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

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2
votes
1answer
65 views

Add to the last digit a number

I have data from a Multimeter that has a precision of 0.6%rdg+4dgt. So i want to take the last digit of, say, 12.007 and add 4 to the last digit like ...
0
votes
1answer
51 views

Speeding up ReplaceRepeated while truncating to desired order

I need to program in an algorithm that recursively makes algebraic replacements which leads to an utterly complicated algebraic function of $x$, but whose final result is only needed at fixed order in ...
3
votes
3answers
181 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 ...
0
votes
0answers
77 views

Using one complicated equation to reduce another

(Relative noob here so don't assume much!) I need to use a vector equation (with $N-1$ entries) in the general form $y_j(x_{i,j},x_{i,j+1},x_{i,j-1})=0, i=1...3, j=1...N-1; x_{i,0}=x_{i,N}=0$, ...
9
votes
3answers
991 views

Replace expressions with symbols

First of all: I'm new to Mathematica, so I would appreciate it if the answers are quite complete. I have the result of calculation that is expressed in $\sin$ and $\cos$. Now, all of these can be ...
0
votes
0answers
27 views

How to prevent function from substituting the part of the parameter name if it has a subscript? [duplicate]

I have the following problem, when defining and using a function, that's supposed to have an outside parameter xmin: A[x_] := Exp[-x/Subscript[x, min]] A[5] ...
1
vote
3answers
136 views

How to ReplaceAll independent of depth?

I have a big list which in principle can be described as this more compact list : ...
5
votes
1answer
188 views

How can I make a “self-referential” replacement operation self-contained?

Fairly often I find use for replacement rules that call themselves on the right-hand side of the rule, e.g.: ...
0
votes
3answers
74 views

Unexpected behavior of rule-replacing with string concatenation [closed]

Here's the code. Clear[x,y,z]; {x,y,z} /. x_ :> ("^_^" <> ToString[x]) I get the result ^_^{x, y, z}, which is not ...
1
vote
1answer
77 views

How can I write a function which takes another function of a variable and the variable itself as arguments and gives a list of critical points?

This is an exercise from Power Programming with Mathematica by Wagner. I need a function which takes a function and a variable as arguments, like this: critpts[f_,x_]:= and then on the rhs I've got ...
3
votes
4answers
195 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
3answers
82 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 + ...
7
votes
3answers
157 views

Substituting values in 3j symbols

I believe these three expressions should give the same answer of -1/Sqrt[3], but the first gives +1/Sqrt[3] whereas the second ...
1
vote
1answer
95 views

Solving equations with QuantityVariable

I'm trying to understand how to easily solve equations that use QuantityVariable. Why does this fail? ...
5
votes
3answers
248 views

When I use Rule, why doesn't this work?

k.k.k //. {k.k -> k} works perfectly, so the output is just k. But, when I use ...
4
votes
3answers
86 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: ...
1
vote
2answers
60 views

How do I format expressions so that replacement rules correctly apply to exponents? [duplicate]

I have a series of rules that I would like to apply to a series of expressions with variables, but cannot get Mathematica to correctly recognize that these rules should apply when my variables have ...
2
votes
3answers
171 views

What is the order of faces on a D20? How can I optimize the order of faces on a D-n?

I am creating a customizable D20 for 3D printing using OpenSCAD, with the following code: ...
3
votes
2answers
219 views

Using ReplaceAll to replace a head

Why would this work to replace the head b with newHead: ...
7
votes
2answers
153 views

Probablity weighted ReplaceAll

I'm looking for an elegant way of doing a probability weighted replacement rule, something like this ...
5
votes
1answer
152 views

Is there a faster way to Map an Association?

Consider mapping an existing Association in a manner such as this: ...
1
vote
1answer
38 views

Automatic substitution of fractions

I have several equations with many variables, and here and there I would like to do a variable substitution. For example, in the equation: a (b + 42 c / d) I ...
1
vote
4answers
116 views

Replacing more than one element from a Sublist

The following expression allows me to extract the value of x and y: ...
0
votes
1answer
38 views

Replace all instances of Integrate by NIntegrate and add a Method option

I have an analytic expression which involves several integrals Integrate[f[x], {x, lb, ub}] where the functions f and bounds ...
3
votes
3answers
155 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 ...
5
votes
2answers
157 views

How to make Missing[] invariant for some functions without redefining them

Missing[...] "is a symbolic object, with no default evaluation rules defined." Missing data can be tagged and enables use of filters like DeleteMissing. So it's worthwhile to normalize various types ...
1
vote
1answer
161 views

Efficient way to give symbolic elements symmetry

I have a symbolic expression that represents a set of operators. These operators have quite a bit of symmetry to simplify expressions: ...
10
votes
2answers
165 views

Dispatch displays rules incorrectly and evaluates when displaying

Consider this a = 3; disp = Dispatch[Unevaluated[{a -> 2}]] We see the rule 3 -> 2, rather than ...
2
votes
3answers
165 views

Simplify system functions

Consider the code defining two constants: a=Cos[t] b=Sin[t] Suppose that we have another value in terms of Cos[t] and ...
9
votes
1answer
71 views

Using PatternSequence on its own in ReplaceAll?

