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

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4
votes
2answers
147 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
2answers
651 views

Replace expressions by self defined symbols

I have a list of expressions like the following: ...
4
votes
4answers
111 views

How to replace all symbols except several?

I'm doing some heavy symbolic calculations and for preliminary simple tests I need to replace all coefficients except chosen with 1, e.g.: ...
4
votes
2answers
296 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
3answers
89 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
2answers
89 views

Creating functions from output of other calculations

Apologies in advance if the title is vague, I'm not really sure what to call this. I have a function (call it 'foo') that generates a largeish polynomial, and it is natural to make the variables be ...
4
votes
2answers
100 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
140 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
1answer
113 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 ...
4
votes
1answer
297 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, ...
3
votes
6answers
168 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
4answers
197 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
159 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
3answers
167 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
130 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
229 views

Using ReplaceAll to replace a head

Why would this work to replace the head b with newHead: ...
3
votes
2answers
240 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 ...
3
votes
2answers
117 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
4answers
440 views

Replace symbols with values without evaluation

If I have assigned values to a, b and c: ...
3
votes
3answers
86 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
972 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? ...
3
votes
1answer
107 views

obtaining real roots of negative numbers in a long expression [duplicate]

I have a complicated symbolic expression which contains many terms like $(a/(a-2))^{1/m}$, where $a/(a-2)$ could be positive or negative after replacement. I only need the real root after ...
3
votes
1answer
171 views

Shortening the syntax {var1, var2} /. # & /@ list

I noticed that I often turn out to have a list of substitutions in the form ...
3
votes
1answer
150 views

Set promotes Rule to RuleDelayed?

In mentally building a model of how Mathematica works, I naively expected Set to create a Rule in a symbol's ...
3
votes
2answers
173 views

Efficient way of setting up a rule

I tried to define a simple rule defining how λ acts on ψ[n]: myrule1 = λ ψ[n_] -> α[n + 1] ψ[n + 1]; The result I get is ...
3
votes
1answer
193 views

Pattern matching a multivariate derivative

I can extract the parameters of a single variable derivative with the rule D[f[x], x] /. Derivative[order_][fcn_][variable_] -> {order, fcn, variable} ...
3
votes
4answers
118 views

How to Remove Undirected Edges that are the Same [duplicate]

Given: theData = {1 <-> 2, 2 <-> 3, 2 <-> 1} I would like to remove any duplicates where I define a duplicate to be: ...
3
votes
1answer
63 views

Replacing multiple instances of a symbol with different variables and assignments

I have an expression of the form: expr=x+y+n+Sin[n]+n^2 I would like to replace each of the variables n with a different ...
3
votes
1answer
89 views

some confusion about ReplaceRepeated

This is Right. NestWhile[Log, 100, # > 1 &] {100} //. {x_} /; x > 1 :> {Log[x]} 100. //. x_ /; x > 1 :> Log[x] This is wrong,why? ...
3
votes
1answer
172 views

Is there a way to simplify this replacement rule

I have a rather big expression which contains many sums of form q x + w y. I know that y is much smaller than ...
3
votes
1answer
112 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: ...
3
votes
3answers
156 views

Multivariate Polynomial Manipulation

I have a large homogenous multivariate polynomial in, say, 5 variables $a,b,c,d,e$. As an example take the polynomial $$a^4+2abcd+a^2 b^2+e^4+cde^2.$$ Now I would like to replace $k$-th power of any ...
3
votes
2answers
120 views

Replacing Patterns In a List of Varying Length

I am working with an array of data that may look something like this. ...
3
votes
3answers
470 views

StreamPlot using NDSolve gives error messages

StreamPlot[{x[0], x'[0]} /. NDSolve[{x''[t] == 1, x'[0] == v0, x[0] == x0}, {x, x'}, {t, 0, 1}], {x0, -2, 2}, {v0, -2, 2}] The above code gives me the errors ...
3
votes
1answer
87 views

Why does the replace all (/.) evaluate differently in plot

I'm plotting this function: Plot[(2 (l/2 π)^2 ((2 π f)/c)^2 /. {f -> (f 10^9), l -> .01, c -> 3 10^8}), {f, 17.5, 26.5}] but it does not plot ...
3
votes
3answers
204 views

Transformation of Derivatives under change of coordinates [duplicate]

I have an expression depending on the two variables z and t, and the derivative of a known function $f(t,z)$. Something like: $$myexp \ =\ t\ z\ \partial_z\ f(t,z)$$ Now I want to make a change of ...
3
votes
1answer
231 views
3
votes
2answers
151 views

truncating terms in symbolic expression basing on their values

I need to simplify big symbolic solution of equations system by truncating terms that are orders of magnitude less then others in subexpressions like a + b. The solution is big fraction with sum of ...
3
votes
1answer
259 views

Replace elements in a list

Suppose that I have a list: ...
3
votes
2answers
408 views
3
votes
3answers
83 views

Replacing a sum of expressions

I have an equation which Mathematica solves but gives a huge output. However I notice in this output that the same expression occurs many times. Namely: $E=864 a^6 - 432 a^4 b^2 + 54 a^2 b^4 - b^6 + ...
3
votes
1answer
54 views

Select a Placeholder in a Notebook?

How might I select the 1st Placeholder in a Notebook? SelectionMove Has no option. ...
3
votes
1answer
45 views

Table doesn't replace inside conditionals (If, Which) [duplicate]

This: Table[If[cond, y], {y, 1, 3}] outputs this: {If[cond, y], If[cond, y], If[cond, y]} The ...
3
votes
1answer
69 views

Have transformation rules to be explicitly set?

I think this problem is not the hardest one, but I can't hit on it. I want to NSolve an equation. For this equation I use a transformation rule "ampTimes". First I used it like this: ...
3
votes
0answers
64 views

Why doesn't ReplaceAll work on Image? [duplicate]

ReplaceAll seems not to reach inside Image, and I don't see why it shouldn't. Here is an example. Create a black and white image ...
2
votes
3answers
193 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? ...
2
votes
3answers
488 views

Replace all integers by 1?

Let list1 be a list of variables of the form {c1 a, c2 b, ...} where $a,b,...\in\mathbb{C}$ and $c1,c2,...\in\mathbb{Z}$. I am ...
2
votes
3answers
88 views
2
votes
2answers
180 views

Create Table of replacement rules

I want to create a table of replacement rules. ...
2
votes
2answers
494 views

How to do substitution of variable with power in Mathematica?

I want to replace $x^{i+1}$ with $z_i$. EDIT: I have some latex equations, and wish to import them to MMa. How to replace x_i (NOT $x_i$) with $x[i]$ But it seems the underscore "_" has a special ...