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

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2answers
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Replacing symbol with vector

Consider example: expr := {-1, 1} * value expr /. {value -> {10, 20}} (* {{-10, -20}, {10, 20}} *) however, I expect to have something like ...
3
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2answers
39 views

Setting initial conditions of a domain grid?

So I'm writing some code to try and produce a cellular automaton which will model an oscillatory chemical reaction, but I'm trying to set up the initial conditions of the grid so that I can vary them ...
1
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0answers
36 views

replace a^2 and b^2 with a different rule to ab [closed]

I have an expression and need to replace all instances of $x^2$ with $a$, all instances of $y^2$ with $b$ and all instances of $xy$ with $0$, but I only know how to replace $x$ or $y$ individually. ...
0
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1answer
52 views

Filter pattern from a list

I have group elements, that I'd like to condense down according to group rules. Take the modular group. This has generators $S,T$ that satisfy the groups relations $S^2 = I, (ST)^3 = I$. So given a ...
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1answer
52 views

Rule for multiplying of Kronecker-delta-type functions

I am trying to introduce a rule of multiplication of entries of a matrix which would have the following property: delta[k,i]*delta[k,j]=delta[k,i]*delta[i,j] and ...
1
vote
1answer
48 views

How to substitude a^n->D[u[t,x],{x,n}] for n=0,1,2,

I have expression which have terms a, a^2, a^3, ..., How can I replace a^n -> D[u[t,x],{x,n}] for n=0,1,2,....
1
vote
1answer
76 views

Why does using replacement rules give a different result than setting the variable? [duplicate]

These two methods should give the same result, but one terminates when the result is unchanged (to machine precision) while the other continues for the full 100 allowed cycles. Why? ...
7
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1answer
169 views

Remove leading and trailing zeros from a list

This functions and gives the desired result: {1, 0, 2, 0, 0} /. {x___, 0 ...} :> {x} {1, 0, 2} But with leading zeros it doesn't: ...
0
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1answer
56 views

Issue with replacement rules screwing up a matrix [closed]

I am pretty new to using Mathematica as more than a calculator. Essentially my problem is that I am trying to calculate a quantity vector quantity $x=x_0 - A^{-1}F(x_0)$ where $x_0$ is a 3-entry ...
4
votes
1answer
88 views

Why a part of this expression, similar to another, is left unchanged?

An expression has one of its part replaceable and one, strictly similar, that isn't. Please, can you point at the difference ? I can't pinpoint the complication ... The FullForm of ...
12
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2answers
287 views

why MatchQ is fast and Replace slow?

Consider a large expression, say a polynomial with 25 terms expr=Product[Unique["a"],{i,1,25}]; to which you apply the following replacement rule: ...
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2answers
87 views

Symbolic derivatives and substitution [duplicate]

I have troubles substituting functions when I have symbolic derivatives and I need to substitute more symbolic derivatives in my expression. Take for example ...
5
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3answers
180 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 ...
7
votes
1answer
118 views

Using two rules for the same variable

I am a newbie in the field so please not angry. I am reading the tutorial below and having a problem. Link: How to Create and Use Rules From the link: If you give two rules for the same ...
2
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2answers
70 views
3
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1answer
50 views

Replacing elements of a matrix

I'm trying to write a program that takes a given matrix $A$ of 1's and 0's and flips a designated amount of 1's to -1's. ...
3
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2answers
69 views

Assigning multiple solutions different replacement names

Let's say we are solving a homogeneous differential equation, such as $$y^{\prime\prime} + a y^{\prime} + b y = 0$$ with characteristic equation $$s^2 + a s+ b = 0.$$ Using the ...
6
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2answers
96 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 ...
8
votes
2answers
120 views

Why is a longer pattern matched rather than a short pattern?

