Questions about Mathematica's rule-based replacement functionality, including functions such as Rule (->), Replace, ReplaceAll (/.) and ReplaceRepeated (//.).

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-2
votes
1answer
72 views

Transform differential equation to get equilibrium position

I would like to determine the equation in the equilibrium position. eq = 1/2 g l m Cos[θ[t]] == J (θ^′′)[t] ...
1
vote
2answers
110 views

How to select elements from a list by condition?

Considering I have a list: F = {f1, f2, f3, f4} I want to perform exponent on the list F = Power[#, 2]& /@ F Now I ...
0
votes
2answers
44 views

FilterRules with pattern for name [duplicate]

I have a list of rules and I want to filter it using a pattern for the initial string representing the name of the rule. So that in the example below it filters linearBool even if it is called ...
0
votes
1answer
53 views

How to speed up ReplaceAll in Table?

I am writing a code which solve a list of equations and then create a Table with resulst of equation solving. In applications where I use this code, I create a bunch of equations and my computations ...
0
votes
1answer
48 views

EdgeRenderingFunction For 2 Vertices in a Loop

Assume there is a set of rules: set = {"A" -> "B", "B" -> "A", "B" -> "C", "C" -> "D", "D" -> "A", "C" -> "C"}; which we want to graph using ...
1
vote
1answer
46 views
0
votes
0answers
106 views

A faster way to replace variables than ReplaceAll

I noticed that for large matrix, the rule /. is very slow. In the example below, the combination of memorization and the replacement "/.sol[n-1]" inside the definition of sol[n] turns the code ...
0
votes
1answer
73 views

Variable change

If I have the following Hamiltonian: ...
0
votes
1answer
46 views

Use of Replace function [closed]

I would like to replace in an expression two functions x[t] and D[x[t],t] by two variables x ...
0
votes
2answers
36 views

ReplaceAll with serveral variables. get all combinations

How can achieve something like this? a * b /. {a -> {1, 2, 3}, b -> {10, 100}} I want {10,100,20,200,30,300} or ...
9
votes
1answer
285 views

How can I replace the Greek symbols in an expression with their plain text names?

Suppose I have an expression with Greek symbols in it. How would I convert that expression to another expression where the Greek symbols have been replaces with their names in plain text? I've ...
0
votes
0answers
69 views

Replace Part of a matrix

I have to generate a ne numbers of matrices with 1 and 0 and then replace some elements. ...
1
vote
1answer
53 views

Don't understand why my Taylor expansion results in a message and an unexpected result

Trying to do something simple: Taylor expand a generic function of t around a point t and substitute ...
6
votes
3answers
201 views

Obtaining the number of iterations used in ReplaceRepeated

ReplaceRepeated has an option MaxIterations which indicates Mathematica is keeping track of the number of iterations. Is there ...
7
votes
1answer
113 views

Why does an inert ReplaceAll modify this Dataset?

I cannot see any reason for this behavior besides a bug, but I've been wrong before so I put it before the community: ...
3
votes
3answers
117 views

ReplacePart equivalent for Associations

I have a fairly complicated Association. I wish to use/create a function such as ReplacePart to operate on certain Keys within this multi-level Association. Here's ...
5
votes
2answers
88 views

Replace expression in series expansion [duplicate]

A test case: I'm trying to replace an expression inside a series expansion: Series[f[x],{x,x0,4}] ./ (x-x0)->h but it still returns ...
0
votes
1answer
44 views

Replace bunch of numbers by letter

I want to do following: ArcTan[1/Sqrt[-1 + (4 Sqrt[2] x^2)/(5 xc^2)]] /.4 Sqrt[2]/5->a^2 I expect to have ...
6
votes
5answers
124 views

Setting value of a rule

How do I change the value of a rule? For example, If I have a = x->3, how do I reset the value of x/.a to 4, so that ...
1
vote
1answer
50 views

Correct interpretation of /. (ReplaceAll)

Consider that you have a complicated expression, which is expensive in terms of running time and memory, and depends on some (initial) parameters. You want to test it for some simple values of the ...
11
votes
2answers
187 views

Dispatch displays rules incorrectly and evaluates when displaying

Bug introduced in 10.0.0 and fixed in 10.0.1 Consider this a = 3; disp = Dispatch[Unevaluated[{a -> 2}]] We see the rule ...
1
vote
2answers
65 views

Why doesn't ToString work in ReplaceAll? [duplicate]

I'm trying to use ReplaceAll in a subscript. However, it doesn't seem to work as expected if I want to use the output the subscript in a ...
0
votes
0answers
34 views

Substitute all known values in matrix [duplicate]

There is probably a very simple way of doing the following, but I cannot find it. I have a an array of symbolic variables: coefficients = Array[c, 3] I then ...
2
votes
1answer
87 views

Limits and transformation rules

I can calculate this limit: Limit[(c^(1 - g) - 1)/(1 - g), g -> 1] and obtain the correct value: Log[c] but if I try ...
0
votes
1answer
65 views

What to use instead of ReplaceAll to replace a parameter with a number before the evaluation of an integral?

