explain xkcd - User contributions [en]

Talk:3062: Off By One

2025-03-13T16:47:20Z

Snaxmcgee:

But what about floats? [[User:GreyFox|GreyFox]] ([[User talk:GreyFox|talk]]) 20:01, 12 March 2025 (UTC)

Is this dithering? [[User:Hcs|Hcs]] ([[User talk:Hcs|talk]]) 21:19, 12 March 2025 (UTC)

: Could be. --[[User:PRR|PRR]] ([[User talk:PRR|talk]]) 22:19, 12 March 2025 (UTC)

This language has a huge off by one error: the docs don't explicitly say if the random range is inclusive. EDIT: the comic description above now includes this, thx --[[User:Snaxmcgee|Snaxmcgee]] ([[User talk:Snaxmcgee|talk]]) 22:22, 12 March 2025 (UTC)

But if it's adjusted both on store and on read, then there is a chance (of about 1 in 22) that the value after read will be exactly the same as the value before store. This does not eliminate pre-existing off-by-one errors, and in fact, introduces new ones if the adjustment on read is off by one from the adjustment on store, when there was no off-by-one error in the original code. And what's worse - with a single store-read cycle, the value can never be off by 40 to 50. It can be off by up to 10, or by between 80 to 100, in either direction. --[[User:NeatNit|NeatNit]] ([[User talk:NeatNit|talk]]) 22:42, 12 March 2025 (UTC)
:I was ''just'' adjusting the explanation to imply this sort of thing (without having read your comment, just yet). Given the assumption that n=n±(40+rand(11)) at every stage (I'm assuming 'inclusive', Snaxmcgee!), two steps of 'intentional adjustment' might result in: -100 (x1), -99 (x2), -98 (x3), -97 (x4), -96 (x5), -95 (x6), -94 (x7), -93 (x8), -92 (x9), -91 (x10), -90 (x11), -89..-80 (x10..x1), -10 (x2), -9 (x4), -8 (x6), -7 (x8), -6 (x10), -5 (x12), -4 (x14), -3 (x16), -2 (x18), -1 (x20), ±0 (x22), +1..+10 (x20..x2), +80..+90..+100 (x1..x11..x1).
:This gives a chance of being entirely correct as 22/484 (4.5454...%) and ''each'' off-by-one as ''very'' slightly less (though ±1, in total is almost twice as likely!).
:Adding further steps (skipping odd step-cummulations, at least at first, until you get to nine of them and everything entirely stops being discontinuous) just spreads out an increased number of highs right next to zero deflection... [[Special:Contributions/172.70.86.129|172.70.86.129]] 23:38, 12 March 2025 (UTC)

::Obligatory quote:
There are two hard things in computer science: cache invalidation, naming things, and off-by-one errors.
::See here for a full story of this quote: https://twitter.com/codinghorror/status/506010907021828096
::--[[Special:Contributions/162.158.129.64|162.158.129.64]] 08:28, 13 March 2025 (UTC)
:::And 3 hard things in distributed computing: 3. Delivering messages exactly one time, 2. Making sure things happen in the correct order, and 3. Delivering messages exactly one time [[User:Jamcdonald|Jamcdonald]] ([[User talk:Jamcdonald|talk]])
::::Presumably 1 is not losing data? --[[User:NeatNit|NeatNit]] ([[User talk:NeatNit|talk]]) 10:19, 13 March 2025 (UTC)