Say I have this list: list = {0, 1, 2, 0, 1, 3, 0, 1, 4} and say I want to remove consecutive sequences of 0 and 1. I'd normally go for this: ...
9
votes
1answer
268 views

Performance of Dispatch and lists of Rules

The issue outlined here seems to be no longer present in version 10, if you use a Dispatch table! This because a Dispatch table ...
11
votes
3answers
229 views

Equivalent of ReplaceAll for Associations?

Is there a nice (built-in) way to apply Replace-style rules to key-value pairs in an association? Something like ...
6
votes
1answer
415 views

ReplaceAll inside an Iterator

I'd like to apply a set of rules to an expression defining the iterators of the table, like this: ...
0
votes
1answer
76 views

How can I represent this automorphism of Q(i,a)?

Let $a$ represent some real irrational number. I am trying to perform computations with an automorphism $f:\mathbb{Q}(i,a)\to\mathbb{Q}(i,a)$ which fixes $\mathbb{Q}(i)$ pointwise and maps $a$ to $i ...
1
vote
4answers
192 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 ...
8
votes
8answers
868 views

How can a power of 0 or 1 be replaced?

Consider this given example for ReplaceAll 1 + x^2 + x^4 /. x^p_ -> f[p] This returns ...
0
votes
2answers
99 views

how can tangent half angle substitution be done in Mathematica?

Is it possible to replace sine and cosine in terms of tangent half-angles and again the tangent half-angles by a variable say t?
2
votes
2answers
123 views

Looking for an elegant way to solve a (system of) ODEs/functional equations with undetermined coefficients

I want to solve an ODE using undetermined coefficients/guess-and-verify and am looking for an elegant way to use this technique. I am running into a few ugly warts. Here is a basic implementation for ...
1
vote
1answer
86 views

Nest Replace Function with different rules

I have a table of data that I want to filter. Any value of zero in the table means that value was not calculated, so it should get populated with the tag "NC". Next the table is filtered for all ...
2
votes
1answer
80 views

Table with some editable cells

I have a table: I want it to be editable only where the zeros appear. I read this post Generating an editable table that can be used as input because it's almost exactly what I want, but I couln't ...
4
votes
2answers
143 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 ...
18
votes
3answers
291 views

Options which require RuleDelayed

For some functions in Mathematica it is important to use RuleDelayed. One prominent example is the EvaluationMonitor for ...
6
votes
5answers
208 views

Lowering the degree of an polynomial with an assumption that the polynomial has a factor x^2+ax+b

Let's assume $ x^{10}+x^5+1 $ has a factor $x^2+ax+b$ Then, If $x^2=-ax-b$, $ x^{10}+x^5+1 =0$. If we successively apply $x^2=-ax-b$ to $ x^{10}+x^5+1 $, we can make $ x^{10}+x^5+1 $ to degree 1, ...
1
vote
2answers
70 views

Replacing a specific pattern of values at level 2 in a list

I've a list of lists, tl = {{True, False, m12}, {False, m22, m33}, {m32, False, True}} and each nested list has the same length, What is the proper way to ...
2
votes
2answers
67 views

Pattern evaluation in nested function call

I'll put the question right here, but will add some context below in case someone wonders why the heck I'm trying to do this. Why is it that I have to do two passes to get the value of the arguments ...
1
vote
4answers
75 views

Replace a sum of squared variables by a new variable squared

Is there any way to tell Mathematica to replace $\kappa^2 + k_z^2 \rightarrow k^2$ in an expression? I want to make a result more readable, e.g. something like this: $$\frac{2 k_z^3 \sqrt{k_z^2 + ...
1
vote
1answer
111 views

Matrix/tensor addition behaving funny under replacement?

I have two matrices I want to add, and one of the matrices is a tensor product of two vectors. I've used a SetDelayed to define the summed matrix, because I want to evaluate it for different values of ...
2
votes
3answers
378 views

How to manipulate nested expressions?

I have the following expression, only slightly complicated, and I'd like to do two things: Plot the elements of the lists (though messy, all are in the same ...
5
votes
2answers
107 views

ReplaceAll performance problem: packed arrays on the LHS are unpacked when the RHS is too long

The following is one of the most mysterious performance problems I encountered and came up while extending the booleval function mentioned here. I am looking for a ...
2
votes
4answers
173 views

Replacing High Densities of 1.s in Very Large Binary Arrays

I am working with very large binary arrays on the order of {100,100000}. It may look something like this. ...