*run time of math problem@ 2021-03-19 23:18 Ray Andrews2021-03-19 23:46 ` Lawrence Velázquez 2021-03-19 23:52 ` Oliver Kiddle 0 siblings, 2 replies; 6+ messages in thread From: Ray Andrews @ 2021-03-19 23:18 UTC (permalink / raw) To: Zsh Users This question might not even be answerable and it's only a point of interest: I was working out some probabilities and the algorithm looks like this: for ((level=1; level<100; level++)); do sum= remainder=$(( (level - 1.0) / level )) for ((terms=level; terms; terms--)); do sum+=$(( remainder**(terms - 1) )) done divided=$(( sum * (1.0 / level) )) echo for level: $level, survival: $divided done Now if I avoid the 'remainder' variable: for ((level=1; level<100; level++)); do sum= for ((terms=level; terms; terms--)); do # 'remainder' calculation done directly here: sum+=$(( ( (level - 1.0) / level )**(terms - 1) )) done divided=$(( sum * (1.0 / level) )) echo for level: $level, survival: $divided done ... I'd expect the thing to run a teeny bit faster but in fact it runs about 15% slower. Is that explicable? Does zsh prefer calculations done in steps with variables for each step? Or is this just some little anomaly? Things might not optimize every time. ^ permalink raw reply [flat|nested] 6+ messages in thread

*Re: run time of math problem2021-03-19 23:18 run time of math problem Ray Andrews@ 2021-03-19 23:46 ` Lawrence Velázquez2021-03-19 23:51 ` Ray Andrews 2021-03-19 23:52 ` Oliver Kiddle 1 sibling, 1 reply; 6+ messages in thread From: Lawrence Velázquez @ 2021-03-19 23:46 UTC (permalink / raw) To: Ray Andrews;+Cc:zsh-users > On Mar 19, 2021, at 7:18 PM, Ray Andrews <rayandrews@eastlink.ca> wrote: > > ... I'd expect the thing to run a teeny bit faster but in fact it > runs about 15% slower. Is that explicable? Does zsh prefer > calculations done in steps with variables for each step? Or is this > just some little anomaly? Things might not optimize every time. Think about how your change affects your algorithm. > for ((level=1; level<100; level++)); do > sum= > remainder=$(( (level - 1.0) / level )) > for ((terms=level; terms; terms--)); do > sum+=$(( remainder**(terms - 1) )) > done > divided=$(( sum * (1.0 / level) )) > echo for level: $level, survival: $divided > done Here, you evaluate "(level - 1.0) / level" 99 times. > Now if I avoid the 'remainder' variable: > > for ((level=1; level<100; level++)); do > sum= > for ((terms=level; terms; terms--)); do > > # 'remainder' calculation done directly here: > sum+=$(( ( (level - 1.0) / level )**(terms - 1) )) > done > divided=$(( sum * (1.0 / level) )) > echo for level: $level, survival: $divided > done Here, you evaluate it 4950 times. vq ^ permalink raw reply [flat|nested] 6+ messages in thread

*Re: run time of math problem2021-03-19 23:46 ` Lawrence Velázquez@ 2021-03-19 23:51 ` Ray Andrews0 siblings, 0 replies; 6+ messages in thread From: Ray Andrews @ 2021-03-19 23:51 UTC (permalink / raw) To: zsh-users On 2021-03-19 4:46 p.m., Lawrence Velázquez wrote: > > Here, you evaluate it 4950 times. > > vq > Nuts, it's just that simple. I'm not as sharp as I once was. Thanks Lawrence, but that's embarrassing and I'm only 64. ^ permalink raw reply [flat|nested] 6+ messages in thread

*Re: run time of math problem2021-03-19 23:18 run time of math problem Ray Andrews 2021-03-19 23:46 ` Lawrence Velázquez@ 2021-03-19 23:52 ` Oliver Kiddle2021-03-20 2:40 ` Ray Andrews 1 sibling, 1 reply; 6+ messages in thread From: Oliver Kiddle @ 2021-03-19 23:52 UTC (permalink / raw) To: Ray Andrews;+Cc:Zsh Users Ray Andrews wrote: > for ((level=1; level<100; level++)); do > sum= > for ((terms=level; terms; terms--)); do > > # 'remainder' calculation done directly here: > sum+=$(( ( (level - 1.0) / level )**(terms - 1) )) > done > divided=$(( sum * (1.0 / level) )) > echo for level: $level, survival: $divided > done > > ... I'd expect the thing to run a teeny bit faster but in fact it > runs about 15% slower. Is that explicable? Does zsh prefer Given that the latter approach has moved the remainder calculation inside a loop and it needs to be repeated 50 times (on average), it should be no surprise that it is slower. If you want to optimise for speed, avoid string conversions and do, e.g. (( divided = sum * (1.0 / level) )) You also may want to make sure to declare some of the variables as float or integer or whatever. The sum+= line might end up being a string concatenation if not. Oliver ^ permalink raw reply [flat|nested] 6+ messages in thread

*Re: run time of math problem2021-03-19 23:52 ` Oliver Kiddle@ 2021-03-20 2:40 ` Ray Andrews2021-03-20 3:59 ` Lawrence Velázquez 0 siblings, 1 reply; 6+ messages in thread From: Ray Andrews @ 2021-03-20 2:40 UTC (permalink / raw) To: zsh-users On 2021-03-19 4:52 p.m., Oliver Kiddle wrote: > > If you want to optimise for speed, avoid string conversions and do, e.g. > (( divided = sum * (1.0 / level) )) Thanks, that's easy to miss but it makes sense. One of those invisible things. Doing it that way throughout is fully 3X faster. Running it up to a billion itterrations it starts to matter. As to the rest I was just stupidly forgetting that I'm in a loop. I'm faking a summation. We don't have summation do we? > You also may want to make sure to declare some of the variables as float > or integer or whatever. The sum+= line might end up being a string > concatenation if not. Yeah, I left out the declarations but it was all kosher. ^ permalink raw reply [flat|nested] 6+ messages in thread

*2021-03-20 2:40 ` Ray AndrewsRe: run time of math problem@ 2021-03-20 3:59 ` Lawrence Velázquez0 siblings, 0 replies; 6+ messages in thread From: Lawrence Velázquez @ 2021-03-20 3:59 UTC (permalink / raw) To: Ray Andrews;+Cc:zsh-users > On Mar 19, 2021, at 10:40 PM, Ray Andrews <rayandrews@eastlink.ca> wrote: > > As to the rest I was just stupidly forgetting that I'm in a loop. > I'm faking a summation. We don't have summation do we? Not that I'm aware of, but you're computing partial sums of geometric series [*], so you have a closed-form expression available to you: for (( level = 1; level < 100; level++ )); do (( remainder = (level - 1.0) / level, sum = level - level * remainder**level, divided = sum * (1.0 / level) )) echo for level: $level, survival: $divided done [*]: https://mathworld.wolfram.com/GeometricSeries.html -- vq ^ permalink raw reply [flat|nested] 6+ messages in thread

end of thread, other threads:[~2021-03-20 4:00 UTC | newest]Thread overview:6+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2021-03-19 23:18 run time of math problem Ray Andrews 2021-03-19 23:46 ` Lawrence Velázquez 2021-03-19 23:51 ` Ray Andrews 2021-03-19 23:52 ` Oliver Kiddle 2021-03-20 2:40 ` Ray Andrews 2021-03-20 3:59 ` Lawrence Velázquez

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