# Tagged Questions

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

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### Replacement of a group of variables in trigonometric expression

I would like to make simplification of trigonometric expressions by using a replacement a group of variables. Here the equation I have to simplify : ...
105 views

### How to put replacement rules into a package

I have a replacement rules list, but its quite long and therefore I would like to make a standalone package of it. I tried but haven't succeeded. A simple example of my approach is ...
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### Generating lists of data with ReplacePart

I have the following example: ...
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### How to make a postordered ReplaceAll that operates just once per subtree?

The answers to this question provide code for a function I'll call postOrderReplaceAll to replace from the inside-out: ...
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### A kind of function optimization problem [duplicate]

I have the following problem. Out of a calculation I got the four functions shown at the end of the post. These functions depends on many variables, qk, pk etc (all of them are actually scalar ...
63 views

### Using a rule to increase precision [closed]

I'm sure this is a very simple question, but I can't work out how to do it. Construct a rule to set x=0.32 to a precision of 20 digits. The rule is to be applied ...
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### Why doesn't FullSimplify find simpler expression given a set of transformation rules?

I've defined a set of rules, which I want to use with FullSimplify, as follows: ...
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### Symmetric properties of KroneckerDelta under replacement

The consider the following code: ...
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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 ...
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### 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 ...
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### 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. ...
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### 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 ...
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### 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? ...
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### 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 ...
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### 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 ...
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### 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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### 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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### 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 ...
123 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 ...
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### Rewriting the set of equations in terms of new variables, using replacement rules when the expression contains partial derivatives [duplicate]

I am working with cumbersome equations, that contain a lot of terms. Here I will consider simple example: ...
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### 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. ...
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### 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 ...
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### 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 ...
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### 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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### Simplifying self-reciprocal rational function [duplicate]

I have trouble transforming self-reciprocal rational functions in one variable like ...
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### 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 ...
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### 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 ...
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Executing: ...
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### 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,....
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### Replace variable with other variable containing a constant in differential expression [duplicate]

I have the following differential expression: ...
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### Symbolic differentiation [closed]

I have two symbolic differential equations and I want to used them in another equation as follows: ...
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### 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 ...
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### Check an Option without Evaluation

Consider the following: ...
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### 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 $Ex^2+Ey^2$....
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### How to force a replacement in an order that I want?

This is a follow-up to this question. I have a big sum, with lots of terms of the form ...
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### 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 ...
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### Replacement producing unexpected result

If I use a replacement rule like ...
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### 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 ...
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### What kind of patterns in a list of rules would benefit from being Dispatched?

I have a long list of rules for manipulating expressions. They are of the form: ...
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### 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, ...