Questions tagged [replacement]
The replacement tag has no usage guidance.
1,370
questions
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33 views
Evaluate, NDSolve and ReplaceAll do not work with tables of solutions
Why does the following report an error even though it returns a (seemingly valid) result? It's also weird that it returns a list of lists, whereas I would expect just a list.
...
2
votes
1answer
113 views
Combining VectorPlot and Graphics3D
I have a simple VectorPlot3D
VectorPlot3D[{x, y, z}, {x, -1, 1}, {y, -1, 1}, {z, -1, 1}]
Can I substitute the default arrows with Graphics3D object like:
...
0
votes
1answer
57 views
Forcing mathematica not to substitute some terms in an expression
I have an expression c which should be defined before the function and a function f defined as follows:
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0
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4answers
41 views
Simplifying an equation by factoring the current coefficients in order to substitute a new coefficients
I have an equation in the following form:
a*x + b*y + c*z == d
I want to substitute the following constants instead of a,b,c,d:
...
5
votes
1answer
89 views
Patterns are not Sequences are not Lists, or: Global Function Argument Reversal
H[z_] := z /. {F[p__] -> F[Reverse[p]]}
F[{x, y}] + F[{x, z}] + F[{}] // H
Almost. It gives an annoying "Nonatomic Reverse" error message at the ...
0
votes
1answer
38 views
Formatting expressions with numeric exponents using pattern matching
Thank you all in advance.
Question:
I want to change the format of this expression (n + r)^3/n^3 into this (1+r/n)^3using rules ...
1
vote
1answer
46 views
How to create a simple transformation function on lists using short form of ReplaceAll?
I can easily do this with Map so the real goal of this exercise is to get a better grip on using transformation rules. Please help with this. The outputs should be ...
1
vote
2answers
61 views
Unable to plot a solution of NDSolve whilst being able to evaluate it
I want to define a function $f_c(x,y)$ as the solution of the PDE $\Delta f_c(x,y) = g_c(x,y)$, where $g_c$ is a given function depending on a constant $c$. I tried the following:
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0
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1answer
73 views
Can someone please help me in breaking down what this code means?
I hope someone can assist me as I am pretty new to Mathematica and understanding the language is a bit of a hurdle.
I am trying to rotate a plot of interlaced circles by some angle without rotating ...
1
vote
1answer
99 views
6
votes
1answer
78 views
unexpected StringCases behaviour inside function module
First, Let use see
a = {"wx", "wy", "wz"};
AssociationThread[Flatten[StringCases[a, "w" ~~ d_ :> d]] -> a]
gives
<...
0
votes
1answer
31 views
Setting rules for Gamma functions
I have some expressions involving lots of Gamma functions. Now the arguments of the functions sometime go to zero, in which case I want to use the factorial instead of the Gamma function (since it ...
1
vote
1answer
57 views
Denoting the whole in Position/Extract/ReplacePart
Position returns lists of numbers consumed by Extract, ReplacePart and others as "...
5
votes
2answers
142 views
Replace terms without symbolic term
I have a variety of terms that have coefficients of what I'll call $c_x$ and terms that are without this coefficient. I have a variable $U$, and would like to substitute powers of $U^2 \to d$ but only ...
0
votes
0answers
51 views
Block Behavior, but for Function Assignment instead of Variable Assignment
We can define a function using a variable b that does not appear as an argument, and then using Block, change the output of the ...
0
votes
1answer
56 views
Single computation is fast, but 6 computations are extremely slow
I have an expression that involves many Sign functions. I want to expand this in the piecewise format.
My function is:
...
2
votes
0answers
34 views
Trouble using pattern matching with derivatives
I'm going to pre-reference this similar question as its very nearly what I need but my attempt at extending it to my case doesn't seem to quite work: Pattern matching a multivariate derivative
In my ...
1
vote
2answers
147 views
0
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0answers
68 views
What is the difference between using replace all and using the value of a function?
For a project I'm working on, I'm using some functions that have a Chop[] to get rid of small terms that complicate things numerically. My function is ...
2
votes
2answers
68 views
Cutting of terms on denominators of giant expression
I have an immense expression of the form
$$
\sum_i \frac{f_i(X,Y,x,y)}{(a_iX^2+b_iY^2+c_ix^2+d_iy^2)(e_iX^2+g_iY^2+h_ix^2+k_iy^2)},
$$
where at $a_i$ or $b_i$ AND $e_i$ or $g_i$ is different from zero....
2
votes
1answer
122 views
How to replace the arguments of a function?
I have the following type of (very long) expression:
$$\ldots + X[1,2,3,5] + X[2,4,5,6] + \ldots \tag{1}$$
where $X[a,b,c,d]$ is some undefined function. I would like to replace all the occurrences of ...
0
votes
1answer
89 views
4
votes
1answer
76 views
How to act with replacement on an association with syntax most similar to ReplaceAll
Replacements acting on Associations confuse me. I have read Pattern matching to a function evaluation inside an Association and How to use ReplaceAll and RuleDelayed with an Association? and I ...
3
votes
4answers
191 views
List all substrings of a number
Need some help listing all substrings of a number
X=219
The output should be
Output= {2,21,219,1,19,9}
Order does not matter for this problem
Any help is appreciated
1
vote
1answer
90 views
Rewrite a variable in terms of another variable
I have two variables
t = 1/Sqrt[2] (Sqrt[1 + Cos[a]] + Sqrt[1 - Cos[a]] +
Sqrt[1 + Cos[b]] + Sqrt[1 - Cos[b]] + Sqrt[1 + Cos[c]] +
Sqrt[1 - Cos[c]])
and
<...
