Questions tagged [assignment]

For questions relating to assignments to symbols, patterns, or expressions.

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179
votes
2answers
10k views

What is the distinction between DownValues, UpValues, SubValues, and OwnValues?

When reading through the documentation, you often encounter the phrases DownValues, UpValues, ...
45
votes
7answers
68k views

Assign the results from a Solve to variable(s)

I understand Mathematica can't assign the results of a Solve to the unknowns because there may be more than 1 solution. How can I assign the 4 values of following result to variables? ...
14
votes
3answers
7k views

Assigning values to a list of variable names

As part of a calculation I need to do something like this Evaluate[{aaa, bbb, ccc}[[ index]]] = {1, 2, 3, 4, 5} so if index is ...
32
votes
2answers
5k views

What is the difference between Set and SetDelayed?

I have a problem with defining a function and most of time I get confused by Set or = and ...
46
votes
2answers
2k views

How do you set attributes on SubValues?

SubValues, as discussed in a previous question, are declared as follows ...
25
votes
5answers
5k 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 ...
22
votes
5answers
911 views

Reassign values to symbols

I've got a situation where I have, say 4 symbols, a, b, c and ...
49
votes
3answers
4k 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: ...
18
votes
1answer
458 views

How to make a function like Set, but with a Block construct for the pattern names

How can we define a function that works like f[x_]=ComputeSomething[x] and treats x as a variable that does not have a value? ...
23
votes
3answers
742 views

Nested definition: How can I define a function with a passed-in expression?

Here's a simplified version of what I'm trying to do: ...
13
votes
2answers
2k views

Scoping in assigning a derivative

While answering another question, I stumbled upon a problem I cannot easily resolve. To assign the derivative of a function to another function, typically one can do this with a ...
10
votes
6answers
870 views

Generate list of strings from a list of assigned variables

How do I generate a list of strings from a list of assigned variables? E.g. convert ...
40
votes
1answer
2k views

What is the story with Removed symbols?

The system function Remove evidently exists primarily to turn a fatal problem into an annoying one, by giving the user a (rather blunt) instrument with which to ...
10
votes
3answers
443 views

How to write a function-defining function which stores the function arguments in a stack?

I have a function-defining function SetAttributes[DefFn, HoldAll]; DefFn[f_[args___], body_] := f[args] := body; I am trying to modify this function to record ...
11
votes
3answers
619 views

MapThread gives different results from ToExpression when trying to assign variables from a list

I have the following data set: data = {{"Jakarta","Surabaya","Bandung"},{1,2,3}} and I'd actually like to assign the numerical values to the categorical data so ...
6
votes
4answers
1k views

Delayed evaluation of assignment

I have a set of PDE's that depend on parameters. They depend on many parameters, but for simplicity we can here assume that they depend only on one, say a. I want ...
24
votes
1answer
599 views

Why does list assignment with a packed array result in unpacked values?

Bug introduced in 5.2 or earlier and fixed in 10.1.0 This is another question on the design choices in Mathematica. I understand that without direct reply from the developers it may not be possible ...
47
votes
1answer
5k views

++ is dangerous for C programmers

I noticed this fact, that may be misleading for programmers used to C language. In Mathematica, if you have a function f[] and an array ...
9
votes
1answer
348 views

Part not behaving the way I expect it to

p = {{1, 0, 0}, {0, 1, 0}, {0, 1, 0}}; p[[All, 3]][[1 ;; 2]] = {1, 1}; p I want to replace the first two elements of the third column of a 3x3 identity matrix. ...
37
votes
1answer
869 views

How to augment the realm of functions Mathematica thinks it knows how to integrate symbolically

My question involves extending the functionality of Integrate over specific integrals in the most generic manner. Specifically, is it possible to "hack into" ...
20
votes
4answers
2k views

How to implement the Observer design pattern?

As a programmer with a history with procedural languages, the Observer design pattern inmediately springs to my mind when attacking certain class of problems, mostly UI/notifications related. A two-...
21
votes
2answers
1k views

Does Set vs. SetDelayed have any effect after the definition was done?

I was always assuming that the only difference between Set (=) and SetDelayed (...
8
votes
2answers
866 views

How to replace a variable that was assigned a value before

Question How to make the replacement $f[x] \to f[y]$ for an abitrary function $f$ if $x$ was assigned a value before? Example 1 How can I make the replacement $x +1 \to y + 1$ if I have assigned ...
4
votes
2answers
244 views

why set values in this way doesn't work?

question is as follows define a list ttt={1,2}; and if I set values in this way {ttt[[1]],ttt[[2]]}={3,4} then the value ...
12
votes
1answer
534 views

Triggering actions when a variable is set

Some built-in variables trigger actions when their values are changed: ...
12
votes
3answers
8k views

Assign output of solve to variables [duplicate]

I have this input: Solve[x^2 + 3 x + 2 == 0, x] which gives this output: {{x -> -2}, {x -> -1}} I want the first x ...
14
votes
3answers
294 views

Why is the first argument of the SetDelayed evaluated?