This is probably very simple but I don't see the solution now. Let us say I have a variable e defined as e=n ReplaceAll can subsitute n with value 3 using ...
5
votes
4answers
312 views
0
votes
0answers
80 views

Writing rules for Einstein summation

I'm trying to write a list of rules for tensor manipulations and in particular, Einstein summation convention. What I've tried, so far is to write something that would take a generic functions with ...
7
votes
1answer
213 views

Is there a faster way to Map an Association?

Consider mapping an existing Association in a manner such as this: ...
1
vote
2answers
178 views

Turn list of rules from radians to degrees, match all greek letters

Is there a easy way to match Greek letters only. I am trying to lower the angst of bachelors level students by keeping things as low level familiar as possible. Now understandably Mathematica counts ...
7
votes
2answers
111 views

Is Replace doing double work?

In an answer to this question, rm -rf used the function Replace with level spefication to do a series of replacements inside out. As far as I can see, the order of substitutions when more than one ...
3
votes
2answers
136 views

Replace expression, but only “outside” of a function [duplicate]

I have a horrid expression, which is basically a sum of exponents with coefficients. Here's a taste: What I want to do is to replace ks with another expression, ...
3
votes
4answers
156 views

ReplaceImageValue and ReplacePixelValue extremely slow

I have a grey scale image: and I am doing the following: bin = Binarize[image, 0.18] ...
1
vote
0answers
45 views

How to substitute inside Hold? How to create table of summation (with Sigma symbol) expressions? [duplicate]

I would like to create table of summation expressions, which are not evaluated, but summation limits are substituted. I can't accomplish this so far. If I write I get summation expanded. If I ...
1
vote
0answers
76 views

Speed up replacement using Rule Dispatch Associtation

I knew that this has asked many times, but I hope there is something more could be done to speed this up. The expression kappa is here. ...
5
votes
2answers
116 views

Why does Rule behave like this?

Why does {x, x^2, x^3, a, b} /. x^n_ -> f[n] result in {x, f[2], f[3], a, b} when ...
3
votes
4answers
179 views

Question about ReplacePart

This is a question about ReplacePart but if there's a better that's great. If I have a list a= {1,2,3,4,5} and I want to replace elements ...
3
votes
3answers
186 views

How to Update 2D List based on a binary array

I have 3 lists: numbers = {{1, 2}, {3, 4}, {5, 6}}; toUpdate = {{1, 0}, {0, 0}, {1, 1}}; newValues = {{10, 20}, {30, 40}, {50,60}}; I have to update the list ...
2
votes
1answer
43 views

Substitution breaking the scope

A novice's problem, perhaps: it seems that /. substitution does weird things to the scope. ...
2
votes
1answer
72 views

Return the incomming expression on false

when I have an expression, that is unknown to mathematica, it is returned exactly the same, as it was called. For example SeeFlyingPigs[1,2] called on the line ...
0
votes
2answers
141 views

Replace variables and redefine a function

I would like to redefine a function f(x)=1+2x^2 to a new function g(y)=1+2y by using the replacement ...
8
votes
5answers
284 views

Select a particular rules out of a list

rule = {beta -> 4, alpha -> 2, x -> 4, z -> 2, w -> 0.8} Select[rule, beta] Select[rule, beta &] Select[rule, beta -> _ &] How would I ...
2
votes
0answers
56 views

Drop all powers from multinomial [closed]

How to automatically replace an expression like x^3 y + x y z^2 by x y + xyz in Mathematica, i.e. replace all ...
0
votes
1answer
42 views

How to use rules in simplify this expression?

D[1/(1 + Exp[-x]), x] % /. {1/(1 + Exp[-x]) -> p} Suppose I know that the rule can be applied, but Mathematics just won't do it straight away as the expression ...
2
votes
2answers
111 views

Derivative of Vectoral NDSolve Interpolated Function / Rule Solutions,/., and Vector NDSolve [duplicate]

I need to find the instantaneous phase and instantaneous frequency for a rotating system. I am having difficulty making the math work with interpolating functions. First, here is a problem without ...
5
votes
3answers
344 views

Simple image processing very slow

I have compared the speed of some simple image processing routines between Mathematica an IDL. 1. Reading of a grey scale png image of (720,577) pixels: The image is: Mathematica: ...
6
votes
1answer
206 views

Better replacement rules to perform this change of variables?

In cosmology, it is common to work with both a normal time variable t and a “conformal” time variable τ, which is defined such ...
4
votes
4answers
127 views

How to replace all symbols except several?

I'm doing some heavy symbolic calculations and for preliminary simple tests I need to replace all coefficients except chosen with 1, e.g.: ...
1
vote
1answer
96 views

What's wrong with this tutorial? [closed]

Disclaimer: This code is not mine. Source. I'm trying to follow along with it, though, but it seems that this code no longer produces a working manipulatable plot at the end -- and I'd like to know ...