In the comic, Cueball clearly says the adjustment amounts is ‘’between’’ 40 and 50, yet this explanation says the adjustment is from 40 to 50, ironically making an off-by-1 error on both ends of the range. Neither integers 40 nor 50 are “between 40 and 50”. [[Special:Contributions/172.71.154.39|172.71.154.39]] 10:43, 13 March 2025 (UTC)
:English language is imprecise with its use of "between", but it's usually taken as inclusive. Most people, when asked, "Pick a number between 1 and 10," will assume that 1 and 10 are both valid choices. Even in computing, you have things like Excel's RANDBETWEEN function to generate random integers between two bounds, which is inclusive. {{unsigned ip|104.23.187.72|13:28, 13 March 2025}}
::Interestingly, in German such ranges are defined as including the borders, in Dutch they're defined as excluding the borders. (hence the Dutch t/m ("tot en met" - "up to, and including") [[Special:Contributions/104.23.170.81|104.23.170.81]] 15:28, 13 March 2025 (UTC)
::Yes, see {{wiktionary|between#Usage notes}} as one overview. Between as in "within the bounds defined by" is different from "amongst those things of which these items are the defining outer examples". Especially, but not exclusively, when that's just two distinct items which have ''no'' valid intermediate states betwixt the two to choose from ("you have to choose between me and my sister" isn't usually satisfactoraily answerable by choosing a different sibling, or perhaps parent, of the two). [[Special:Contributions/172.69.195.54|172.69.195.54]] 15:24, 13 March 2025 (UTC)

It's easy to make an off-by-one error without using a computer at all. Ask a friend how many fenceposts are needed for a 100-foot fence if the rails are ten feet long. [[Special:Contributions/172.71.30.199|172.71.30.199]] 12:58, 13 March 2025 (UTC)
:And how wide are the posts..? ;) [[Special:Contributions/141.101.98.6|141.101.98.6]] 15:27, 13 March 2025 (UTC)
::If I ever answer this question 100, it's obviously because in my mind each post (or more accurately, the distance between the grooves cut into the post) is a foot wide, which is slightly larger than usual but not totally unreasonable size, and not because I fell for the trick.--[[User:Snaxmcgee|Snaxmcgee]] ([[User talk:Snaxmcgee|talk]]) 16:47, 13 March 2025 (UTC)

I don't understand - what's all this got to do with water balloons? [[Special:Contributions/172.70.86.129|172.70.86.129]] 15:37, 13 March 2025 (UTC)
:(ISWYDT...) [[Special:Contributions/173.70.195.206|171.68.193.204]] 16:47, 14 February 2026 (UTC)

Talk:3062: Off By One

2025-03-13T16:32:56Z

Snaxmcgee: Someone else referenced this comment so now there's a bunch of confusion. I'm undoing it's removal but editing to clarify. Undo revision 368789 by 172.70.176.15 (talk)

But what about floats? [[User:GreyFox|GreyFox]] ([[User talk:GreyFox|talk]]) 20:01, 12 March 2025 (UTC)

Is this dithering? [[User:Hcs|Hcs]] ([[User talk:Hcs|talk]]) 21:19, 12 March 2025 (UTC)

: Could be. --[[User:PRR|PRR]] ([[User talk:PRR|talk]]) 22:19, 12 March 2025 (UTC)

This language has a huge off by one error: the docs don't explicitly say if the random range is inclusive. EDIT: the comic description above now includes this, thx --[[User:Snaxmcgee|Snaxmcgee]] ([[User talk:Snaxmcgee|talk]]) 22:22, 12 March 2025 (UTC)

But if it's adjusted both on store and on read, then there is a chance (of about 1 in 22) that the value after read will be exactly the same as the value before store. This does not eliminate pre-existing off-by-one errors, and in fact, introduces new ones if the adjustment on read is off by one from the adjustment on store, when there was no off-by-one error in the original code. And what's worse - with a single store-read cycle, the value can never be off by 40 to 50. It can be off by up to 10, or by between 80 to 100, in either direction. --[[User:NeatNit|NeatNit]] ([[User talk:NeatNit|talk]]) 22:42, 12 March 2025 (UTC)
:I was ''just'' adjusting the explanation to imply this sort of thing (without having read your comment, just yet). Given the assumption that n=n±(40+rand(11)) at every stage (I'm assuming 'inclusive', Snaxmcgee!), two steps of 'intentional adjustment' might result in: -100 (x1), -99 (x2), -98 (x3), -97 (x4), -96 (x5), -95 (x6), -94 (x7), -93 (x8), -92 (x9), -91 (x10), -90 (x11), -89..-80 (x10..x1), -10 (x2), -9 (x4), -8 (x6), -7 (x8), -6 (x10), -5 (x12), -4 (x14), -3 (x16), -2 (x18), -1 (x20), ±0 (x22), +1..+10 (x20..x2), +80..+90..+100 (x1..x11..x1).
:This gives a chance of being entirely correct as 22/484 (4.5454...%) and ''each'' off-by-one as ''very'' slightly less (though ±1, in total is almost twice as likely!).
:Adding further steps (skipping odd step-cummulations, at least at first, until you get to nine of them and everything entirely stops being discontinuous) just spreads out an increased number of highs right next to zero deflection... [[Special:Contributions/172.70.86.129|172.70.86.129]] 23:38, 12 March 2025 (UTC)

