Questions about functions which maintain an expression in unevaluated form, including the use and behavior of Hold, HoldForm, HoldComplete, and HoldPattern. Also questions concerning the attributes HoldFirst, HoldRest, HoldAll, etc.

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58
votes
3answers
2k views

Replacement inside held expression

I wish to make a replacement inside a held expression: f[x_Real] := x^2; Hold[{2., 3.}] /. n_Real :> f[n] The desired output is ...
33
votes
3answers
1k views

Changing Values in an Association using Map

I am trying to change the value of one key in an association based on the value of another key in that association. So if my association starts as: ...
21
votes
2answers
793 views

How to inject an evaluated expression into a held expression? [duplicate]

I know that there are methods to structurally manipulate held expressions (discussed e.g. here), but I failed to apply those for this particular problem: ...
20
votes
5answers
2k views

How to pass a symbol name to a function with any of the Hold attributes?

Given a function with the attribute HoldFirst, HoldAll or similar, and a variable, list, how ...
15
votes
4answers
421 views

Prevent graphics render inside held expression

Assuming each odd line is input I would like the even lines to be the output of the following lines. In the above the Graphics are inside ...
12
votes
2answers
780 views

Displaying a series obtained by evaluating a Taylor series

Description of problem I would like to use Mathematica to display the series obtained by substituting a value for $x$ in a Taylor series expansion. The terms of the series will be rational numbers, ...
10
votes
2answers
269 views

Held keys in associations

Jacob Akkerboom commented: Note that PositionIndex does work correctly with held expressions, whereas this is a bit painful to implement using ...
8
votes
4answers
209 views

Hold expression with RandomInteger

One more question for today: I'm trying to show two random integers with a plus (+) sign between them, in an unevaluated form. I know how Hold and HoldForm work, but they hold everything, including ...
7
votes
3answers
191 views

Converting StringJoin to StringForm inside Hold

Let's say I have a piece of code: Hold[{code1, "asdad " <> ToString[testa] <> " adsd " <> ToString[testb], code2}] (*MWE ofc*) ...
19
votes
2answers
305 views

Pattern does not match with Orderless head

Why does the following pattern matching not succeed? ...
7
votes
2answers
267 views

HoldForm[Operator ##] on some list

Recently in response to this question Mr.Wizard suggested an unusual way to summing numbers. This doesn't seem to be documented. ...
2
votes
1answer
299 views

Why is this expression being held, and how can I evaluate it?

I can't figure out why an expression isn't evaluating, even though there's no Hold* in its FullForm: ...
6
votes
4answers
331 views

Is there a more Mathematica-y way to label these plots?

I am studying an infinite square well in the context of quantum mechanics. ...
9
votes
2answers
156 views

Passing an unevaluated part of an association to a function

I'm making a UI widget that controls sound volume on a speaker using a slider, e.g.: ...
5
votes
4answers
301 views

HoldForm does not Hold Form for fractions sometimes

I'm facing a strange behavior of HoldForm. I need to display 1/2*3/4 in LaTeX like this : $$ \frac{1}{2} \times \frac{3}{4} $$ ...
4
votes
2answers
76 views

MatchQ failing in some cases with Hold [duplicate]

Could someone clarify why the first of these MatchQ finds a match whereas the second does not? (I'm using version 10.0, in case that matters.) ...
11
votes
1answer
500 views

Get the name of a symbol passed to a function

I'm trying to get the name of a symbol passed to a function with this: f[x_] := {SymbolName[x], x} SetAttributes[f, HoldFirst] x = 5; f[x] But ...
7
votes
6answers
214 views

How to join held Lists?

Let's say I have a = Hold@{2+2} b = Hold@{4+4} How can I get Hold@{2+2,4+4} ie join the two held lists ? More ...
1
vote
1answer
184 views

direct assignment to the element in a Table

I have already asked a related question here why set values in this way doesn't work? But I think I have to write the question which I encountered explicitly? ...
4
votes
3answers
154 views

Getting terms and only evaluate specific parts

I want to expand the expression but I do not want it to evaluate. Sum[(HoldForm[1/(2 # - 1) - 1/(2 # + 1)] &)[i], {i, 1, 6}] However if you evaluate this you ...
2
votes
1answer
283 views

Plot, HoldAll and the color problem

I have a similar problem as posted in Difference in Plot when using Evaluate vs when not using Evaluate, however due to a numerical evaluation I cannot use ...
15
votes
1answer
247 views

Why are the attributes of Plot changing?

In version 9, with a newly started Mathematica session, try the following code: ...
11
votes
3answers
231 views

Creating a custom context with a Hold construct

I'm trying to create a function "InContext" to execute a piece of code inside a certain context (and to return the result of evaluating that piece of code). One approach is to place the code inside a ...
21
votes
3answers
427 views

Can't inject EncryptedObject

Bug present from 10.0 to 10.3.1 and fixed in 10.4.0 !! Here are minimal examples: ...
13
votes
4answers
356 views

Replacing parts of a held expression with held parts of another expression

I'm applying the Trott-Strzebonski technique in order to replace inside a held expression. ...
12
votes
7answers
624 views

How to properly inject iterators into table? [duplicate]

Goofing off on a prior question, I was fiddling with other methods, which led me to the need to inject a constructed set of iterators into a table construct. Now, ...
17
votes
2answers
471 views

When should I, and when should I not, set the HoldAll attribute on a function I define?

I have seen a number of examples on this site (such as in syntax highlighting and checking evaluation status) in which functions definitions are preceded with ...
3
votes
4answers
624 views

Make function return unevaluated equation

I have made this function which outputs the equation of a nth degree polynomial: ...
6
votes
2answers
94 views

Pattern matching an expression involving Times

I'm trying to write a Mathematica function that will format expressions according to my preferences, for example displaying (5 c x)/Sqrt[2] as ...
5
votes
4answers
325 views

Concatenating strings, unevaluated expressions and evaluated expressions/values

Maybe this is something very simple to some of you. Anyway, I think this can be useful to many others like me. I would like to build an output that involves strings, unevaluated expressions and ...
2
votes
1answer
136 views

Need to take infinite sum of residues, is there a way to choose the order of operations for ReleaseHold?

I'm computing an infinite sum of residues. I want to do something like this: ...
2
votes
2answers
136 views

Extracting variables from Hold expression

For example I have some global assignments x=1;y=2;z=3 and an expression Hold[x*y/z]. How can I now extract variables ...
1
vote
1answer
183 views

How to evaluate some parts of an expression inside Hold? [duplicate]

I want to return a Hold expression but with some parts pre-evaluated. The easiest way to explain this is probably through code example. Here is what I have ...
1
vote
2answers
122 views

Pattern matching to head with HoldFirst

I want to define a function that only operates on expressions with a head from a given list, such as f[expr:(_And|_Or)] := Print["LHS is ", expr[[1]]]; This of ...
1
vote
1answer
211 views

Passing a joined list of symbols for Module or Block to treat as its own local symbols

The Question I'd like to write a function which itself takes a list of symbols, e.g. Foo[{x, y, z}] to pass to Module ...
1
vote
2answers
765 views