In[1]: f=g; In[2]: SetDelayed[f[x_],x^2]; In[3]: ?f Global`f f=g In[4]: ?g Global`g g[x_]:=x^2 However, SetDelayed has the ...
9
votes
2answers
532 views

How to name a variable with the value of another variable?

A simple but very important (at least for me) question. In some languages for defining a variable, for example, we can do: i=10 "d"&i=30 So I have defined the variable d10, and... d10=30 ...
17
votes
4answers
346 views

Setting parts of a list

Suppose I have list a = Range[10] {1, 2, 3, 4, 5, 6, 7, 8, 9, 10} in which I want to set some elements to be a list ...
16
votes
2answers
297 views

Clearing a specific definition

Consider the case where a symbol has multiple definitions attached to it, a /: Subscript[a,2] := 1 a[b_] := 2 a[b_, c_] := 3 How does one clear (/unset/remove) ...
12
votes
2answers
692 views

How to ignore list elements when extracting with pattern matching

If I have a simple list, say {1, 2, 3}, and I want to extract a subset of the elements, what is the correct syntax? e.g. { i, _, j } = {1, 2, 3} I want i = 1 &...
9
votes
2answers
290 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.: ...
8
votes
1answer
207 views

Different behaviours of Default Argument

I don't really understand the behaviour of Default Argument. If I execute this command in Mathematica: In: {f[a], f[a + b]} /. f[x_ + y_.] -> p[x, y] <...
4
votes
3answers
382 views

Using Block with a list of variables

I have a list of variables var={v1,v2}; which I want to protect/localize during a particular operation. I have decided that I want to use ...
0
votes
2answers
269 views

Associate the elements of a table

I have a table form as: table = {{a, 6}, {b, ff}, {c, 2}} I want to associate the first element of each pair from the table to the second element of the pair as ...
13
votes
1answer
258 views

Why can AppendTo modify a referenced list in-place but Part cannot?

Part, AppendTo, PrependTo, AddTo, etc. allow in-place modification of a list, but only Part requires that the list be referenced through a simple symbol, e.g. the following all does what you'ld expect:...
9
votes
2answers
3k views

Modifying parameters: Set::setps in the part assignment is not a symbol [duplicate]

Ok, this will probably be a silly question, but I can't get my head around it. I'm making a simple module that, given a list and two integers, will swap the position of the elements in the list.(...
29
votes
3answers
533 views

Assign a randomly generated list to variable

Bug introduced in 10.0.0 and fixed in 10.0.2 I don't know if this is a silly question, but it really baffles me. So please help me understand this behavior. ...
8
votes
1answer
840 views

How is LHS = RHS; … ; LHS (nontrivially) different from … ; RHS

I'm trying to understand how Mathematica thinks about the results of Set. Suppose we perform LHS = RHS. I'm focusing on when ...
10
votes
3answers
413 views

Why does ++++x return an increment of 2 when the value of x is only incremented by 1?

This line returns 3: x = 1; ++++x However, the value of x after the increment is only 2. ...
9
votes
2answers
173 views

DownValue assignment using Apply

Why does assigning a DownValue using Apply, e.g., ...
4
votes
3answers
610 views

Altering values in Manipulate with dynamically generated controls

I have a Manipulate whose controls are generated based on a parameter in a containing Manipulate: ...
1
vote
1answer
238 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? ...
3
votes
1answer
66 views

Using HoldFirst at the same time as Apply

I have some code which pops the first number from a list, displays that number, and updates the list: ...
3
votes
1answer
271 views

Is there any way to implement a “sequential” With[] in Mathematica? [duplicate]

I want the equivalent of Scheme's let*, or basically, a sequential With that works like this: ...
18
votes
4answers
1k views

How can I randomly generate a unique, unused symbol?

In a previous question on collecting terms for a multivariable polynomial, I answered with a solution that required a unused symbol to be used for a temporary substitution and back substitution. It ...
7
votes
2answers
198 views

Problem with Simplify, Sqrt, and Set: What's going on?

I am having some trouble getting Mathematica to simplify an expression of the form Sqrt[x]*Sqrt[1/x], where x>0. The problem ...
5
votes
1answer
445 views

Assign variables automatically to a function

Lets consider that I've this function: fn = a*x^2 + b*x + c; The idea here is to assign automatically the variables a, ...
8
votes
2answers
442 views

Problem with function inside brackets. Bug? [duplicate]

This code adds random digits to lists, and it works fine: ...
6
votes
3answers
808 views

Understanding Function and Variable in Mathematica for beginner [closed]

I'm reading the book of Leonid Shifrin, and I love the way the book explains the core in Mathematica. As everything is an expression, now I understand that, there ...