# Questions tagged [assignment]

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

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

When reading through the documentation, you often encounter the phrases DownValues, UpValues, ...
• 34.1k
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### 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? ...
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### 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 ...
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### 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 ...
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### How do you set attributes on SubValues?

SubValues, as discussed in a previous question, are declared as follows ...
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### 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 ...
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### Reassign values to symbols

I've got a situation where I have, say 4 symbols, a, b, c and ...
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### 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: ...
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599 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? ...
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### 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: ...
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### 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 ...
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### 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 ...
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### 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 ...
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534 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. ...
• 123
1k 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 ...
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### 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 ...
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725 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 ...
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### ++ 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 ...
682 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 ...
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### Does Set vs. SetDelayed have any effect after the definition was done?

I was always assuming that the only difference between Set (=) and SetDelayed (...
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### Why is the first argument of the SetDelayed evaluated?

In[1]: f=g; In[2]: SetDelayed[f[x_],x^2]; In[3]: ?f Globalf f=g In[4]: ?g Globalg g[x_]:=x^2 However, SetDelayed has the ...
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### 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" ...
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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-...
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### 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 ...
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### 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 ...
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### 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 ...
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605 views

### Triggering actions when a variable is set

Some built-in variables trigger actions when their values are changed: ...
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### 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 ...
450 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) ...
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### 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 ...
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### 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.(...
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### 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:...
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### How to pass variables by reference and by value [duplicate]

I haven't seen this question specifically addressed before in this site, although some hints and traces have been given, for instance, here: Pass by reference for an option argument Does pass-by-...
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### 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 &...
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### 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.: ...
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242 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] <...
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### 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: ...
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556 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 ...
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1 vote
936 views

### Variable dependency...easy one

I'm brand new to Mathematica...should be an easy one. Instead of writing u[x] everywhere in the code, how do I assign the dependency early on like this: u=u[x] ...
328 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 ...
706 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. ...
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500 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. ...
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### 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 ...
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### Why Optional in Set is not held by the HoldFirst attribute?

When evaluatingf[x_:x0] := x, x0 will be evaluated, regardless of the HoldAll attribute of <...
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### DownValue assignment using Apply

Why does assigning a DownValue using Apply, e.g., ...
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### Altering values in Manipulate with dynamically generated controls

I have a Manipulate whose controls are generated based on a parameter in a containing Manipulate: ...
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104 views

### Set on symbol with OwnValues skips UpValues

Can anyone provide insight on what's going on here? First I define a new Set behavior for a symbol: ...
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### Assignment with Span to Indexed Variable

If I use span to assign a value to every element in a "column", it miraculously works: ...
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160 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: ...
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1 vote