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

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6
votes
2answers
88 views

Condition and RuleDelayed

On this site I found, for example here, some very interesting questions and answers on the use of delayed rules with conditions. I would very much like to understand these answers, but so far I failed ...
5
votes
3answers
771 views

Replacing Tan[x] with Sin[x]/Cos[x]

Why doesn't this work? (1 - Tan[x])/(Sin[x] - Cos[x]) /. Tan[x] -> Sin[x]/Cos[x] I get back: ...
5
votes
4answers
319 views

ReplaceAll is not working correctly

The line of code -E^(-I k) x /. {-x -> -x, -E^(-I k) x -> -E^(I k) x} is not properly replacing the exponential of (-I k) with the exponential of (I k), as ...
5
votes
5answers
418 views

Random character change in a random place

I would like to ask for help in a Random Replace of character. For example: ORANGE and change 1 character randomly (random place and random character from ORANGE) in it into any character of ORANGE, ...
5
votes
3answers
2k views

How to replace all occurrences of an element in a list?

Consider the following: list={1,2,2,2,3}; I would like to replace all 2 with the string "Test". ...
5
votes
4answers
327 views

Long list of similar rules

Given the following: ...
5
votes
4answers
285 views

How to split a list with respect to its nested sublists?

I have a problem with the following task: having some nested list list = {{k6},{k1,k3,k5},{k7}} I would like to replace all sublists by their permutations and ...
5
votes
3answers
456 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: ...
5
votes
3answers
164 views

How to replace an element in a list with an element in another list with matching indices

I have a list like this l = {1 + 2*s, 2 + 3*s, 4 + 1*s}, and I have calculated s = {1, 2, 3}. I need to replace ...
5
votes
2answers
349 views

Replace rule with function? Derivatives don't evaluate

Say I have an expression (call it expr) involving a function, f[x]. I'd like to be able to evaluate that for a particular choice of ...
5
votes
1answer
396 views

How to speed up calculations with large number of replacement rules?

In my program I have an expression which looks like: ...
5
votes
3answers
341 views

Replace x, but not x[a] [duplicate]

I want to replace all occurences of the symbol x with x[a], as long as it is not in the form ...
5
votes
3answers
873 views

Assign output of solve to variables [duplicate]

I have this input: Solve[x^2 + 3 x + 2 == 0, x] which gives this output: {{x -> -2}, {x -> -1}} I want the first x ...
5
votes
2answers
127 views

ReplaceAll performance problem: packed arrays on the LHS are unpacked when the RHS is too long

The following is one of the most mysterious performance problems I encountered and came up while extending the booleval function mentioned here. I am looking for a ...
5
votes
3answers
177 views

How to change symbols in differential equation with a single rule?

Problem Suppose I have a differential equation such as $\qquad f'(x)+f(x)=0$ and I want to change it into $\qquad y'+y=0$ My Method Using ...
5
votes
1answer
200 views

How can I make a “self-referential” replacement operation self-contained?

Fairly often I find use for replacement rules that call themselves on the right-hand side of the rule, e.g.: ...
5
votes
1answer
325 views

replace items in a list with items in a another list

Suppose I have a list A={x1,x2,x3}, a list B={3,4,5}, and a function f[x]=x1+x2+x3. Is there ...
5
votes
1answer
305 views

Embedding StreamPlot in Graphics3D

I am trying to visualise the action of the infinitesimal generator of a Lie group on the solution of a 2-ODE system drawn in the 3D space (t,y1,y2): t is time and y1,y2 are the two dependent ...
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 ...
5
votes
3answers
305 views

When I use Rule, why doesn't this work?

k.k.k //. {k.k -> k} works perfectly, so the output is just k. But, when I use ...
5
votes
2answers
191 views

BezierCurve in EdgeShapeFunction fails when used on multiedges graph

I would like to create a network graph with curved edges. To do this, I wrote this function: ...
5
votes
1answer
212 views

Replace rule does not match

I have the following expression -3 I Ez Re[Ex] + 3 I Ex Re[Ez] And I wish to write this in the form ...
5
votes
3answers
73 views

How can I pass replacement rules to a package-defined function and have it understand their context?

I am writing a package where I would like to be able to pass replacement rules to a function inside the package and have the function manipulate its symbols using those replacement rules. However, I'm ...
5
votes
2answers
101 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 ...
5
votes
1answer
193 views

Simplifying a b = 1 in non-commutative products?

Yesterday, I try to define some rules for non-communicative multiplication, here. I find some quite helpful rules, but things still do not work properly; the distributive rule can't be applied (e.g.: ...
5
votes
1answer
570 views

Apply color function to matrix plot

I want to apply the following color rules to a large table of numbers. I have extracted a part of the table to use as an example. I would like to assign 5 different colors to 5 different ranges of ...
5
votes
1answer
384 views

Why does this pattern for combining two lists require triple underscores (___) to work?

