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

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11
votes
2answers
193 views

Dispatch displays rules incorrectly and evaluates when displaying

Bug introduced in 10.0.0 and fixed in 10.0.1 Consider this a = 3; disp = Dispatch[Unevaluated[{a -> 2}]] We see the rule ...
1
vote
2answers
78 views

Why doesn't ToString work in ReplaceAll? [duplicate]

I'm trying to use ReplaceAll in a subscript. However, it doesn't seem to work as expected if I want to use the output the subscript in a ...
0
votes
0answers
34 views

Substitute all known values in matrix [duplicate]

There is probably a very simple way of doing the following, but I cannot find it. I have a an array of symbolic variables: coefficients = Array[c, 3] I then ...
2
votes
1answer
91 views

Limits and transformation rules

I can calculate this limit: Limit[(c^(1 - g) - 1)/(1 - g), g -> 1] and obtain the correct value: Log[c] but if I try ...
0
votes
1answer
68 views

What to use instead of ReplaceAll to replace a parameter with a number before the evaluation of an integral?

This is probably very simple but I don't see the solution now. Let us say I have a variable e defined as e=n ReplaceAll can subsitute n with value 3 using ...
5
votes
4answers
316 views
0
votes
0answers
116 views

Writing rules for Einstein summation

I'm trying to write a list of rules for tensor manipulations and in particular, Einstein summation convention. What I've tried, so far is to write something that would take a generic functions with ...
7
votes
1answer
236 views

Is there a faster way to Map an Association?

Consider mapping an existing Association in a manner such as this: ...
1
vote
2answers
187 views

Turn list of rules from radians to degrees, match all greek letters

Is there a easy way to match Greek letters only. I am trying to lower the angst of bachelors level students by keeping things as low level familiar as possible. Now understandably Mathematica counts ...
7
votes
2answers
113 views

Is Replace doing double work?

In an answer to this question, rm -rf used the function Replace with level spefication to do a series of replacements inside out. As far as I can see, the order of substitutions when more than one ...
3
votes
2answers
143 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
162 views

ReplaceImageValue and ReplacePixelValue extremely slow

I have a grey scale image: and I am doing the following: bin = Binarize[image, 0.18] ...
1
vote
0answers
53 views

How to substitute inside Hold? How to create table of summation (with Sigma symbol) expressions? [duplicate]

I would like to create table of summation expressions, which are not evaluated, but summation limits are substituted. I can't accomplish this so far. If I write I get summation expanded. If I ...
1
vote
0answers
94 views

Speed up replacement using Rule Dispatch Associtation

I knew that this has asked many times, but I hope there is something more could be done to speed this up. The expression kappa is here. ...
5
votes
2answers
120 views

Why does Rule behave like this?

Why does {x, x^2, x^3, a, b} /. x^n_ -> f[n] result in {x, f[2], f[3], a, b} when ...
3
votes
4answers
190 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
188 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 ...
2
votes
1answer
43 views

Substitution breaking the scope

A novice's problem, perhaps: it seems that /. substitution does weird things to the scope. ...
0
votes
2answers
172 views

Replace variables and redefine a function

I would like to redefine a function f(x)=1+2x^2 to a new function g(y)=1+2y by using the replacement ...
8
votes
5answers
295 views

Select a particular rules out of a list

rule = {beta -> 4, alpha -> 2, x -> 4, z -> 2, w -> 0.8} Select[rule, beta] Select[rule, beta &] Select[rule, beta -> _ &] How would I ...
2
votes
0answers
58 views

Drop all powers from multinomial [closed]

How to automatically replace an expression like x^3 y + x y z^2 by x y + xyz in Mathematica, i.e. replace all ...
0
votes
1answer
42 views

How to use rules in simplify this expression?

D[1/(1 + Exp[-x]), x] % /. {1/(1 + Exp[-x]) -> p} Suppose I know that the rule can be applied, but Mathematics just won't do it straight away as the expression ...
2
votes
2answers
123 views

Derivative of Vectoral NDSolve Interpolated Function / Rule Solutions,/., and Vector NDSolve [duplicate]

I need to find the instantaneous phase and instantaneous frequency for a rotating system. I am having difficulty making the math work with interpolating functions. First, here is a problem without ...
5
votes
3answers
384 views

Simple image processing very slow

I have compared the speed of some simple image processing routines between Mathematica an IDL. 1. Reading of a grey scale png image of (720,577) pixels: The image is: Mathematica: ...
6
votes
1answer
210 views

Better replacement rules to perform this change of variables?

