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

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4
votes
4answers
180 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
3answers
248 views

How to set multiple parameters to zero?

Say I have the matrix A = {{a,b,c,d,e}, {f,g,h,j,k}} and I want to set some of these to zero. I have been doing this via ...
4
votes
2answers
188 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
137 views

Alternative forms to Table for iterating over replacement rules

I have a multivariate polynomial x. I get coefficients of various monomials using CoefficientRules, which returns a list of ...
4
votes
3answers
229 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
229 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
166 views

Appending to sublists using “anchor” values

Say I have a set of lists that look like this: ...
4
votes
2answers
240 views

How do I encode this replacement rule?

I'm pulling a 5x5 array from a larger array. The small sub-array (call it S) can contain any combination of the symbols A, B or 0. What I'd like to do is: if the array contains any matching pairs ...
4
votes
4answers
1k views

Replace symbols with values without evaluation

If I have assigned values to a, b and c: a=7; b=5; c=6; ...
4
votes
2answers
139 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
128 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
292 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
1answer
131 views

How can one match a function in a replacement rule

I'm trying to write a delayed replacement rule that is capable of matching both the functions and their parameters, like ...
4
votes
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 ...
4
votes
2answers
66 views

Type error before replacement rule is applied

When I type a <> "0" /. {a -> "1"} I get the error ...
4
votes
2answers
172 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
131 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
899 views

Replace expressions by self defined symbols

I have a list of expressions like the following: ...
4
votes
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 ...
4
votes
1answer
140 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 ...
4
votes
2answers
374 views

Returning Replacement Rules in a function and using it later on

I would like a function which returns replacement rules as some internal mathematica functions are doing and use the results in other functions. I could not find any information on that and I'm ...
4
votes
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: ...
4
votes
3answers
106 views

Question about Replacements and String Manipulations

I'm trying to get better at Mathematica and in the book I'm using (by Nancy Blachman) is the following question (which apparently was taken from another book: The Art and Science of C) which reads: ...
4
votes
1answer
89 views

Better use of pattern matching in replacing elements of a list

I have a list of reals and I'm replacing those which are contained in a given interval with two different values depending on wheter the selected reals are positive or negative. I came up with this ...
4
votes
2answers
197 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]? ...
4
votes
2answers
178 views

Remove redundant parameters from equation

I have random expressions like (b1 x + b2 x)/(b3 + b4/b5 x) + Sin[b6 x] where $x$ is a variable and {b1, ..., b6} are parameters to be fitted. This expression is ...
4
votes
2answers
188 views

Replacing a function in defined expressions

I'm having a little trouble with substituting functions into an expression. A minimal example is as follows: ...
4
votes
2answers
587 views

Replace a column in table with a list of numbers?

I have a Table of values e.g. {{x,y,z},{x,y,z},{x,y,z}…} How do I replace the the "z" column with a List of values?
4
votes
1answer
151 views

Generalization of a rule to N arguments

I am trying to apply a series expansion on a function x[t1,t2,...tn], with an expansion parameter a. For ...
4
votes
1answer
125 views

Replacing a combination of terms

I have a quite simple question. I have an expression that looks similar to $T=\frac{1}{a_1}-\frac{1}{a_2}+1$ and I'd like Mathematica to use $A=\frac{a_1}{a_2}$ to reformat the expression (if possible)...
4
votes
2answers
63 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 ...
4
votes
1answer
388 views

Replace rule also matching complex numbers

I'm having the replace rule: rule = {-g_ x_^4 - 2 g_ x_^2 y_^2 - g_ y_^4 -> -g (x^2 + y^2)^2}; Now, ...
4
votes
0answers
82 views

Obtain a minimal set of parameters from a large number of rules

Problem statement I have a matrix $$D(\vec{k})=\sum_\vec{R}\tilde{D}(\vec{R})\cos(\vec{k}\cdot\vec{R})$$ where $\vec{R}$ is summed over all atoms positions and $\tilde{D}$ are 3x3 matrices. $D(k)$ is ...
4
votes
0answers
73 views

Why isn't SparseArray working in this case?

This works: SparseArray[{{1, 1} -> 1, {2, 2} -> 2}]\ (* {{1, 0}, {0, 2}} *) This works: ...
4
votes
0answers
92 views

Optimization of recursive functions by using the //. operator [duplicate]

In some cases, recursion in Mathematica (without storing values) is not very efficient. For example, the recursive implementation of Fibonacci numbers (version which doesn't store the values) : <...
3
votes
6answers
197 views

Change last column of a matrix

Given a (rectangular) matrix of integers, i.e. RandomInteger[4, {10, 3}] I want to apply the following rules only to the last column ...
3
votes
3answers
113 views

How to select the value of a substitution rule for a particular variable?

Say I have the following lists of rules: ...
3
votes
4answers
236 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
4answers
247 views

Replace all symbols in a matrix with their numbered forms

I have a matrix made mostly from "x" with a few 0's thrown in: ...
3
votes
4answers
331 views

How to find all symbols in an expression and perform an operation on them?

========== motivation =========== Suppose, for example, I have an incoming expression like m = (3-I)x + 4(x y)/(Cos[y]) I want to find all the symbols in ...
3
votes
3answers
274 views

Slow image processing using ReplaceAll

Can anyone explain me, why this line of simple code is so slow? And can it be improved somehow? ...
3
votes
3answers
202 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 <...
3
votes
4answers
209 views

ReplaceImageValue and ReplacePixelValue extremely slow

I have a grey scale image: and I am doing the following: bin = Binarize[image, 0.18] ...
3
votes
2answers
554 views

Using ReplaceAll to replace a head

Why would this work to replace the head b with newHead: ...
3
votes
2answers
378 views

How to replace subscript symbols

I have an expression with subscripted variables. I would like to replace all those terms with other symbols. For example, $P_1 + P_2^3 \rightarrow P1 + P2^3$ I used the following code but it doesn't ...
3
votes
2answers
505 views

Can I ask Mathematica to replace the arguments of my functions with a replacement rule?

I have an equation that I evaluate at some point (let's say $x=1$) that have terms of the form f[1,y] D[g[1,y],{y,2}] Is there an easy way to replace [1,y] by [x,...
3
votes
2answers
184 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, ...
3
votes
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 ...
3
votes
3answers
111 views

Operations on expressions within Hold [duplicate]

I want to convert {Hold[1 + 2], Hold[3 + 4], Hold[5*6]} into Hold[{1 + 2, 3 + 4, 5*6}]. I have tried ...
3
votes
3answers
1k views

Changing variables algebraically

Suppose one has two functions, $y(x)$ and $z(x)$, and one seeks to obtain $y(z)$ by substituting $x(z)$ into $y(x)$. Can this be done in a single step? Or must $z(x)$ first be inverted independently? ...