# Tagged Questions

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

2answers
89 views

### Need explanation on strange behavior of Do together with AppendTo

We didn't expect a replacement of B. ...
2answers
147 views

1answer
35 views

### 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 : ...
2answers
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 ...
1answer
59 views

### Generating lists of data with ReplacePart

I have the following example: ...
1answer
109 views

### 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: ...
0answers
52 views

### 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 ...
2answers
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 ...
2answers
123 views

### 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: ...
0answers
28 views

### Symmetric properties of KroneckerDelta under replacement

The consider the following code: ...
2answers
76 views

### 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 ...
2answers
42 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 ...
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. ...
1answer
66 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 ...
1answer
77 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? ...
1answer
57 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 ...
1answer
91 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 ...
2answers
300 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: ...
1answer
53 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 ...
2answers
100 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 ...
1answer
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 ...
2answers
73 views

### 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: ...
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. ...
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 ...
2answers
105 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 ...
2answers
121 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 ...
0answers
25 views

### Simplifying self-reciprocal rational function [duplicate]

I have trouble transforming self-reciprocal rational functions in one variable like ...
3answers
104 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 ...
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 ...
1answer
194 views

Executing: ...
1answer
49 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,....
0answers
34 views

### Replace variable with other variable containing a constant in differential expression [duplicate]

I have the following differential expression: ...
3answers
125 views

### Symbolic differentiation [closed]

I have two symbolic differential equations and I want to used them in another equation as follows: ...
3answers
239 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 ...
0answers
91 views

### Check an Option without Evaluation

Consider the following: ...
1answer
46 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 $Ex^2+Ey^2$....
2answers
119 views

### 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 ...
0answers
59 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 ...
1answer
72 views

### Replacement producing unexpected result

If I use a replacement rule like ...
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 ...
1answer
177 views

### 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: ...
1answer
121 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, ...
5answers
682 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 ...