Questions tagged [hold]
Questions about functions which maintain an expression in unevaluated form, including the use and behavior of Unevaluated, Hold~ functions, Inactive related functions or Hold~ attributes.
259
questions
0
votes
0answers
43 views
Odd ordering of evaluation for piece-wise functions, element-wise multiplication and dot product
I am constructing functions that call on multiple other functions and I want to understand how the ordering of evaluation of functions works and what is the best practice when writing many component ...
4
votes
3answers
83 views
Is it possible to use a previously defined value in subsequent definitions of a `Block`?
Is it possible to use a previously defined value in subsequent definitions of a Block?
According to syntax coloration, this should work as expected, that is, ...
3
votes
7answers
222 views
Obtaining the terms of a summation alongside the result
I have a Sum expression:
s := Sum[2x,{x,0,3}]
As an exercise to explore the rewriting capabilities of the Wolfram Language, I ...
0
votes
1answer
59 views
Through does not have hold attribute?
Possibly Related to Unexpected behavior of KeyTake.
Consider the following example, where I input #11 (last) is the misbehaving one. Am I missing something or do you agree there is a bug?
...
1
vote
1answer
25 views
Import expressions from a file without it getting evaluated?
How can I import a file without its expressions being evaluated ?
E.g. assume that the file "test.m" contains just the list
{a, a + b + b + c, d}
If I use
<...
0
votes
0answers
37 views
After activating, inactive integral, output is coming same as input
I am trying to plot s w.r.t r (0,10). But because of inactive integral I am not able to. When I activate inactive integral, output is coming same as input. When I am trying to plot graph w.r.t r(0 to ...
3
votes
1answer
35 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 ...
0
votes
0answers
47 views
3
votes
2answers
84 views
Construct a function that maps strings to functions
The main question is the last 5 lines (including codes) of this article.
A[x_, y_] := Evaluate[ToExpression["x^2+y^2"]]
A[3,4]
Then the output becomes
...
2
votes
2answers
41 views
Plot[Evaluate[compiled functions… causes problems
I'm sure there is an easy solution to this, but I have beaten my head against a wall for a couple of days, so I hope someone can help.
I have compiled functions. For a minimal example let's take:
<...
14
votes
3answers
283 views
Evaluated vs. unevaluated Association
This question might have a remote connection to my older question, it is also about sameness of expressions and subtle differences observable only in certain forms.
I encountered this strange ...
4
votes
1answer
47 views
Evaluate calls my function multiple times when the function generates an error
If I execute a line of code with Evaluate I find some unexpected behavior when the code returns an error. Here is a minimal example. Let ...
1
vote
2answers
56 views
InputField with Boxes-held formatting fails inside Dynamic
This is an outgrowth of an earlier question.
I have an extensive user interface that has InputFields that I want to hold their specific structure as input by the user (e.g., keeping scientific ...
5
votes
1answer
92 views
Hold[Expression] (or similar) in InputField that truly holds the input unmodified
I am trying to design a user interface where the specific form that the user enters in InputFields are left unchanged. For example, if the user puts in a number in scientific notation, it would be ...
2
votes
0answers
95 views
Non constant coefficient in heat equation
I have to solve the following heat equation over a cylindrical domain.
In cylindrical coordinates the PDE Equation reads:
...
5
votes
2answers
87 views
Obtaining head and parts without evaluation
I have been trying to write a routine MyHead and MyPart
which would have the following properties.
MyHead[2+3]
evaluates to Plus
...
1
vote
1answer
69 views
2
votes
1answer
32 views
2
votes
3answers
104 views
Preventing evaluation of $e^{2i \pi}$
I want to prevent Mathematica from using the identity $e^{2i \pi} = 1$ because at a later step I will evaluating something like $\left(e^{2i \pi}\right)^\alpha$ for some non integer $\alpha$. I wish ...
0
votes
1answer
44 views
Inactive / Activate : basic question of how it works
I would like to understand how Inactive and Activate function works because I'm disturbed.
If I need to inactivate a function and then re activate it how to do it ?
I tried the following which doesn'...
2
votes
1answer
38 views
How can I properly use Catch[] to prevent my program from quitting?
I am getting an error depicting an Uncaught Throw, with the accompanying code being returned:
Hold[Throw[$Failed, CURLLink`Utilities`Exception]]
I have managed ...
0
votes
1answer
76 views
Evaluating function before its plotting
Suppose I want to plot a function $f(x,y)$ but first need to calculate its symbolic expression. An easy solution is to copy the output of my calculation by hand and then paste into ...
0
votes
0answers
65 views
Replace Integrate with NIntegrate
I have a symbolic integral with a pretty complex expression. I need it in symbolic form because I want to compute its derivative.
...
0
votes
1answer
50 views
Evaluation Control for Equation Input to NDSolve
I am trying to numerically integrate a system of equations using NDSolve and am having issues with symbolic matrix inversion taking a long time. The actual system is much more complicated, but I have ...
1
vote
2answers
82 views
Function returning a pattern with condition - how to prevent condition to evaluate prematurely?
I figured it would be convenient to have a function
that returns a pattern, that match equally nested brackets, of the specified type. Such a pattern is easy to create, but I get some error messages, ...