::Obligatory quote:
There are two hard things in computer science: cache invalidation, naming things, and off-by-one errors.
::See here for a full story of this quote: https://twitter.com/codinghorror/status/506010907021828096
::--[[Special:Contributions/162.158.129.64|162.158.129.64]] 08:28, 13 March 2025 (UTC)
:::And 3 hard things in distributed computing: 3. Delivering messages exactly one time, 2. Making sure things happen in the correct order, and 3. Delivering messages exactly one time [[User:Jamcdonald|Jamcdonald]] ([[User talk:Jamcdonald|talk]])
::::Presumably 1 is not losing data? --[[User:NeatNit|NeatNit]] ([[User talk:NeatNit|talk]]) 10:19, 13 March 2025 (UTC)

In the comic, Cueball clearly says the adjustment amounts is ‘’between’’ 40 and 50, yet this explanation says the adjustment is from 40 to 50, ironically making an off-by-1 error on both ends of the range. Neither integers 40 nor 50 are “between 40 and 50”. [[Special:Contributions/172.71.154.39|172.71.154.39]] 10:43, 13 March 2025 (UTC)
:English language is imprecise with its use of "between", but it's usually taken as inclusive. Most people, when asked, "Pick a number between 1 and 10," will assume that 1 and 10 are both valid choices. Even in computing, you have things like Excel's RANDBETWEEN function to generate random integers between two bounds, which is inclusive. {{unsigned ip|104.23.187.72|13:28, 13 March 2025}}
::Interestingly, in German such ranges are defined as including the borders, in Dutch they're defined as excluding the borders. (hence the Dutch t/m ("tot en met" - "up to, and including") [[Special:Contributions/104.23.170.81|104.23.170.81]] 15:28, 13 March 2025 (UTC)
::Yes, see {{wiktionary|between#Usage notes}} as one overview. Between as in "within the bounds defined by" is different from "amongst those things of which these items are the defining outer examples". Especially, but not exclusively, when that's just two distinct items which have ''no'' valid intermediate states betwixt the two to choose from ("you have to choose between me and my sister" isn't usually satisfactoraily answerable by choosing a different sibling, or perhaps parent, of the two). [[Special:Contributions/172.69.195.54|172.69.195.54]] 15:24, 13 March 2025 (UTC)

It's easy to make an off-by-one error without using a computer at all. Ask a friend how many fenceposts are needed for a 100-foot fence if the rails are ten feet long. [[Special:Contributions/172.71.30.199|172.71.30.199]] 12:58, 13 March 2025 (UTC)
:And how wide are the posts..? ;) [[Special:Contributions/141.101.98.6|141.101.98.6]] 15:27, 13 March 2025 (UTC)

I don't understand - what's all this got to do with water balloons? [[Special:Contributions/172.70.86.129|172.70.86.129]] 15:37, 13 March 2025 (UTC)
:(ISWYDT...) [[Special:Contributions/173.70.195.206|171.68.193.204]] 16:47, 14 February 2026 (UTC)

Talk:3062: Off By One

2025-03-12T22:22:01Z

Snaxmcgee: