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

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2
votes
2answers
59 views

Trouble simplifying using Replace

I have the following expression: w.a.b.r+w.c.d.r I want to get: w.(a.b+c.d).r So far I used ReplaceRepeated: ...
2
votes
2answers
93 views

Replace elements in some region in a list

I want to replace some elements in list. Assume that we have A={{1,2,3,4}, {5,6,7,8}, {9,10,11,12}, {13,14,15,16}} and we want to replace elements ...
25
votes
3answers
2k views

Extracting values from nested rules in JSON data

I have been using Mathematica to analyse some data from the StackExchange API. It is conveniently available in JSON form, which Mathematica interprets as replacement rules. However, some of the rules ...
3
votes
1answer
73 views

Correct interpretation of /. (ReplaceAll)

Consider that you have a complicated expression, which is expensive in terms of running time and memory, and depends on some (initial) parameters. You want to test it for some simple values of the ...
5
votes
4answers
277 views

ReplaceAll is not working correctly

The line of code -E^(-I k) x /. {-x -> -x, -E^(-I k) x -> -E^(I k) x} is not properly replacing the exponential of (-I k) with the exponential of (I k), as ...
1
vote
1answer
47 views

How to rationalize real number in string?

Considering we have: in = "F 18/10" How can I make this to be: Thanks!
8
votes
2answers
279 views

Unexpected behavior of rule matching a pattern

I am a beginner exploring the world of Mathematica. I expected the following code T[6, 5, 4, 1, 2, 3] /. {T[a___, 1, b___] -> Length[List[b]]} should return ...
8
votes
1answer
229 views

Why is `Block` significantly faster than `ReplaceAll` for inputting variable values

Given that the documentation for FindMinimum uses ReplaceAll for its various examples of what to do with the found solution to ...
0
votes
1answer
52 views

Replace instanced variables within module-return functions for dynamic Tooltip

Currently I am stuck with the issue of renaming an instanced variable like r1$38655 to a global one. The function is created within a "localizeAll" Module. Here's ...
2
votes
1answer
66 views

Define (conditional) substitution rule for sum/difference of radicals

I want to tell Mathematica to simplify some expressions applying the following substitutions, conditioned on the fact that $a\geq 0,b \geq 0, a^2-b\geq0 $ $$ \sqrt{a+\sqrt{a^2-b}} \pm ...
6
votes
5answers
319 views
1
vote
1answer
68 views

Apply solution rules to new functions as parameters [duplicate]

Update: I put an screenshot of how my code and problem look like at the end. Say that Mathematica solved a system of equations and got two answers {{a -> 3, b -> c, d -> 0}, {a -> -3, b -> 0, c -> ...
2
votes
4answers
140 views

Symbolic cut-off of high-order terms

I know that I can cut-off high-order terms of a $1$-variable polynomial P = a0 + a1*x + a2*x^2 + a3*x^3 + a4*x^4 + a5*x^5; simply by doing for example ...
0
votes
4answers
51 views

How to trim out sequence of blank-spaces?

Considering we have few strings: s1 = "dasbdk " s2 = "asdnkal asn knkl nkn dvklsn " s3 = " jbjkbasd nklnkln " Q1: How can the blank-spaces in ...
2
votes
1answer
86 views

Return the incomming expression on false

when I have an expression, that is unknown to mathematica, it is returned exactly the same, as it was called. For example SeeFlyingPigs[1,2] called on the line ...
0
votes
0answers
59 views

Replacing function in Sum

I have this expression, which, after execution, gives these $F[s,r]$ in output. $$ S_{1,1}=\sum _{s=1}^{16} \sum _{r=1}^{16} F(s,r) \text{Tr}\left[G_r.G_1.G_s.G_1\right] $$ Here is a piece of ...
1
vote
1answer
45 views

Concatenated rule applications (applying rules to rules correctly)

I am trying to apply a series of individual rules onto an expression sequentially, but also not have to write out the repeated ReplaceAll functions each time I want ...
0
votes
1answer
56 views

Replacing every nth comma in string output with “\n” [closed]

Feels like a strange question to ask, but I basically have a string with sets within sets, and it seems like the simplest formatting solution to seeing the data without needing to import everything ...
2
votes
1answer
42 views

Conditional replacement syntax

Why does the following expression not do what I expect Array[a, 6, 0] /. a[__?MemberQ[{1, 2}, #] &] -> 0 I expect ...
2
votes
1answer
77 views

What's the difference between functions and replacement?

