# Questions tagged [sow-reap]

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

Consider the following trivial example: ...
• 4,777
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 ...
• 333
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### 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 ...
• 912
1 vote
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### 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 ...
• 37
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### 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 ...
255 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 ...
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### 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, ...
• 11
1 vote
104 views

### How to use Reap and Sow with ParallelDo

Consider the two cases: ...
• 6,174
171 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 ...
• 1,032
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### 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
108 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. ...
136 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]]]]] ...
129 views

### Reap and sow for BreadthFirstSearch

The following Code highlights the breadth-first scan tree ...
• 1,086
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### 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-...
• 4,361
191 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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### 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 ...
• 183
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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
224 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. ...
494 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 ...
104 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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238 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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283 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: ...
• 325
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### Timing functions with Sow / Reap and AbsoluteTiming

I have a main function f[x] that calls three sub-functions: ...
• 1,509
1 vote
115 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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### 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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### 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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158 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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