# Tagged Questions

Questions about transformation rules in Mathematica--constructs that let you set, or programatically modify, local values for expressions.

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### Rule for derivatives

I have a differential equation of the form $$f'(x)=g(x,f(x))$$ (where $g$ is has a known explicit form, e.g. $x f(x)$) and I have an expression which contains several derivatives of $f$. I want to ...
40 views

### Generate rule that replaces if string matches pattern

I have let's say a function f[x], but for x I pass a string. And I want to replace that string with a number, if that string ...
45 views

### What is the function of OrderedQ in these expressions?

Could someone please let me why the output of the two expressions change with OrderedQ in place. The first expression is as follows: ...
39 views

### Possible bug with KeyValuePattern and Optional

I expect to get 0 on every line of output of this: ...
58 views

### Why does syntax highlighting in Set and Rule not color pattern names on the RHS?

The mathematica front-end colors x on the right hand side of x_ :> x and f[x_] := x but ...
146 views

### How to scope Pattern labels in rules/set?

Module[{x}, f@x_ = x; p@x_ := x; {x, x_, x_ -> x, x_ :> x} ] ?f ?p gives ...
83 views

### Association matching still not working perfectly?

Up until 10.4 you could not match on internal Association structure at all: http://mathematica.stackexchange.com/a/119542/6804 Now you can, but it seems flawed ...
97 views

### Defining a symbolic inner product

So I have some rules for a symbolic inner product that work for quite a lot of cases: ...
46 views

### How to replace the argument of a function?

For the following expression, b'[x y/z]+z b''[x y/z] I want to replace the argument of b such that the above expression becomes ...
39 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 ...
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### What are rules in Mathematica and what are its uses? [closed]

I'm new to Mathematica. I have been going through the official reference and came across this article on rules to which I was led by a Google search on how to ...
41 views

### Why doesn't recursive transformation stop when rules no longer match?

In an exercise where I was trying to pick out consecutive words starting with the same letter, i.e. I wanted: ...
62 views

### Pattern which will transform a given expression [closed]

Find the Pattern which when applied to this expression expr = {a^2, a^b, x^4, (x - 1)^2}; yields this result: ...
43 views

### Rule/Replace/Patterns [closed]

I need help with doing this question. given: Sin[x+y] Cos[x+y+z] Define using Patterns a rule (specifically a list of 2 rules ) , the two adding rules for trigonometric functions sin (a + b )...
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### Module output rules like Solve FindRoot etc built-in functions

How do I print rules {x -> values} as an output from my module? I'd like it formatted just like those from the built-in functions, so that users can use Out[ ] or % to directly apply the rules. ...
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### Select and use only one solution in a list solutions [closed]

I would like to use and plot only one of the two solutions of a quadratic equation. In particular this is my equation ...
240 views

### How do I encode this replacement rule?

I'm pulling a 5x5 array from a larger array. The small sub-array (call it S) can contain any combination of the symbols A, B or 0. What I'd like to do is: if the array contains any matching pairs ...
112 views

### Replace list element with element from another list based on condition

I Have a list on integers and another list of "duplicates". E.g. start={{3},{1},{2},{4}}; duplicates={{1,4,7},{5,6}} I would like to substitute each element of ...
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### How to use the function Predict[] with variable-length dataset?

I can use the function Predict[] with pre-defined dataset successfully. A = {1, 2, 3, 4, 5, 6, 7, 8, 9} -> {1, 2, 3, 4, 5, 6, 7, 8, 9}^3; p = Predict[A] ...
221 views

### Pattern Matching for Rules [duplicate]

Consider a set of rules, e.g. {a -> aa, b -> bb, c -> cc, d -> dd, e -> ee} I want to remove from this list all patterns of the form e->_ and to ...
188 views

### How to prevent substitution in a part of an expression?

Background: I am trying to write a program that can execute scheme in Mathematica. Now I have things like {"+", 3, {"-", 2, {"'", {"*", 1, 3}}}}. The syntax ...
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### RSolve - basic question about using rules as functions

I think this is basic question, but I can't find it in Help... (I am begginer :P) I have this expression ...
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### How does one exchange an option with another one?

