# Questions tagged [sow-reap]

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### Extract result of Reap in a natural way no matter if it's empty

This feature has long troubled me, I would like to share it. If Reap has non-empty set of sown, result looks like: ...
• 1,729
177 views

### How to parallelize Reap and Sow in Mathematica 14.0.0?

I'm working with a huge amount of data, so I'm avoiding using AppendTo. I used to use the Reap and ...
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279 views

### How to effectively guess the initial values for a Poincaré section?

I have the following code, which plots the Poincaré section for the trajectory of particles around a symmetric black hole: ...
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473 views

### Poincaré section of the particle motion near the black hole horizon

I am trying to recreate the Poincaré sections given in Figure 2 of this paper. For this purpose, I use the following two methods: Method1: Using ResourceFunction["ClickPoincarePlot2D"] <...
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41 views

### Optimization of conditions applying

Let we have some data generator, e.g. lazy tuples or permutations. I need to select data by some conditions cond1, cond2, …, up to 5-6 conditions. All of them need ...
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527 views

### Recreating Poincaré section for a double pendulum

I am trying to recreate the Poincaré Sections for the double pendulum as described in this paper. I am unable to figure out a way to include energy values in the code. The code that I used until now ...
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146 views

### How do I create a Table from a For loop?

I've been trying to create a table that has the % of the cycle of a sample and the module of acceleration for the first 100 values, but I don't know how to create it. Here's the code: I have imported ...
303 views

### Join not working

I am trying to append elements to a list in Mathematica using the join function but it is not working. Here is a small working example: ...
328 views

### What Reap/Sow tricks do you use?

I admit that I have been reluctant to use Reap and Sow for years and that I used it only when I had no choice with ...
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### Using Reap Sow to define unsorted union [closed]

In the documentation of Reap under "Applications" paragraph there is this example: ...
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41 views

### What is the best memory utilization and efficiency considerations for simulating particles that are randomly created and removed?

I am wanting to run a simulation involving a varying number of "particles" (points defined by 2D coordinates $(x,y)$ within some region). At each time step, there is a probability that a new ...
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44 views

### Keep Reap / Sow results after breaking out of a loop [closed]

I am doing a long calculation where I create a list using Reap and Sow. However, sometimes this calculation gets too wild and I ...
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2k views

### What happens if you don't Reap what you've Sown?

As I understand it, when using Sow[expr] you throw the expr on some private stack which you can ...
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523 views

### Bifurcation diagram for Lorenz attractor

As I was following a similar question I came across a beautiful answer here. However, in that code, he used InternalBag[] which heavily depends on ram that I can't ...
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108 views

### Sowing Once into Each of Many Lists – Any Better than Append?

In general I know to try to use Reap and Sow over Append or AppendTo due to superior performance. However, Reap and Sow are notorious for being difficult to understand how to use and how they work ...
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39 views

### Sow and Reap returning different results for the same expression [closed]

Consider the following trivial example: ...
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1k views

### How do the functions Reap and Sow actually work behind the scenes?

I have read that using Reap and Sow to generate a list of unknown length is much more efficient than using AppendTo over and over. I believe AppendTo works by creating a new list that is one element ...
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67 views

### Can I parallelize this function?

Can I and should I parallelize this function ? my aim is to have faster calculations below is a toy function in which I kept the structure of the real function I am using. It uses ...
• 1,014
1 vote
72 views

### Getting columns (instead of rows) using Reap/Sow

In my program, I'm computing a vector inside a Do loop. The example below is an overly-simplified version, but the structure is the same (the original code can't be ...
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72 views

### Problems with NDSolve and initial conditions (NDSolve::nbnum1:)

this is the situation, I'm trying to solve a system (s1) with two differential equations, called "x[Ne]" and "y[Ne]". They have and initial condition that first need to satisfy ...
388 views

### How to pick consecutive numbers from list?

Question is very simple. If we have tst = {2,3,4,6,7,9,11} result must be {{2,3,4}, {6,7}, {9}, {11}} There are similar ...
• 1,729
80 views

### Why Reap and Sow is not working here [closed]

Here in the first example my code works: Reap[ For[x = 1, x <= 10, x++, f[x]; Sow[{x, f[x]}]]][[2, 1]] and I can plot $x$ vs $f(x)$. However, ...
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1 vote
170 views

### How to use Reap and Sow with ParallelDo

Consider the two cases: ...
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194 views

