All Questions
Tagged with replacement evaluation
65 questions
1
vote
0
answers
60
views
Is there an easy way to make built-in functions do a replacement before throwing an error message?
Many built-in functions throw errors before executing a ReplaceAll. The functions still execute the ReplaceAll as intended, but ...
- 153
0
votes
2
answers
91
views
Same code behaves differently as a pure function and a function, and insider another function
I have a rule-transformer that takes a list of rules (and a list generator = {σ[z], σ[-1], σ[1]})
...
- 555
7
votes
7
answers
487
views
How do I withdraw the rhs from a list of equations?
I have a list of equations that were solved for different values of another parameter and stored in a Table and has the form:
{a==0, a==1, a==2, a==3}
and so on. I ...
- 73
5
votes
2
answers
241
views
Do not understand evaluation order of ReplaceAll [duplicate]
{x, x^2, Length@x, Subsets[x]} /. x -> {1, 2, 3}
(* {{1, 2, 3}, {1, 4, 9}, 0, {{}, {1}, {2}, {3}, {1, 2}, {1, 3}, {2, 3}, {1, 2, 3}}} *)
with error
What ...
- 1,924
4
votes
1
answer
179
views
RuleDelayed (:>) involving Piecewise leads to Piecewise::pairs warning in ReplaceAll (/.)
When I write a replacement rule like this:
Piecewise[{{x^2, x < 0}}] /. Piecewise[a_, b___] :> Piecewise[a, b]
It works fine, but I get the unexpected ...
- 3,043
4
votes
4
answers
193
views
How to use replacement rules in a named expression before a sub-expression evaluates?
I have a named expression: expr := a + b. a and b are also named expressions.
Let's say <...
2
votes
2
answers
158
views
Replacing symbolic variables inside of a compile
As the result of a symbolic integration, I have a very long array with symbolic variables u[i] which I would like to substitute for real number values. Moreover, I ...
- 163
1
vote
0
answers
59
views
How to get rid of "/." in my result, when I evaluate a term containing ReplaceAll?
In the previous code I defined Bchoice and pchoice
In one line I defined
...
- 151
0
votes
0
answers
35
views
Define a new rule to take precedence over the others in a symbol's `Downvalues` list [duplicate]
I have a symbol that has a very large list of Downvalues. One of the rules in this list should be applied before the others because it simplifies many expressions, ...
- 1,854
2
votes
1
answer
360
views
What's the difference between Evaluate and Replace command? [closed]
all, I'm new to Mathematica, I am very confused between Evaluate and Replace (/.) command. I feel they are very similar.
For example,
...
- 33
0
votes
1
answer
73
views
Why does DifferenceRoot evaluate only in this table?
I would like to use a complicated function containing DifferenceRoot as a replacement rule, but somehow it evaluates only in ...
- 711
2
votes
1
answer
43
views
Formula: setting parameters vs replacement, different results?
I have a question that may sound stupid and have an easy answer - but I currently do not see it.
I have defined a function of four variables, $f[a,b,c,d]=...$, in what I think is correct Mathematica ...
- 43
1
vote
1
answer
55
views
2
votes
0
answers
48
views
How to refactor solution verification function for solving simple ordinary differential equations with Dsolve? [closed]
I have tried different approaches but none would work for me as I am new to both functional programming and Mathematica. At bottom is the code I have.
How ...
- 2,467
1
vote
0
answers
41
views
6
votes
0
answers
90
views
In over my head in trying to create a debugger
Apologies for a less than ideal question, but if I could have asked a good question the answer would probably have been revealed in the process...
In answer to this question @LeonidShifrin gave some -...
- 2,570
3
votes
1
answer
100
views
Prevent argument substitution in held expression when injecting into unevaluated code
I want to write a custom sow/reap pair to wrap any piece of code in sow[code] and call reap ...
- 47.2k
2
votes
1
answer
168
views
How to get ReplaceAll to happen before function call
I have a problem I am attempting to solve with NMinimize.
I have a scoring function, which will give me a final score. The problem has many variables, so instead of having them as parameters, they ...
- 21
3
votes
1
answer
226
views
0
votes
1
answer
54
views
Constructing sequence of function with elements depending on the previous one / recursion
Starting from a function $\theta0[x\_ , y\_ , z\_ ]$, the terms $\varphi[x\_ , y\_ , z\_ ]$ constitutes a sequence of functions that is constructed as follows:
Order 1 (one function and its ...
5
votes
3
answers
404
views
First ReplaceAll, then evaluate an expression
Here is a simple example using ReplaceAll:
rvec[[2]] /. rvec -> {x, y}
I think Mathematica first tries to evaluate ...
- 1,991
2
votes
0
answers
49
views
Using Min & Max of an Interval in a replacement [closed]
I want to use a replacement rule to transform a list of equations. The following works as expected:
...
- 20.4k
3
votes
2
answers
219
views
Convert defined variables to rule list
I'd like to make a function that takes a list of variables and returns a corresponding rule list with the current values of the variables. E.g.
...