2
votes
0answers
34 views
Replace element within a list of lists with MemberQ condition [closed]
I'm trying to replace the second element within a list of ordered pairs depending on whether that ordered pair exists within another set.
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0
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0answers
85 views
Part specification is longer than the depth of object
I want to solve the function lns50 all the values are Part 1 of an List
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0
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1answer
79 views
No solution to the equation even though it has one
I was trying out Mathematica with a simple equation. I tried this one out, but it outputted nothing and when I went on WolframAlpha to test it, there was a result (c = 3 btw). Please let me know if ...
1
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0answers
25 views
What subsequences does SequenceReplace consider
Consider SequenceReplace[{a, b, x, x, a, c}, {a, e_} :> e].
What subsequences does SequenceReplace consider?
The ...
0
votes
1answer
60 views
Using :> vs -> in SequenceReplace
The first example on the page for SequenceReplace is the following:
SequenceReplace[{a, b, x, x, a, c}, {a, e_} :> e]
...
0
votes
1answer
68 views
Nonlinear Model Fitting to Numerical Data with automatic elimination
I have an experimental data set {f,x,y,z} as
{{300., 2., 4., 6.}, {500., 0., 4., 25.}, {6600., 1., 15., 9.}, {100.,
5., 0., 2.}, {1100., 10., 8., 1.}, {1300., 7., 8., 18.}, {300.,
23., 5., 0.}, {400....
4
votes
1answer
86 views
Making a rule that only applies when the expression is NOT an argument?
I know we can make rules of the kind:
magic/:f_[x___,magic[a_],y___]:=f[x,arg[a],y]
This will replace magic[a] by ...
1
vote
2answers
66 views
I use the rule, but it cannot work, So I want to know why it can't? [closed]
I make a approximate solution of the differential equations. But when I use the rule for one, and it can work. But when using a list of rules, It cannot. I mean "(Sum[ep^k*Subscript[y, k], {k, 0,...
0
votes
1answer
72 views
How to change a string-formatted element in an array?
I have an array called Array7 in which a typical element, let's say Array7[5][[22]], looks like this:
...
1
vote
1answer
68 views
Tracing my rules
As said in another question, I apply a set of rules to a string (OK, technically written as a function F of several arguments so I can easily apply rules on the fly)...
0
votes
1answer
52 views
Trouble with variable replacement
I'm having a lot of trouble with functions build from other functions. I'm struggling to understand the nuance of variable replacement. Here is an example:
...
2
votes
1answer
62 views
Replace with rule
I have an expression,
$$test=\left(x_{jv}-y_{jv}\right)b+\left(x_{im}-y_{im}\right)c$$
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0
votes
1answer
64 views
Composite function derivatives
I would like to calculate the numerical partial derivatives (numerical gradient) of this function:
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3
votes
0answers
45 views
Preventing $-renaming of pattern names in lexically-scoped constructs
When you use a lexically-scoped construct like With or Function to perform replacements in an expression, Mathematica might ...
2
votes
1answer
56 views
Mathematica association does not work on symbols
Question about using an association within Replace.
If I have an association which contains symbols like so:
...
1
vote
2answers
64 views
How to Replace a Date in String Format with Values in a Table
I have a Table generated by reading a set of Excel files using the code:
...
1
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0answers
49 views
Rule Precedence
Do I understand the following correct ($R$ on-the-fly executable rules, $E$ expressions)? I assume MATHEMATICA uses an unification algorithm like I learnt it for e.g. PROLOG.
If $R_1$ was defined ...
0
votes
0answers
51 views
How to implement integration by parts?
I have been trying to simplify integrand expressions using integration by parts, which schematically is given by $\theta(x)\nabla\phi(x)=-\phi(x)\nabla\theta(x)$. I think this can be done as a ...
1
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0answers
52 views
Avoiding infinite rule recursion
Assume I want to apply rule $R_1$ from a rule set $S$ to an expression $E$ just if it is "half safe", i.e., if the next application of $S$ to $R_1(E)$ does not yield $E$ again. (This ...
2
votes
2answers
68 views
Formal substitution of exponential factors in an expression
Assume that we have some complex algebraic expression, like
Exp[(a t + b s)/w] ( t Exp[ q t] w + q Exp[q s] Exp[-2 s] + a^2 Exp[3 s] + s t + 1 )
Now, this is just ...
0
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0answers
31 views
Substituting factors for letters in expression
I have several large expressions that I'd like to substitute some factors for letter for the sake of simplicity. For example, I have this expression:
$$
\frac{C_3 C_5 (C_1+C_4)-C_1 C_6 (C_2+C_5)}{2 ...
2
votes
1answer
59 views
Replacing multiple variables according to the index
I'm currently working on the Polya's Enumeration Theorem implementation in Mathematica. As an example, of what I want to do, here's a formula I'm working with:
$$P_{C_{12}}(x_1,\ldots,x_{12})=\frac{1}{...
0
votes
2answers
84 views
Replacement of names of variables
First of all, I know replace or plugging input of given function through command Replace[] or /.{x->1} etc.
Here I want to focus not just on plugging or replacing variables into numbers but just ...
4
votes
2answers
74 views
How to replace x[t] with Cos[t] in a differential equation? [closed]
If I use /. only x[t] is getting replaced, but x''[t] is not getting replaced.
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0
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2answers
78 views
Why does this simple code with Eliminate not give any result?
My goal is to obtain the function $s$ defined in the code below as a function of $\chi_1$, $\chi_2$, $\chi_3$. The code runs but never ends, although the problem does not seem too hard to solve for a ...