I understand that __ is a list of one or more elements, and that ___ is 0 or more elements, but when I try this rule with 2 ...
5
votes
1answer
213 views

Generating replacement rules programatically

I would like to generate sets of replacement rules programatically for predefining some permutations. So a function ...
5
votes
1answer
856 views
5
votes
2answers
252 views

How to “ReplaceAll” using a set of conditional expressions instead of a list of rules [closed]

Suppose that I have two linear functions f[x_] := f0 + f1 x g[x_] := g0 + g1 x and a (possibly rather complicated) set of conditional expressions, obtained ...
5
votes
1answer
164 views

How can I substitute something using only map?

For example, I have the equation x^2 + y^2 == x*y and I want to apply the rule y -> s*x I can do it easily by ...
5
votes
2answers
140 views

Trouble with Replacements [duplicate]

I have the following line of code: Plus @@ Table[p[x], {x, 0, 20}] /. p[x_] -> Boole[MemberQ[{0, 5}, x]] The first part of this produces ...
5
votes
1answer
79 views

How to transform abstract finite group to permutation group?

It seems that Mathematica only has group functionality for permutation groups? Then there is a step to transform the abstract finite one to a permutation one. As an example, consider the following ...
5
votes
2answers
1k views

Speeding up mathematica by subsitituting numerical values

According to this Wolfram Blog post, one can speed up Mathematica code by substituting numerical values as soon as possible. How does one substitute before the main expression is evaluated? The ...
4
votes
4answers
173 views

Rearrange graph

I have to rearrange graph described by j = {1 -> 2, 2 -> 3, 3 -> 1}; appling some function to all vertices on the right so rearrange[j] output should ...
4
votes
5answers
170 views

Converting set of characters into numbers using list of replacement rules

I am trying to convert a text, as an example "aaaa" into a set of numbers as defined in a list of replacement rules, for example letterstonum . letterstonum takes in letters a to z and returns the ...
4
votes
2answers
183 views

Confusing ReplaceRepeated & Hold

I've been bootstrapping myself to the very alien world of Mathematica and there came my first WTF moment: ...
4
votes
3answers
206 views

Replace elements in matrix to make it symmetric using automated procedure

I am generating a matrix using (mat2 = Array[Subscript[a, ##] &, {4, 4}]) // MatrixForm but would like to force Mathematica to make it symmetric. Thereby I ...
4
votes
3answers
214 views

Simplifying expression with patterns (Replace)

Q1: I need to simplify some algebraic expression, according to particular pattern, namely: \begin{equation*} x^m = \left\{ \begin{array}{ll} 1 & \text{if } m \text{ is even} \\ x & \text{if ...
4
votes
3answers
165 views

Appending to sublists using “anchor” values

Say I have a set of lists that look like this: ...
4
votes
3answers
1k views

Replace function and its derivatives found with Solve

If I have an equation eq = f[x] + g[x] == 0 and I solve for f[x] sol = Solve[eq,f[x]] ...
4
votes
4answers
978 views

Replace symbols with values without evaluation

If I have assigned values to a, b and c: ...
4
votes
2answers
130 views

ReplaceRepeated as a Looping construct?

There are a lot of discussions about looping constructs (notable example here). Wolfram language guides, as usually, contain most relevant links. Looping constructs guide has the following part on ...
4
votes
1answer
103 views

Substitute a function's definition for its name

I'm having trouble with a function which is supposed to work on another function. Simplified version: ...
4
votes
2answers
288 views

Creating a Nearest function programmatically

I've been trying to work out how to create a Nearest function programmatically. My goal is to produce something similar to this, a hand-assembled function: ...
4
votes
2answers
64 views

Type error before replacement rule is applied

When I type a <> "0" /. {a -> "1"} I get the error ...
4
votes
2answers
163 views

Replacing one element of list

I have a list of sequences of x's and y's. I want to switch the last position to the "opposite" symbol - if ...
4
votes
1answer
125 views

How can I avoid this ReplaceAll trap? [duplicate]

I am having some difficulties with the ReplaceAll function. When I write: {1, 2, 3, 4, 5} /. x_?OddQ -> x^2 I expect: ...
4
votes
2answers
841 views

Replace expressions by self defined symbols

I have a list of expressions like the following: ...
4
votes
1answer
118 views

How to transform a list (of sums) into list of summands

Suppose you have the list list={a,Sin[b+c],d+c}. What I want is to transform all elements of list with head ...