In cosmology, it is common to work with both a normal time variable t and a “conformal” time variable τ, which is defined such ...
4
votes
4answers
136 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.: ...
1
vote
1answer
99 views

What's wrong with this tutorial? [closed]

Disclaimer: This code is not mine. Source. I'm trying to follow along with it, though, but it seems that this code no longer produces a working manipulatable plot at the end -- and I'd like to know ...
0
votes
0answers
53 views

How to solve ReplaceAll problem inside Manipulate

I'm running into some trouble when I try and use Manipulate (code included below). Instead of letting me manipulate the plot, I get an empty plot and this: ReplaceAll::reps: ...
1
vote
0answers
44 views

Question on Function in Replacement Operation

I am trying to create a replacement function which, depending on the input, gives an additional minus. In my example I have an undefined function a with two ...
2
votes
0answers
116 views

Is this a bug in NCAlgebra library?

In course of my work with NCAlgebra package for Mathematica, http://math.ucsd.edu/~ncalg/ , I encountered unexpected behavior of replacements. I am using ...
4
votes
2answers
132 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 ...
2
votes
3answers
513 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
1answer
149 views

replace RHS of rule

is there an elegant way of replacing the RHS of a rule with the value of some function of it. Say I have {{a -> 1.2}, {a -> 2.3}} and I want to replace ...
3
votes
1answer
124 views

How to program a clickPanel for chemical elements (like a periodic table)?

The command ColorData["Atoms", "Panel"] indicates RGBColors of "Atoms" by clicking a panel. As I don't want to reinvent the wheel, I asked myself: How to ...
15
votes
2answers
242 views

Why Infinity /. _?(# < 6 &) -> 0 replaces Infinity with ComplexInfinity?

Suppose I define a matrix: mat = { {Infinity, 4, 9}, {6, Infinity, 10}, {8, 9, Infinity} }; Now I want to replace all values smaller than ...
8
votes
1answer
230 views

Transformation rule on a list of non-negative integers

I am trying to create a transformation rule that takes a list of non-negative integer values of any length, finds a non-zero entry in the list, adds 1 to all preceding numbers, subtracts 1 from the ...
3
votes
1answer
114 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 ...
0
votes
0answers
37 views

How can I replace an expression by another

I'm trying to replace a expression like this: (s[e,f] - I*phi[e,f]) -> M[e,f] Can anyone help me?
1
vote
3answers
171 views

A rule replacement problem: list -> list

Suppose I have a vector function f[x_] := {x[[1]]^2 - x[[2]]^2, Sin[x[[1]]] + Cos[x[[2]]]}; Then I want to make a list of gradients at some values: ...
1
vote
2answers
86 views

replace two elements in a list with the sum of those two elements?

I have a list: G[3] = {2, 4, 2, 4, 2, 4, 2, 4, 2, 4} (there's a 3 in there for the 3rd Prime) Recursively defined by: ...
1
vote
0answers
130 views

Replace part in a matrix that matches specific pattern

I have the following problem. Given a matrix L which looks as follows, Matrix L is a link, route matrix. Further, I got the following vector (d), which indicates the demand per vertex combination. ...
1
vote
0answers
66 views

Understanding the order of applying Rule to expression [closed]

I am a little confused about: In what order is a Rule tried to apply to an expression? In a depth-first order or otherwise? For example, ...
1
vote
2answers
120 views

Replace list of symbols in an expression

I have an expression: exp = a+b+c and a list containing all the symbols of my expression: syms = {a,b,c} and I would like ...
6
votes
2answers
242 views

Apply Trig Identity to Transform Sum of Sinusoids into Phase-Shifted Cosine Function

I'm trying to solve for the output of a simple RC low-pass filter, with Cos[w t] input. The convolution of the input with the RC transfer function gives me the right output, but I'd like to be able to ...
30
votes
2answers
1k views

Best practice of passing a large number of parameters to functions

I have a number of functions that all take a large number of parameters. I am wondering what is the best practice of passing these parameters to those functions. I could, of course, simply specify ...
2
votes
3answers
79 views

How to get an element of a list , which is computed out of substituted variables?

What is the best way to get the element of a list, which is computed out of substituded variables? For example one wants to get the first element of the first row of ...
2
votes
2answers
51 views

replace an interval of values [duplicate]

I want to replace values in an interval [a,b] by 0 in a list of values. Example: {{.5,.2,.49,1,1},{.49,.5,.495,1}} I want to get: ...