbies have marvelous ideas on how programming could be. Okay, I know, this is not the case. 😀

@Zoom Thanks for your handy iterator.

@SolitudeSF @Araq @miran Thanks for the explanation on the design. You are right, countdown does not work, even on the Commodore 64. 🙂 I checked: (see pic)

https://drive.google.com/file/d/1clRLLd4CzA-5x6keqBLECxyD2wnUhEWH/view?usp=sharing

@freeflow You are right. I am catched by Nim’s is intuitiveness and flexibility and explore freely with minimal reading of the manual. I know it is unprofessional but works most of the cases, which is the merit of the language indeed. 🙂

Mirror of forum.nim-lang.org

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