0
votes
2answers
130 views
Mathematica puts function with certain constants on hold, why? [closed]
Here, I probably have a trivial case on how to use functions in Mathematica but I can't figure it out with the help of the Mathematica pages. I define the following function:
...
10
votes
2answers
212 views
How to explain the behaviour of TreeForm? [duplicate]
Just try this:
g=1;a=1;TreeForm[Hold[g[a]]]
The weird thing in the outcome is that the node of g displays 1 ,and a is still a.
I do not know how to explain ...
4
votes
1answer
101 views
Sign of conservative convection coefficient in a formal (Inactive) PDE
Consider the PDE
$$\nabla \cdot (
- \color{blue}{\texttt{c}} \;\nabla u
- \color{blue}{\texttt{alpha}} \; u) = 0
\tag{std}\label{std}
$$
where
...
0
votes
2answers
53 views
How to show the answer in terms of another variable
if we have
A = 1;
B = 44 + 1
output :: B = 45
how we can show B like this
output :: B = 44 + A
2
votes
2answers
84 views
Extract unevaluated definition of function
Say I have a function f[x_]:=a x + b.
Now I want to get the a x + b part from the function to use in e.g. ...
0
votes
1answer
28 views
1
vote
0answers
43 views
Differentiating functions of Set expressions
There are so many things in Mathematica I think I will never understand. Perhaps today there will be one fewer...
B'[y]/.B->Function[y,y^2]
Outputs ...
2
votes
2answers
100 views
How do you change from Traditional form to Input form?
I have something like this:
Hold[TraditionalForm[1/2 + x^2]]
And wish to change it to Input form with the use of code (that is, without clicking Cell > Convert ...
2
votes
1answer
45 views
Strange behaviour (or issue?) in using evaluation of held argument inside a table in Mathematica
I encountered a very strange behaviour on Mathematica using Evaluate on a held argument. Could anyone explain to me what is the subtlety (or is it a bug?)
Here is the expression:
...
10
votes
3answers
181 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 ...
1
vote
0answers
44 views
How to use Evaluate inside function definitions? [duplicate]
How to use Evaluate inside function definition so that is acts as Evaluate only whet function is called?
This is toy exampl, where I want to define function inside another function:
...
7
votes
5answers
431 views
What is the the proper way to define a function from an expression? [duplicate]
Say after some long computation we get an expression
expr=x^2
We do not know what the value of expr beforehand.
Now we want ...
3
votes
2answers
123 views
Display elementary math without evaluating
Given a list of operators and integers, for instance
oper = {Subtract, Times, Plus}
{a, b, c, d} = {3, 5, 4, 9}
I would like to print
...
1
vote
1answer
85 views
How to stop evaluation of Zeta with even arguments
It could be a duplicate question. However I could not find much after a huge searching.
I have a huge expression in mathematica involving many Zetas.
I want not to evaluate the even Zetas which ...
9
votes
3answers
259 views
HoldForm: Display exactly as entered?
Sometimes, Mathematica's internal representation of expressions can give somewhat surprising results. Specifically, I noticed this when I tried to preserve products of fractions for display.
E.g. ...
0
votes
2answers
55 views
Evaluation control - Hold (?) argument name not argument values
I wanted to produce a list of statistics labelled with the statistic name, for exchange with a database.
I defined
...
2
votes
3answers
106 views
Globally Defining a Variable that Depends on Arguments of Functions [duplicate]
I have many delayed-defined functions of many variables, and all the functions share a term containing some of these variables. Thus, I would like to just call the term ...
3
votes
1answer
53 views
Apply on Message resolves MessageName before applying
I want to make a Message from a List.
Message[LibraryFunction::argx, 1, 3]
...
2
votes
1answer
192 views
Unwanted Hold from recursive function
I am literally copying an example from a tutorial in Mathematica documentation of a recursive function, but when I evaluate I get a Hold. I don't understand why I ...
4
votes
3answers
43 views
Controlling HoldForm while mapping
If I execute
A = {Sin, Cos, Tan}
B = {ArcSin, ArcCos, ArcTan}
#[[1]]@#[[2]]@x & /@ Transpose[{A, B}]
the output is
...
3
votes
1answer
48 views
1
vote
1answer
60 views
RegionPlot: building a list of inequalities
I have a function like this:
Myfunction[x_,y_]:={Max[1, Min[x, 2]], Max[1, Min[y, 2]]}
and I need to do a RegionPlot like ...
3
votes
2answers
99 views
Expanding powers in an expression and Hold
Let an expression like -(1/2) + (3 x^5)/2
be given.
How can this be turned into -(1/2) + (3 x x x x x)/2?
All powers of ...
4
votes
1answer
94 views
How to extract part of a held expression without evaluating it
How can I extract part of a held expression without evaluating it?
Clear[w]
Part[HoldForm[w = 3],1,1]
(* w *)
w = 4
Part[HoldForm[w = 3],1,1]
(* 4 *)
Here I ...
1
vote
1answer
108 views
Replacing one of the integrals in the multiple integral with an analytical solution
Problem consist in simplifying complicated multiple numerical integral, where I know analytical solution for a subset of them. Integrand is of the form
...
