Archive for power
battery recharged!
Posted in pictures, Travel with tags Amazon, battery, DIY, HP EliteBook, laptop, power on September 10, 2022 by xi'an
midnight run
Posted in Running, Travel with tags adaptivity, Banff, Canada, convenience, midnight run, midnight sun, power, UBC, Vancouver on December 8, 2019 by xi'an
Le Monde puzzle [#1114]
Posted in Kids, R with tags brute-force solution, cube, Le Monde, mathematical puzzle, perfect square, power, R on October 16, 2019 by xi'an
Another very low-key arithmetic problem as Le Monde current mathematical puzzle:
32761 is 181² and the difference of two cubes, which ones? And 181=9²+10², the sum of two consecutive integers. Is this a general rule, i.e. the root z of a perfect square that is the difference of two cubes is always the sum of two consecutive integers squared?
The solution proceeds by a very dumb R search of cubes, leading to
34761=105³-104³
The general rule can be failed by a single counter-example. Running
sol=0;while(!sol){
x=sample(2:1e3,1)
y=sample(1:x,1)-1
sol=is.sqr(z<-x^3-y^3)
z=round(sqrt(z))
if (sol)
sol=(trunc(sqrt(z/2))^2+ceiling(sqrt(z/2))^2!=z)}
which is based on the fact that, if z is the sum of two consecutive integers squared, a² and (a+1)² then
2 a²<z<2 (a+1)²
Running the R code produces
x=14, y=7
as a counter-example. (Note that, however, if the difference of cubes of two consecutive integers is a square, then this square can be written as the sum of the squares of two different integers.) Reading the solution in the following issue led me to realise I had missed the consecutive in the statement of the puzzle!
How many subjects? [not a book review]
Posted in Books, pictures, Statistics with tags Brett Kavanaugh, Christine Blasey, power, statistical significance, statistical tests, tests, textbook on September 24, 2018 by xi'an
take those hats off [from R]!
Posted in Books, Kids, R, Statistics, University life with tags exponent notation, exponentiation, functions in R, mantissa, power, R, scientific notation, system.time on May 5, 2015 by xi'an
This is presumably obvious to most if not all R programmers, but I became aware today of a hugely (?) delaying tactic in my R codes. I was working with Jean-Michel and Natesh [who are visiting at the moment] and when coding an MCMC run I was telling them that I usually preferred to code Nsim=10000 as Nsim=10^3 for readability reasons. Suddenly, I became worried that this representation involved a computation, as opposed to Nsim=1e3 and ran a little experiment:
[sourcecode language=”r” gutter=”false”]
> system.time(for (t in 1:10^8) x=10^3)
utilisateur système écoulé
30.704 0.032 30.717
> system.time(for (t in 1:1e8) x=10^3)
utilisateur système écoulé
30.338 0.040 30.359
> system.time(for (t in 1:10^8) x=1000)
utilisateur système écoulé
6.548 0.084 6.631
> system.time(for (t in 1:1e8) x=1000)
utilisateur système écoulé
6.088 0.032 6.115
> system.time(for (t in 1:10^8) x=1e3)
utilisateur système écoulé
6.134 0.029 6.157
> system.time(for (t in 1:1e8) x=1e3)
utilisateur système écoulé
6.627 0.032 6.654
> system.time(for (t in 1:10^8) x=exp(3*log(10)))
utilisateur système écoulé
60.571 0.000 57.103
[/sourcecode]
So using the usual scientific notation with powers is taking its toll! While the calculator notation with e is cost free… Weird!
I understand that the R notation 10^6 is an abbreviation for a power function that can be equally applied to pi^pi, say, but still feel aggrieved that a nice scientific notation like 10⁶ ends up as a computing trap! I thus asked the question to the Stack Overflow forum, getting the (predictable) answer that the R code 10^6 meant calling the R power function, while 1e6 was a constant. Since 10⁶ does not differ from ππ, there is no reason 10⁶ should be recognised by R as a million. Except that it makes my coding more coherent.
[sourcecode language=”r” gutter=”false”]
> system.time( for (t in 1:10^8) x=pi^pi)
utilisateur système écoulé
44.518 0.000 43.179
> system.time( for (t in 1:10^8) x=10^6)
utilisateur système écoulé
38.336 0.000 37.860
[/sourcecode]
Another thing I discovered from this answer to my question is that negative integers are also requesting call to a function:
[sourcecode language=”r” gutter=”false”]
> system.time( for (t in 1:10^8) x=1)
utilisateur système écoulé
10.561 0.801 11.062
> system.time( for (t in 1:10^8) x=-1)
utilisateur système écoulé
22.711 0.860 23.098
[/sourcecode]
This sounds even weirder.
