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

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0
votes
1answer
45 views

Force ReplaceAll to output vector of same length

I would like to do a replace in an expression... a = t; b = 1; a/.t->Range[1,5] (*returns: {1,2,3,4,5} *) b/.t->Range[1,5] (*returns: 1 *) but I ...
-3
votes
1answer
107 views

Transform differential equation to get equilibrium position [closed]

I would like to determine the equation in the equilibrium position. eq = 1/2 g l m Cos[θ[t]] == J (θ^′′)[t] ...
0
votes
2answers
64 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 ...
3
votes
0answers
115 views

Define (conditional) substitution rule for sum/difference of radicals

I want to tell Mathematica to simplify some expressions applying the following substitutions, conditioned on the fact that $a\geq 0,b \geq 0, a^2-b\geq0 $ $$ \sqrt{a+\sqrt{a^2-b}} \pm ...
4
votes
2answers
175 views

Trying to get Arg[1 + I a] -> ArcTan[a]

Can Mathematica evaluate Arg[1+ I a] when a is a positive real in order to get ArcTan[a]? ...
0
votes
1answer
66 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
62 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
87 views

Replacing Logic operator List and generate combination of variables

so I'm trying to do the following starting from a list of inequalities: ...
1
vote
0answers
143 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
78 views

Variable change

If I have the following Hamiltonian: ...
0
votes
1answer
69 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 ...
1
vote
2answers
183 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
42 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
397 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 ...
1
vote
1answer
77 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 ...
1
vote
0answers
125 views

Replace part of a matrix

I have to generate a ne numbers of matrices with 1 and 0 and then replace some elements. ...
6
votes
3answers
254 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
126 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: ...
0
votes
0answers
75 views

Creating a lazy evaluator of the lambda calculus

I have a sub project in my research to write a lazy evaluator of the lambda calculus. How should I approach this problem? I already have a implementation of the lambda calculus in another language. ...
5
votes
2answers
102 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
54 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 ...
3
votes
1answer
82 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 ...
6
votes
5answers
152 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
2answers
97 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 ...
2
votes
1answer
125 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
78 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
327 views

Long list of similar rules

Given the following: ...
11
votes
6answers
405 views

For a given expression: if it appears, remove it, but if it is absent, add it

While reformatting Szabolcs's code from (42660) I noticed this interesting operation: ...
0
votes
1answer
147 views

Replacement of terms/Pattern matching involving products of derivatives of a function

In delving into Ramanujan summation, I'm trying to get a hold of the relations of the form $$\sum_{n=0}^\infty f(n)=\dfrac{h\frac{d}{dx}}{{\mathrm{e}^{h\frac{d}{dx}}}-1}\int_{0}^\infty ...
0
votes
0answers
216 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
2answers
124 views

Why does ToRules return a Sequence?

Why does ToRules return a Sequence expression? Why doesn't it return directly what ...
1
vote
2answers
223 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 ...
3
votes
2answers
162 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, ...
6
votes
2answers
187 views

How can you resize all the images in a notebook section?

I've pasted a large number of photos and they are too big which makes scrolling hard. Is there a way to automatically have Mathematica size them to 10% or something reasonable without have to click on ...
1
vote
0answers
70 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
129 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
123 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
213 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
197 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
87 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 ...
9
votes
5answers
344 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
58 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
2answers
326 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 ...
0
votes
1answer
58 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
154 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 ...
3
votes
3answers
132 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
3answers
458 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
228 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 ...