I used to thought that calling a function is equal to applying a rule, i.e. applying a rule of f[x_]->sth. is equal to calling a function f with definition f[x_]:=sth. However, given ...
0
votes
1answer
56 views

Setting a range of values to one value [closed]

My question is perhaps worded a little vaguely, but it's a problem I'm running into defining my own function. I'm drawing some shapes with variable height, and sometimes this height is 0. For it still ...
0
votes
1answer
64 views

Substituting inside a differential equation

x y''[x]+2y'[x]+l^2 x y[x]==0/.y[x]->Cos[x] I think above code shows what I want to do quite clearly. How do I make it happen. Only y[x] is replaced.
1
vote
1answer
105 views

A puzzling result of the `ReplaceAll` [duplicate]

I tried to do something below Log[PDF[NormalDistribution[m, s], x]] /. Log[u_[x__] ] :> Plus @@ Log[List[x]] which gives a desirable result that splits the ...
10
votes
3answers
362 views

substitution rules excluding subscripted variables

Suppose I have an expression containing p,Subscript[p,1],Subscript[p,2],etc. I want to ...
6
votes
3answers
186 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 ...
5
votes
1answer
269 views

Embedding StreamPlot in Graphics3D

I am trying to visualise the action of the infinitesimal generator of a Lie group on the solution of a 2-ODE system drawn in the 3D space (t,y1,y2): t is time and y1,y2 are the two dependent ...
5
votes
3answers
324 views

Replace x, but not x[a] [duplicate]

I want to replace all occurences of the symbol x with x[a], as long as it is not in the form ...
1
vote
1answer
48 views

Using HoldForm for a changing list

I have a 'list' of the form an expansion: F = (A[1] + A[2])(B[1] + B[2]), such that F[[1]] produces the result ...
1
vote
0answers
98 views

Replace part of a matrix

I have to generate a ne numbers of matrices with 1 and 0 and then replace some elements. ...
1
vote
2answers
86 views

why does replacement rule not work

Why does the following replacement rule not work? term = x^2/y^2 + x^3/y^3 term /. u_^p_/v_^p_ -> z^p I expected it to return ...
1
vote
3answers
118 views

Randomly changing matrix entries

Write a block of code that generates a 4x4 matrix with randomized real number entries each between 0 & 1, then selects 3 matrix entries at random, and finally replaces these 3 entries with ...
3
votes
3answers
67 views

How to apply ReplaceAll to denominator?

Here is the simple task. In reality, it's far more complicated, but it comes down to this. I want to take the expression $\frac{y}{x^2}$ and replace $x^2$ with $k$. This doesn't work: ...
0
votes
0answers
34 views

Mathematical symbol and name for Symbolic Replacement

(Originally from math.SE, but might be better here) I'm searching for a mathematical symbol, that describes the symbolic replacement done for instance in Mathematica: ...
11
votes
6answers
397 views

For a given expression: if it appears, remove it, but if it is absent, add it

While reformatting Szabolcs's code from (42660) I noticed this interesting operation: ...
1
vote
1answer
130 views

Making ReplaceAll faster

I have a list of functions ncalc that is calculated by ...
0
votes
1answer
115 views

Replacement of terms/Pattern matching involving products of derivatives of a function

In delving into Ramanujan summation, I'm trying to get a hold of the relations of the form $$\sum_{n=0}^\infty f(n)=\dfrac{h\frac{d}{dx}}{{\mathrm{e}^{h\frac{d}{dx}}}-1}\int_{0}^\infty ...
2
votes
1answer
45 views

Pattern matching & simple replacement

I have a question about an expression that I want to modify. Here is the expression: ...
10
votes
1answer
255 views

Is there a programmatic equivalent of Edit > Find…?

Most things that can be done via the front-end interface in Mathematica can also be accomplished by some function. Is that the case for find-and-replacing? That is, is there some code I could execute ...
16
votes
1answer
305 views

Why is ReplaceAll behaving like this?

I'm learning to use the ReplaceAll function and I found the behavior of which is quite confusing. For Sqrt[f[x, y]] /. f[___] -> u Mathematica returns ...
0
votes
1answer
73 views
1
vote
1answer
138 views

Why does it matter at what point I replace?

I need to find four orthogonal linear combinations of complicated functions, that vanish at four different points. I use (LK4 is defined below, but its shape should have nothing to do with my ...
10
votes
9answers
986 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 ...
1
vote
1answer
83 views

Why does ReplaceAll behave so oddly?

ReplaceAll can behave a bit oddly. Consider ...
14
votes
4answers
1k views

How to change element color in Periodic Table?

The command ColorData["Atoms", "Panel"] generates a periodic table. As I don't want to reinvent the wheel, I asked myself how to change the color of each cell ...
1
vote
1answer
39 views

What particular situation do we have to use RuleDelayed after replacement? [closed]

On the answer of How to impose normalization condition (not initial condition) on DSolve or NDSolve? k = vars /. sol /. cs[[1]] /. C[_] :> 0 // N; I see that ...
0
votes
0answers
65 views

Creating a lazy evaluator of the lambda calculus

I have a sub project in my research to write a lazy evaluator of the lambda calculus. How should I approach this problem? I already have a implementation of the lambda calculus in another language. ...
2
votes
1answer
52 views

Mathematica does not replace \[ImaginaryI] inside Hold

Mathematica 10.1 does not replace I (i.e. \[ImaginaryI], the square root of -1) inside functions with the Hold attribute. Example: ...
1
vote
1answer
46 views

Replace symbols in output without evaluation

I found the Wolfram example for ExpandNCM but I want a more human-readable output form. I want to evaluate ExpandNCM, for example with ...