Consider an expression with a list of options at its end, like ...
54 views

### Rule for manipulate an expression

like as my precedent post I have this Expression: ...
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### Replacement of a group of variables in trigonometric expression

I would like to make simplification of trigonometric expressions by using a replacement a group of variables. Here the equation I have to simplify : ...
109 views

### How to make a postordered ReplaceAll that operates just once per subtree?

The answers to this question provide code for a function I'll call postOrderReplaceAll to replace from the inside-out: ...
63 views

### Using a rule to increase precision [closed]

I'm sure this is a very simple question, but I can't work out how to do it. Construct a rule to set x=0.32 to a precision of 20 digits. The rule is to be applied ...
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### Setting initial conditions of a domain grid?

So I'm writing some code to try and produce a cellular automaton which will model an oscillatory chemical reaction, but I'm trying to set up the initial conditions of the grid so that I can vary them ...
278 views

### Combinations of multiple matching patterns

I wonder if there is any nice way to combine DownValues (or any other suitable rule-/pattern-/function -based Mathematica construct) when multiple patterns match an expression. Let me explain what I ...
36 views

### How to simplify a ratio [closed]

I have a ration of the form $\frac{p^2}{(p^2+1)^2}$. I can simplify it using $p^2=p^2+1-1$, and to get $\frac{p^2}{(p^2+1)^2}=\frac{1}{p^2+1}-\frac{1}{(p^2+1)^2}$. How can I create the rule to do it ...
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### Using two rules for the same variable

I am a newbie in the field so please not angry. I am reading the tutorial below and having a problem. Link: How to Create and Use Rules From the link: If you give two rules for the same ...
202 views

### Applying rules to expressions [closed]

I'm trying to write a rule that acts on any number, lists, expression and so on to turn any negative numbers positive. The simple r = n_ -> Abs[n] works ...
238 views

### ReplaceAll with Associations instead of Rules

I have just noticed that x /. <|x -> 123|> evaluates to 123 In other words, the association seems to behave as a ...
119 views

### How to force a replacement in an order that I want?

This is a follow-up to this question. I have a big sum, with lots of terms of the form ...
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### Composition of rules: a->b + b->c = a->c

I am trying to compose two rules to form a new one: {a -> b, b -> d} and {b -> c} should become ...
47 views

### Output of Coefficientrules into Jacobian matrix

I have a pair of large polynomials in u, v, a1...a9 and b1...b9, generated by ...
112 views

### why doesn't the replacement rule work?

I have the following code: ...
120 views

### Swap LHS's (or equivalently RHS's) of two rules in a list

Assume I have a list of rules, each of the form $a_i \rightarrow b_i$. What is the best way, for some $i$ and $j$, to swap $a_i$ with $a_j$ (or equivalently, $b_i$ with $b_j$). Currently, I have ...
213 views

### How to make a ReplaceAll Rule for adjacent items in a list?

I would like to make only one pass of a list of 2D items and replace adjacent items that have matching $x$ or $y$ coordinates with a series of linearly interpolated points between the two points. My ...
36 views

### ReplaceAll - Position of list arguments not displayed properly [closed]

This code A[1, 2, 4, 5] /. A[a___] -> Position[{a}, 4] return {} Why this code not return 3 (a position of number 4) ? I ...
55 views

### How could I check if a differential equation crosses a specific value? [closed]

I'm designing a magnetic field to simulate relativistic speeds in a small cyclotron. I need to see if the ratio -R/B * dB/dR crosses 0.2 in a certain range, and right now I'm trying to do that by ...
182 views

### How to change symbols in differential equation with a single rule?

Problem Suppose I have a differential equation such as $\qquad f'(x)+f(x)=0$ and I want to change it into $\qquad y'+y=0$ My Method Using rule = f[x] -> y,...
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### How to replace an element in a list with an element in another list with matching indices

I have a list like this l = {1 + 2*s, 2 + 3*s, 4 + 1*s}, and I have calculated s = {1, 2, 3}. I need to replace ...
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### How avoid error message with Range?

Evaluating Range[1, n] /. n -> 3 produces an error message ...
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### Avoiding misconversion of data with ToExpression function [closed]

I've got a list of data with the following structure which can be easily converted To expresion by some manipulation such as ...
63 views

### Why is the evaluation of f different using rules and using arguments to it as a function? [closed]

I will provide all the code I used at the end of the question. I was trying to evaluate a function f with the following signature (which I'll give the details at ...
196 views

### How does one unpack the contents of a list while using rules to substitute values for each variable?

My intention is having a large collection of variables (symbols to be more specific) in a list and substituting all the variables at once with a value using rule instead of going through each one, one ...
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### How to disqualify a rule [closed]

Executing this line Assuming[x < 0, x -> 10] Expected output: False Say x -> 10 is a root of a polynomial ...