### Accessing sowed data during long computation

This question is very similar to the one posted here: Interrogating a running evaluation but my running computation consists of a backtracking procedure which sows valid solutions. It is hard to tell ...
• 1,580
42 views

### What is the meaning of [ ][[ ]]? [duplicate]

I have a question about these two pieces of code Reap[ For[i = 1, i < 10, i++, Sow[i]; Sow[i^2]; Sow[i!]; Sow[N@Log@i]]; ][[2, 1]] ~ Partition ~ 4 and <...
1k views

### How to Sow[] until I've Reap[]'d enough?

I have a process that returns an unpredictable number of data points, and I'd like to run it repeatedly until I have a certain number of points. My actual code is too complicated to use an ...
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1 vote
140 views

### How to assign output of Last@Reap to a variable? [closed]

I would like to define a variable that equals the solution of a numerical equation. Usually something like smax=zmax/.FindRoot[Exp[-zmax/2]==2,{zmax,0}] would work. ...
158 views

### I need an alternative to AppendTo using Reap and Sow

c = Table[RandomVariate[BernoulliDistribution[1/2], 3], 3] d = Total[c, {2}] list = {}; For[k = 0, k <= Length[d], k++, If[d[[k]] == 2, AppendTo[list, c[[k]]]]] ...
164 views

### Reap and sow for BreadthFirstSearch

The following Code highlights the breadth-first scan tree ...
• 1,688
125 views

### How to control which list Sow puts elements in

I'm trying to Reap/Sow elements of a list into separate sub-lists. In this toy example, I want evens in one list, odds in the other. But whichever sub-list d[[1]] happened to belong in, that sub-...
• 4,612
227 views

### Reap and Sow Tags. How to retrieve part of list belonging to specific tag? [closed]

I'm trying to improve my code's performance by using reap and sow instead of appendto and it works, but I hit a small obstacle. Since my code is building two lists simultaneously, I need to use sow ...
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2k views

### How to use Reap and Sow instead of Append to

I have matrix and and i want to do increment in a do loop and and i want to store in list using append to. It works fine for less values, But when i want to do it for values around 1 million the ...
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### Understanding Sow and Reap documentation

Both in Sow and in Reap documentation there is the example: ...
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1 vote
293 views

### WhenEvent, Storing Sow data, Loops

My problem is essentially to do with manipulating and storing Sow data in a nested loop structure. I have a set of differential equations describing time-evolution of a bunch of variables. ...
604 views

### Better definitions of Reap and Sow

I am having tremendous difficulty understanding Reap and Sow in Mathematica. The definitions are pretty circular. When I look at the definition of Sow, I get, "specifies that e should be collected by ...
116 views

### Poor performance from Manipulate and Sow-Reap

I am not a mathematician, and I barely know how to use Mathematica. But, over the course of many months and with help from this site awhile back, I managed to generate dynamic output that does exactly ...
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276 views

### Sow in WhenEvent can't extract parameter value of ParametricNDSolve

This is a problem I noticed when trying to answer this question. Just consider the following toy example: ...
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291 views

### How to keep building list until manually stopped?

I would like to run a numerical experiment for as much as it can run in a reasonable time. This is a sketch of how I currently do it: ...
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### Timing functions with Sow / Reap and AbsoluteTiming

I have a main function f[x] that calls three sub-functions: ...
• 1,641
1 vote
121 views

### Reap unpacks packed arrays

I noticed that Reap unpacks packed arrays. I've looked at previous answers but am still an unsure whether this a serious issue. E.g. consider Ex 47.3 from EIWL: ...
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807 views

### Writing Faster Mathematica Code - Sow and Reap?

I am applying Select function to a large list called xyCoordinateCentreCircle. The dimensions of the list are: ...
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508 views

### Why is Reap result nested? [closed]

A minimal (toy) example for my question: i= 1; Reap[Do[Sow[i = (i + 1)^2], {4}]] (* {Null, {{4, 25, 676, 458329}}} *) I presume the entire result is a ...
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### Reap, Sow with Parallelize: bad performance, why?

I have a question about the performance of Reap and Sow with Parallelize. I am aware of the following questions How to collect result continuously (interruptible calculation) when running parallel ...
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163 views

### Is Sow/Reap compatible with Composition?

I'm trying to figure out why these two pieces of code return different results. As expected, f[Reap[Sow[x]]] gives ...
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