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

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7
votes
4answers
152 views

Replacing elements in lists of a nested list

I have the following nested list ...
3
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: ...
0
votes
0answers
28 views

Gradient Descent's Update Rule for Multiparameter Functions

I'm sure this has been answered before, but I haven't been able to find it. I'm trying to manually implement gradient descent for some function, f, with a vector of any length. I define the update ...
1
vote
0answers
72 views

Speed differences between Table, Replace, Array, Map, and AssociationMap

EDIT: Actually, it looks like part of this is bogus—Array doesn't actually work on this input (I'm often a little shaky on what kind of input, exactly, works for ...
1
vote
1answer
31 views

Substitute when hold is on

I have an expression for $div$ with Lame coefficients. I want to look at the $div$ in different coordinate systems. So I want to substitute the Lame coefficients not letting Mathematica to actually ...
4
votes
3answers
245 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 ...
1
vote
0answers
40 views

Using Replacement Rule on Part of List? [closed]

Why can't I use a replacement rule to replace a variable name standing in for the index of a list? For example: myList={1,2,3,4,5} myList[[index]]/.index->3 ...
1
vote
1answer
51 views

Is there parallel/simultaneous substitution in Mathematica?

I know this is simple but I'm not proficient at Mathematica and cannot seem to find it online or in the help. Also, Replace[] says it does the assignments in order, ...
-1
votes
1answer
34 views

Substitution in a power transformed in product

This could be thought as a duplicate question but it's in fact an extension of an old section. 1 + (a (α + β))/(b^2 + c^2) + ((α + β)^2 a^2)/(b^2 + c^2) could ...
12
votes
2answers
204 views

Opposite of Alternatives in a Rule

Mathematica provides Alternatives, to match one of several patterns. In a rule, this is used as p1|p2|p3:>replacement. I ...
0
votes
2answers
44 views

How to replace the argument of a function?

For the following expression, b'[x y/z]+z b''[x y/z] I want to replace the argument of b such that the above expression becomes ...
0
votes
1answer
37 views

Replace argument of delayed function before it evaluates [duplicate]

How should I use replace so that f[x_] := x^2 Replace[f[3], 3 -> 1] OUT = 9 evaluates to 1, instead of 9? This way, the replacement of the argument is done ...
4
votes
1answer
130 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 ...
3
votes
2answers
110 views

Replace non-zero elements with symbol

I have a very large matrix (tensor) in Mathematica with some zero and non-zero elements. I am interested in replacing the non-zero elements with some symbol or a 1 so that it is easier to view the ...
0
votes
0answers
38 views

Replace higher order derivatives given lower order derivative

I have an unknown function f[x1,x2] whose partial with respect to x1 is identically zero. I would like to make sure that ...
2
votes
1answer
87 views

replacement rule doesn't work

I define a function which counts the number of negative elements in a list as follows: g[x1_, x2_] := Count[{x1, x2}, _?Negative] Plugging in values shows that ...
1
vote
1answer
43 views

How to define a Reduce function in order to define two parameters afterwards

I'm trying to evaluate numerically an equilibrium for a system of ODEs: dA = c A + b B - a A dB = d B + a A - b B So I define a list of known parameters: ...
1
vote
1answer
40 views

Why doesn't recursive transformation stop when rules no longer match?

In an exercise where I was trying to pick out consecutive words starting with the same letter, i.e. I wanted: ...
2
votes
0answers
65 views

Find and Replace in Notebook using a Pattern [closed]

I would love to be able to use the power of Mathematica's patterns to find and replace within my code. I have searched for this on google, and so far I've come up with this: http://reference.wolfram....
1
vote
1answer
71 views

Replace a variable with a list of numbers [closed]

How can I replace a variable with a list of numbers? Example Suppose I have the following list of values V = Table[i, {i, 1, 6.8, 0.19}]; and I want to ...
0
votes
1answer
68 views

Module output rules like Solve FindRoot etc built-in functions

How do I print rules {x -> values} as an output from my module? I'd like it formatted just like those from the built-in functions, so that users can use Out[ ] or % to directly apply the rules. ...
6
votes
3answers
229 views

What's wrong with naming a rule?

