Questions tagged [replacement]
Questions about Mathematica's rule-based replacement functionality, including functions such as Rule (->), Replace, ReplaceAll (/.) and ReplaceRepeated (//.).
7
votes
5answers
230 views
Unexpected behaviour of Nothing inside List inside Association
When evaluating the following input:
<|a -> {c, d, e}|> /. d -> Nothing
I get (expression 1):
...
0
votes
0answers
16 views
Plotting multiples figures using rules [duplicate]
Consider the following:
rule = {a -> x+1, b -> x^2, c -> 3x^3+1}
Plot[{a, b, c} /. rule, {x, 0, 1}]
This, unfortunately, does not color each plot with a ...
0
votes
1answer
33 views
Use of module and nested variables
Consider the following:
rule = x -> 2 para + 1
funct[para_] := Module[{x}, x /. rule]
this unfortunately does not work. If I put the content of "rule" ...
0
votes
1answer
43 views
Replacement of the patterns in the replacement
I want to make such changes:
$(A\, p + B\, q)[i] \to A\, p[i]+B\, q[i]$
But the replacement below can not do this
...
0
votes
1answer
28 views
Expression replacement with variable exponents
I have an expression that consists of a large sum of variables of powers like:
X[1]^(2p) X[2]^2 + X[1]^2 X[2]^(2p) + X1[1]^(1+p) X[2]^(1+p) ...
Now I would like ...
1
vote
1answer
43 views
Replacing powers with ReplaceRepeated
I have question about ReplaceRepeated. Consider
$\qquad\mathsf{FullSimplify}\left[\left\{\frac{q^5-q^4}{q^5-2 q^4+2 q^2-q}\right\}\text{//.}\, \left\{q^3\to 1\...
0
votes
4answers
70 views
Implementing a Dirac Delta like function symbolically
I have an arbitrary expression where some terms are multiplied with a Dirac delta 'like' function d(x,y). For example,
e^(ax+by+cz).d(x,y).d(a,c) + e^(ax+by+cz).d(x,y)+e^(ax+by+cz)
I need to get ...
10
votes
3answers
170 views
Perform localized, evaluation-leak free replacements
Consider the following expression:
expr = Hold[
{
f[
{1, {Print@1}}
],
g[
{{{Print@1}}}
]
}
];
I'm looking for a way to ...
3
votes
1answer
94 views
“Smart” substitution of subexpressions
I have the following question.
An expression, which I want to simplify contains several subexpressions which appear quite frequently all over the place. To optimize simplification I would like to ...
14
votes
1answer
319 views
Is there a difference between Replace with parameter “All” and ReplaceAll
Here is my extremly basic understanding of Replace and ReplaceAll.
This post is also a way for me to check if I understood the mechanism behind, if you see mistakes in my explanation don't hesitate ...
1
vote
1answer
39 views
Replace an If by a combination using replacement rules
Consider the following code :
tttest = a^2 + If[a > 0, a, -a]
a^2 + If[a > 0, a, -a]
I would like to replace my If function by something like fonction @ ...
0
votes
2answers
62 views
Replacement rules with list as matching
How can I use replacement rules to match and work with a list. For example,
{1, {2, 3}} /. {{a_, b_} -> ((a + # &) /@ b)}
outputs
...
2
votes
2answers
47 views
Replace a an expression in Exponential only but not in other terms?
I want to replace the A in the Exponent only but when I use the replace rule it also replaces the Derivative of A.
...
2
votes
2answers
51 views
Expand a recursion relation
Hermite Polynoms have the following recursion relation:
$$ H_{n+1}(x) = 2x H_n(x) - 2n H_{n-1}(x), $$
which can be rearranged to be
$$ x H_n(x) = \frac{1}{2} H_{n+1}(x) - n H_{n-1}(x).$$
Is it ...
0
votes
1answer
61 views
Pattern matching on Orderless functions
I'm trying the following pattern matching command:
ReplaceList[Hold[b + a], Hold[x___ + y___] -> {{x}, {y}}]
And Mathematica returns:
...
2
votes
1answer
43 views
ReplaceAll rules for Power function
I have the following function:
eW = fL11(u1L^r + u2L^r) + fM11(u1M^r+ u2M^r) + fH11(u1H^r + u2H^r)
The full form of eW is then:
...
3
votes
2answers
103 views
Replace $ z z^* $ by $ |z|^2 $
I have a long expression involving different complex numbers. I want to force Mathematica to replace z Conjugate[z] by Abs[z]^2 ...
0
votes
3answers
63 views
Transformation rule for a sum of functions
When applying the rule:
exp[c[x] + d[x] + e[x]] //. exp[a_[x] + b_[x]] -> exp[a[x]] + exp[b[x]]
I would like to get
...
1
vote
2answers
68 views
Replacement of the variables
I have lots of functions of the form $-x_4^2+x_0 x_4-x_1 x_3-x_5 x_7+x_1 x_{13}+x_2 x_{22}$ (this is $eq_4$ see below). I want to replace each pair $x_i x_j$ by $y_l=x_i x_j$. How is it possible to do ...
2
votes
1answer
33 views
Defined equality does not show in ContourPlot
I have some function definitions that ultimately define an equality through Solve and Replace:
...
2
votes
2answers
38 views
Symmetry of a Dot with functions with arguments
Seemingly an easy question, but I'm trying to avoid using a straight replacement rule.
As an example, I could take a term like
p[i].p[j]-p[j].p[i]
where this ...
