Skip to main content

bugs - RegionMeasure is wrong for simple 1D Path


Bug introduced in 11.1




Take the simple path defined by a line through a set of points


pts = {{0, 0}, {0, 1.5}, {0.05, 1.5}, {0.05, 0.5}, {0.1, 0.5}, {0.1,  1.5},
{0.15, 1.5}, {0.15, 0.5}, {0.2, 0.5}, {0.2, 1.5}, {0.25, 1.5}, {0.25, 0.5},
{0.3, 0.5}, {0.3, 1.5}, {0.35, 1.5}, {0.35, 0}}


reg = Line[pts];

This can be visualized using Graphics


Graphics[{Blue, reg}, Axes -> True]

image


Both RegionMeasure and ArcLength should give us the length of the line and in fact it does give us a number


RegionMeasure@reg
(* 7.85 *)

ArcLength@reg
(* 7.85 *)

This is wrong however!


This gives the sequence of the individual arc lengths:


FoldPairList[{Abs@Total[#2 - #1], #2} &, {0, 0}, pts]
(* {0., 1.5, 0.05, 1., 0.05, 1., 0.05, 1., 0.05, 1., 0.05, 1., 0.05, 1., 0.05, 1.5} *)

The sum of this sequence is $9.35$ not $7.85$.


Sanity check: Count the line segments on screen. There are two segments of length $1.5$, seven segments of length $0.05$, and six segments of length $1$. So in total: $2\times1.5 + 7\times0.05 + 6\times1 = 9.35$





I use Mathematica 11.1.1.0 on Mac OS 10.12.6




Edit in response to comments:


When I copy and paste pts from SE I also get $9.35$. Try the following which I used to generate pts in the first place:


pts2 = AnglePath[{{1.5, 90.°}, {0.05, -90.°}, {1, -90.°}, {0.05, 90.°}, {1, 90.°},
{0.05, -90.°}, {1, -90.°},{0.05, 90.°}, {1, 90.°}, {0.05, -90.°}, {1, -90.°},
{0.05, 90.°}, {1, 90.°}, {0.05, -90.°}, {1.5, -90.°}}] //Chop

and check that pts and pts2 are in fact equal



pts == pts2
(* True *)

Now try RegionMeasure@Line@pts2. For me this gives $7.85$. Seems the culprit is AnglePath.




Comments

Popular posts from this blog

plotting - How to draw lines between specified dots on ListPlot?

I would like to create a plot where I have unconnected dots and some connected. So far, I have figured out how to draw the dots. My code is the following: ListPlot[{{1, 1}, {2, 2}, {3, 3}, {4, 4}, {1, 4}, {2, 5}, {3, 6}, {4, 7}, {1, 7}, {2, 8}, {3, 9}, {4, 10}, {1, 10}, {2, 11}, {3, 12}, {4,13}, {2.5, 7}}, Ticks -> {{1, 2, 3, 4}, None}, AxesStyle -> Thin, TicksStyle -> Directive[Black, Bold, 12], Mesh -> Full] I have thought using ListLinePlot command, but I don't know how to specify to the command to draw only selected lines between the dots. Do have any suggestions/hints on how to do that? Thank you. Answer One possibility would be to use Epilog with Line : ListPlot[ {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {1, 4}, {2, 5}, {3, 6}, {4, 7}, {1, 7}, {2, 8}, {3, 9}, {4, 10}, {1, 10}, {2, 11}, {3, 12}, {4, 13}, {2.5, 7}}, Ticks -> {{1, 2, 3, 4}, None}, AxesStyle -> Thin, TicksStyle -> Directive[Black, Bold, 12], Mesh -> Full, Epilog -> { Line[ ...

list manipulation - Selecting multiple columns from a matrix?

Sample data: data = { {{2013, 1, 1}, 24.13, 167.67, 231.82}, {{2013, 1, 2}, 32.15, 170.92, 225.99}, {{2013, 1, 3}, 35.43, 172.68, 221.67}, {{2013, 1, 4}, 36.73, 173.05, 218.32}, {{2013, 1, 5}, 58.19, 165.96, 197.05}, {{2013, 1, 6}, 69.99, 163.50, 187.52}, {{2013, 1, 7}, 71.37, 154.21, 175.58}, {{2013, 1, 8}, 72.51, 149.66, 163.25}}; I want a DateListPlot with three graphs, so for a matrix formed by columns 1 and 2, one for columns 1 and 3, and 1 for columns 1 and 4. At the moment I'm using this code: data2 = Transpose[{data[[All, 1]], data[[All, 2]]}]; data3 = Transpose[{data[[All, 1]], data[[All, 3]]}]; data4 = Transpose[{data[[All, 1]], data[[All, 4]]}]; DateListPlot[{data2, data3, data4}, Joined -> True, Filling -> {3 -> {1}}] but I have a hunch that this can be done more efficiently. I don't like the Transpose s in particular. Any ideas? edit (for extra credit) What if I need to multiply the second column by 2, which in my solution is simp...

calculus and analysis - Integral of the Sinc product

Let us consider the following integral $$ B_n = \int_0^\infty \prod_{k=1,3,5,\dots}^n\frac{\sin (x/k)}{x/k}dx $$ By definition, Sinc[x] == Sin[x]/x , therefore B[n_?OddQ] := Integrate[Product[Sinc[x/k], {k, 1, n, 2}], {x, 0, Infinity}] B /@ Range[1, 13, 2] {Pi/2, Pi/2, Pi/2, Pi/2, Pi/2, Pi/2, Pi/2} OK, everything is fine. But... B[15] 467807924713440738696537864469 Pi/935615849440640907310521750000 What's going on? Answer As Eckhard wrote in comments B[n] is the n-th Borwein integral . (The letter B was not accidental :) ) This funny properties of Borwein integrals is related to the Fourier transform of Sinc function FourierTransform[Sinc[x], x, k] 1/2 Sqrt[Pi/2] (Sign[1 - k] + Sign[1 + k]) Plot[%, {k, -2, 2}, Filling -> 0] which is the box function. The result is $\pi/2$ while the sum $$ 1/3+1/5+\dots+1/n $$ B_n = \frac{\pi}{2} - \pi \bigg(\sum_{k=3,5,\ldots}^n\frac{1}{k} -1\biggr)^\frac{n-1}{2}\prod_{k=3,5,\dots}^n\frac{k}{k-1}. $$ For $n=15$ it is equal to $$ \frac{46780...