Please have a look at the code below. The second Manipulate uses the name rl of the replacement rule given in the first line, ...
4
votes
2answers
239 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 ...
2
votes
2answers
459 views

What is the meaning of “/.”? [closed]

I've looked to the built-in help of Mathematica, but I still can't understand the usage of /., like in the following expression: ...
3
votes
1answer
108 views

Replace list element with element from another list based on condition

I Have a list on integers and another list of "duplicates". E.g. start={{3},{1},{2},{4}}; duplicates={{1,4,7},{5,6}} I would like to substitute each element of ...
2
votes
0answers
39 views

Replacement rule of pure function not working [closed]

The following code works g[x_, y_] := x^2 + y Thread[xxx[{1, 3, 4}, {10, 20, 30}]] /. xxx -> g I would like to use ...
0
votes
1answer
35 views

Restricting the evaluation of variables [closed]

Hopefully you understand my question. I m calculating inverse kinematics of a robot and each subsequent value is dependant on the one before. For simple porting into c++ I would like something like ...
2
votes
0answers
44 views

Replacement rule for summation with Kronecker delta

I want to perform some symbolic computations in index notation, without explicit reference to sums, so that in expressions of the type f[i,j]g[j,k] summation over <...
4
votes
3answers
135 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 ...
0
votes
1answer
36 views

Subbing in parameter values [closed]

I have the following code ...
9
votes
1answer
199 views

Fast implementation of Metropolis-Hastings update with multiple sets

This is an optimisation question rather than something I am unable to do. I have a naive, primitive working version of the code (I submit it below) but I am trying to make it work faster. I am asking ...
1
vote
1answer
53 views

Rearrange the list with some rules

I am trying to solve physic problem on operators which are not commute. However, I am not good at coding, so I am having some problem with Mathematica code. Let's define my list such that ...
1
vote
0answers
39 views

Pretty equations in HoldForm[]

I have been generating equations programmatically, resulting in lists that look like this: {Log[2,(4^4)^3, 3 (Mod[2,4] 4), Log[root[2, 3], 4^4]} where 'root[x,y]'...
1
vote
1answer
76 views

How to replace a variable in a function?

I'm new to Mathematica and am using it for algebraic manipulation. I'm trying to figure out how to replace a variable in a user-defined function. I defined a derivative function like this: ...
0
votes
1answer
51 views

Error in partial derivative and replace?

After run my code, output is ...
0
votes
0answers
42 views

Using ParametricPlot to Display result of Maximize

One would expect that: Plot[f[x],{x,0,1/2}] and ParametricPlot[{x,f[x]},{x,0,1/2}] would always yield identical outputs. ...
7
votes
3answers
187 views

How to prevent substitution in a part of an expression?

Background: I am trying to write a program that can execute scheme in Mathematica. Now I have things like {"+", 3, {"-", 2, {"'", {"*", 1, 3}}}}. The syntax ...
2
votes
2answers
87 views
6
votes
2answers
117 views

What is the Difference Between Rule (->) and RuleDelayed (:>)?

I am new to Mathematica so this question may seem rudimentary. Sorry for that! :) I want to implement the well-known property of Kronecker's Delta $$\Sigma_{i=1}^{n} a_i \delta_{ij}=a_j,\quad 1 \le ...
-2
votes
1answer
26 views

How to renew a value when a value is larger than it, counting the time of the record being broken at the same time [duplicate]

I have a given value and is it possible that after some calculation, when the calculated result is larger than the given value, the given value can be set as the value that is larger than it? Other ...
1
vote
1answer
79 views

Replacing negative components of a matrix with zeros [closed]

How can I replace the negative components of a matrix with zeros?
0
votes
1answer
50 views

Extract solutions of linear system to variables

I need to output the answers from solve into the variables named just as they were named in the solve equations. I have checked out this thread Assign the results from a Solve to variable(s) but ...
3
votes
1answer
26 views

Possible incompatibility between ReplaceAll and LaplaceTransform

I have a large symbolic expression containing many terms of the form, LaplaceTransform[u2[z], z, s], with various functions for the first argument. I wish to ...
0
votes
1answer
66 views

Replacing terms in a sum according to a condition

If I have a generic expression such as ...
1
vote
0answers
31 views

How Can I combine the equations' results in a table without “x->” [closed]

For example: I have a series of functions: F[x_,o_]=x+o And I want to combine the result into a table with o=0.3,0.6,0.9.. ...
0
votes
2answers
59 views

replace a part of equation by a symbol

I have an equation: $$ \frac{d\text{$\phi $0}}{dx} \left(-\frac{\text{A1}(x) \frac{d\text{A0}}{dx}}{\text{A0}(x)^2}+\frac{\frac{d\text{A1}}{dx}}{\text{A0}(x)}+\frac{\text{B1}(x) \frac{d\text{B0}}...
0
votes
1answer
35 views

Replacement of a group of variables in trigonometric expression

I would like to make simplification of trigonometric expressions by using a replacement a group of variables. Here the equation I have to simplify : ...
4
votes
2answers
104 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 ...
1
vote
1answer
59 views

Generating lists of data with ReplacePart

I have the following example: ...
5
votes
1answer
108 views

How to make a postordered ReplaceAll that operates just once per subtree?

The answers to this question provide code for a function I'll call postOrderReplaceAll to replace from the inside-out: ...