I was reading the Mathematica V5 Book and am a bit perplexed on the behavior of a particular pattern matching example where it seems to be choosing a longer pattern rather than a shorter pattern. The ...
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0answers
25 views

Simplifying self-reciprocal rational function [duplicate]

I have trouble transforming self-reciprocal rational functions in one variable like ...
5
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3answers
95 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 ...
1
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1answer
62 views

how to replace one calaculation result to another one in a loop?

I am doing the calculation in a loop. I gain a digit for rzin in ...
14
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3answers
230 views

ReplaceAll with Associations instead of Rules

I have just noticed that x /. <|x -> 123|> evaluates to 123 In other words, the association seems to behave as a ...
1
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3answers
125 views

Symbolic differentiation [closed]

I have two symbolic differential equations and I want to used them in another equation as follows: ...
1
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1answer
38 views

ReleaseHold NIntegrate after Replace variable

I would like to calculate the sum of square of two integral(Ex and Ey). I first hold this two integral because two variables(tx, ty) are not specified.Then, I replace tx,ty and releasehold ...
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0answers
57 views

How do I use Replace All? [duplicate]

I have an expression: a + b 2^(1/2) + (c + d 2^(1/2))*I I want to replace each occurrence of 2^(1/2) with ...
2
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1answer
71 views
3
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1answer
34 views

TensorContract with KroneckerProduct instead of ordinary Times

Is it possible somehow to contract two tensors, say A={{a1,a2,a3},{a4,a5,a6}} and B={b1,b2,b3}, where the elements ...
9
votes
1answer
172 views
2
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1answer
120 views

Function not returning number [closed]

I am quite new to Mathematica and I guess this question is really basic, but I can't figure it out myself. I'm making a simple notebook that plots a Hamiltonian function, a plane of constant energy, ...
6
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4answers
624 views

How to replace a space by a comma

in this expression = "{1 2 3 4 5 6 7 8}", I want to replace each space by a comma , to get ...
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0answers
56 views

Can't operate on vector within a list of rules

I'm trying to set up a list of rules which includes the following: ...
0
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2answers
44 views

Unevaluated derivatives of known functions after replacement

I want to make the following replacement work correctly: Array[(2 D[xM[[#1]][t], t] * D[xM[[#2]][t], t]) &, {3, 3}] /. {xM -> {Sin[ω t], Cos[ω t], 0}} ...
5
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2answers
450 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 ...
0
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0answers
36 views

Replace function and its derivatives [duplicate]

When I have a complicated expression that depends on some variable, say, $f$, the following usually works ok: Expression ./ { f-> x^2 } But if $f$ is a ...
6
votes
3answers
888 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: ...
0
votes
1answer
94 views

How to make this particular replacement involving exp

I apologize if similar questions have already been asked, but I have some difficulties with replacements in Mathematica and I would like to make a substitution like ...
0
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1answer
44 views

Solving a trigonometric equation for all $x$ using constant coefficients

I want to find the coefficients $a$, $b$, $c$, etc. that make an equation like $a+bx=x$ true for all $x$, but for trigonometric functions of $x$. For example ...
4
votes
2answers
58 views

Composition of rules: a->b + b->c = a->c

I am trying to compose two rules to form a new one: {a -> b, b -> d} and {b -> c} should become ...
3
votes
1answer
67 views

Convert an expression to use a specific analytic form

I have an expression that evaluates to an expression containing multiple ExpIntegralEi expressions. However, I would prefer that Mathematica use ...
3
votes
0answers
124 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 ...
11
votes
6answers
371 views

Fill out blanks with a upcoming number in a list?

I have a bunch of arrays looking like this: {"", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", ...
4
votes
2answers
135 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 ...
1
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1answer
43 views

Pattern matching for product of two terms fails if one factor is 1

I have a long set of replacement rules that I utilize to solve some longish integrals over trigonometric functions. Just look at this snippet (from this answer): ...
14
votes
4answers
400 views

A replacement problem

list = {1, x, 3, x, 4, x, x, 3, 4, 3, x, 2, 3, x, 8, 7}; If there is exactly one integer between two x I want to replace it with y: ...