# Tagged Questions

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

69 views

### Why isn't SparseArray working in this case?

This works: SparseArray[{{1, 1} -> 1, {2, 2} -> 2}]\ (* {{1, 0}, {0, 2}} *) This works: ...
65 views

### Why does a badly conditioned matrix give a totally different results in LinearSolve function? [closed]

Here I define a vector: C1p[n_, h_] := C1p[n, h] = LinearSolve[D1p[n, h], F1p[n, h]]; with 4*4 Matrix D1p[n,h] badly ...
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 ...
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 ...
67 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. ...
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 ...
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 ...
202 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 ...
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 ...
192 views

Executing: ...
668 views

### How to replace a space by a comma

in this expression = "{1 2 3 4 5 6 7 8}", I want to replace each space by a comma , to get ...
109 views

### How to force a replacement in an order that I want?

This is a follow-up to this question. I have a big sum, with lots of terms of the form ...
49 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, ...
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 ...
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 ...
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 ...
152 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 ...
247 views

### How to program a click panel 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 ...
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....
129 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 ...
86 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 ...
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 ...
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 ...
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: ...
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: ...
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 ...
87 views

### Solve equation with two different sets of replacement rules

I want to solve eqn for c0 in the following code. I want to do it with both replacement rules. I realize I could just use solve ...
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, ...
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 ...
458 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: ...
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 ...
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 ...
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 ...
198 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 ...
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 ...
43 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 <...
36 views

### Subbing in parameter values [closed]

I have the following code ...
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 ...
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]'...
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: ...
321 views

### Pattern does not match with Orderless head

Why does the following pattern matching not succeed? ...
51 views

### Error in partial derivative and replace?

After run my code, output is ...
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. ...
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 ...
86 views

### Need explanation on strange behavior of Do together with AppendTo

We didn't expect a replacement of B. ...
116 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 ...
79 views

### Replacing negative components of a matrix with zeros [closed]

How can I replace the negative components of a matrix with zeros?
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 ...