2
votes
1answer
45 views
2
votes
0answers
35 views
Fold/ NestList/ etc in MCMC loop for optimisation
I am trying to simplify and optimise an old loop code I wrote. Since Fold and NestWhile use the results of the last iteration in the new iteration, I would like to use them in order to get rid of Do ...
1
vote
1answer
32 views
Replacement into a list
I have the following polynomials:
pol = {x^2 + 2 x^3 + 4 x^7, x + 2 x^5 + 4 x^6, 3 x^6 + 2 x^9};
and following list
...
4
votes
1answer
148 views
Efficient way to add new rules to a sublist within a nested list of rules
I often build nested lists (as a way to keep data sets organised) with some form like:
...
3
votes
4answers
101 views
How can I replace a function? Something like f[x] /. f -> -g. ReplaceAll doesnt seem to work properly when there is a minus sign
How can I replace a function with minus another function?
This:
Sin[x] /. Sin -> Cos
works just fine.
But this:
...
1
vote
1answer
73 views
2
votes
1answer
68 views
How do you find a sub-graph and replace it with another subgraph?
I saw a video where SW shows how he has a large graph and some rules for finding a given subgraph in a graph and replacing it with another subgraph.
How would one go about doing this in Mathematica?
...
5
votes
1answer
34 views
Overriding TwoWayRule for in-place element swapping
I'd like to make \[TwoWayRule] work for things other than graph edges, for example, for swapping i.e. Replace, ReplaceAll, StringReplace, etc:
...
2
votes
2answers
168 views
Wrestling with substitution rules
Summary
I am struggling with substitution rules.
Example
Here are several cases which are problematic:
...
2
votes
1answer
35 views
Replace rule using a wildcard
I have a complicated expression which I want to simplify following a rule. For example, if I want to make the following replacement:
A[t] - B*A[t - d1] = Y[t]
...
2
votes
1answer
56 views
Understanding conditional replacement
I have encountered recently a replacement and I tried to look up documentation but could not quite find anything and hope to get some explanation from experts.
The replacement is the following:
...
0
votes
1answer
34 views
Substituting values
I have a little program that should find values that Solve a system of polynomic equations and then substitute those values function. It solves the system but then, the substitution doesn't happen. I ...
3
votes
4answers
240 views
Basic substitution
I am relatively new to Mathematica, so I am sure this is basic, but I lack the basic terminology to be able to google it.
How can I make this command work?
...
5
votes
1answer
40 views
how to comfortably concatenate substitutions
Often I have to do perform a succession of substitutions on some expression, and in a notebook the way I know to do that is
...
6
votes
3answers
141 views
1
vote
1answer
21 views
Substitution of expressions for the output
I have a code which gives this kind of output :
W = Sum[v[j]*b[j], {j, 1, 3}];
AA = (v[1]*b[1] + v[2]*b[2] + v[3]*b[3])*2 + 3 ;
I know that if i want to ...
3
votes
2answers
173 views
Substituting 0 for even and 1 for odd natural numbers using EvenQ[] [closed]
I am trying to define a substitution that gives 1 for odd numbers and 0 for even numbers. My guess was (in a particular example)
...
0
votes
0answers
21 views
Define a function using pattern matching and repeated replacement to sum the elements of a list.(exercise in Paul Wellin's Book)
I am reading Paul Wellin "Essentials of Programming in Mathematica".
I have a question about his answer to 4.1 Exercise 5.
4.1 Exercise 5:
Define a function using pattern matching and repeated ...
1
vote
0answers
31 views
ReplaceAll does nothing with the rgb values of a picture [closed]
In the book "Programming with Mathematica: An Introduction" at the beginning of the chapter on Patterns and rules an example is given where the colors of an image are transformed according to
...
1
vote
1answer
37 views
Discrepancy between ReplaceList and ReplaceAll?
Consider the following
ClearAll[eqv, not, neqv, p, q, r]
ReplaceAll[eqv[neqv[p, q], r], neqv[p_, q_] :> not[eqv[p, q]]]
...
1
vote
1answer
40 views
Why does ReplaceAll match pattern while MatchQ fails
Why does the following pattern with MatchQ fail
MatchQ[1+Sin[1] I, a_Complex /; Im[a] > 0]
(* False *)
Whereas, the same ...
0
votes
1answer
41 views
1
vote
1answer
29 views
Replace function arguments multiple times?
I have a very large function f[x_,y_,z_] and it's too cumbersome to type it directly. So I wish to break the definition and use (...
5
votes
1answer
106 views
Replace targeted elements in first level of list only
I have a list where some elements are strings or sublists:
...
0
votes
3answers
98 views
Replacements within a mathematica expression?
Say I have many expressions of the form $$\text{expr} = \frac{1}{AB(C+D)^n E^m}$$ where $A,B,C,D$ and $E$ are symbols and $n,m$ arbitrary powers. In order to convert between the mathematica output and ...
4
votes
3answers
96 views
Repeated replacing elements in the list
I have a large list of the following form
{{1, 2} -> -1, {1, 1} -> 1, {1, 2} -> -1, {1, 6} -> 1}
and I want to combine all of the pairs ...
4
votes
3answers
353 views
How to replace a comma “,” with “||” in a list
I have a list of equations:
dene = {x1== 0.2, x2== 0.4, x3==0.5};
I would like to convert this list into a rule:
...
1
vote
1answer
44 views
Simple way to apply a rule (from list of rules) based on the presence of the a rule key within a string
If I have some strings eg:
str1 = "12-26b-14a";
str2 = "12-21h-14";
and I want to replace the string with a value based on the following rules:
...
6
votes
2answers
98 views
How to use ReplaceAll and RuleDelayed with an Association? [duplicate]
I expected that something like this
...