- 20.4k
10
votes
3
answers
204
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 ...
- 34.4k
3
votes
4
answers
401
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?
...
- 133
6
votes
2
answers
246
views
How to use ReplaceAll and RuleDelayed with an Association? [duplicate]
I expected that something like this
...
- 4,225
1
vote
1
answer
56
views
Setting the parameters in a defined function vs replacement rule in the formula
I have the following definition in my Mathematica notebook.
...
user49048
3
votes
0
answers
51
views
How does the code idiom for replacing part of a held expression work? [duplicate]
I've seen this idiom used in different programs but never fully understand why it is necessary. Could someone explain? Is it still the recommended way to replace some part of a held expression to its ...
- 8,674
1
vote
0
answers
255
views
Problem in evaluating nested anonymous functions [duplicate]
It seems that I have found a bug (or a fundamental problem in the design of the programming language of Mathematica) with functions defined as rules.
Define the function g as g(1)=2 and g(2)=1. We can ...
2
votes
0
answers
118
views
Avoid evaluating left-hand side of rule
I am trying to force some integrales in the form $\int \mathrm{d}x f(x)+g(x)$ into $\int \mathrm{d}x f(x)+\int \mathrm{d}x g(x)$. My first naive solution was
...
- 1,617
1
vote
0
answers
51
views
Evaluation of an expression for different functions [closed]
Suppose I have function
a[t_]:=f'[t]/f[t].
The actual expression is more complicated but the idea is the same.
I want to evaluate ...
18
votes
1
answer
405
views
Pattern matching to a function evaluation inside an Association
I find the following behaviour deeply counter-intuitive:
<|1 -> 123.456 |> /. {x_?NumericQ -> Round[x]}
(* <|1 -> Round[123.456]|> *)
That ...
- 10.3k
2
votes
2
answers
313
views
Evaluating ReplaceAll inside SetDelayed, first
Consider the following (simplified for the purpose of example) definitions:
f[x_] := y /. y -> x
g[x_] := x
Where for example, the definition of ...
- 517
5
votes
2
answers
105
views
Why does `Times` evaluate within `Hold` when doing replacement?
Hold @ t[t[1,1],2]/.t-> Times
Hold @ t[t[1,2],2]/.t-> Times
Hold[(1 1) 2]
Hold[2 2]
Why doesn't the second one give <...
- 5,779
19
votes
1
answer
940
views
Is this a bug in Replace?
Consider the following minimal examples. This one works as expected, replacing trees starting from the inside with a numerical value.
...
- 47.2k
2
votes
0
answers
159
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
...
- 907
0
votes
1
answer
1k
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 ...
- 687
1
vote
2
answers
247
views
How to define a function returning x -> 2 while x = 1?
What I want is an f that will work like this:
...
11
votes
2
answers
2k
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 ...
- 1,737
3
votes
0
answers
133
views
0
votes
2
answers
162
views
Unevaluated derivatives of known functions after replacement
I want to make the following replacement work correctly:
Array[(2 D[xM[[#1]][t], t] * D[xM[[#2]][t], t]) &, {3,
3}] /. {xM -> {Sin[ω t], Cos[ω t], 0}}
...
- 743
17
votes
1
answer
325
views
When is the righthand side of a RuleDelayed evaluated?
The documentation on RuleDelayed states that lhs :> rhs represents a rule that transforms lhs to rhs, evaluating rhs only after the rule is used.
Consider the ...
- 10.2k
1
vote
1
answer
219
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 ...
- 842
0
votes
2
answers
110
views
Different behaviour of Map and Inner in replace rules
Why does
x[{1, 2}] /. {x[l_] -> Map[func, l]}
{1, 2}
not behave as expected, while for example
...
- 63
5
votes
2
answers
145
views
Why does Rule behave like this?
Why does
{x, x^2, x^3, a, b} /. x^n_ -> f[n]
result in
{x, f[2], f[3], a, b}
when ...
- 437
2
votes
1
answer
120
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 ...
- 23
3
votes
3
answers
250
views
ReplacePart equivalent for Associations
I have a fairly complicated Association. I wish to use/create a function such as ReplacePart to operate on certain Keys within this multi-level Association.
Here's ...
- 11k
5
votes
2
answers
184
views
ReplaceAll performance problem: packed arrays on the LHS are unpacked when the RHS is too long
The following is one of the most mysterious performance problems I encountered and came up while extending the booleval function mentioned here. I am looking for a ...
- 236k
1
vote
2
answers
120
views
Evaluate differentiation after replacement
The problem is
Find two linearly independent solutions of $2x^2y'' -xy' + (-4 x + 1)y = 0, \, x>0$ of the form $y_1 = x^{r_1}(1 + a_1 x + a_2 x^2 + a_3 x^3 + \cdots)$ and
$y_2 = x^{r_2}(1 + b_1 x ...
- 749
1
vote
1
answer
180
views
Commutative algebra by generators and quadratic relations
My apologies if this has been asked before. Please also be gentle, as this is my first question here.
I am trying to define a commutative algebra by generators and relations in the following way. ...
