Questions about Mathematica's rule-based replacement functionality, including functions such as Rule (->), Replace, ReplaceAll (/.) and ReplaceRepeated (//.).

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0
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1answer
63 views
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3answers
176 views

Conditional Check on Matrix

Suppose I have the following matrix (i,j): ...
0
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0answers
82 views

Back Linear Substitution

I have an equation set Ax=b and A(n*m),n<m . To lower down the dimension, I express some vars by other vars, which gives ...
0
votes
0answers
19 views

Manipulating named patterns inside replacement rules [duplicate]

When I evaluate the following expression, {a -> "value1", b -> "value22"} /. {Rule[x_, y_] -> Rule[x, y]} I get ...
3
votes
0answers
64 views

Why doesn't ReplaceAll work on Image? [duplicate]

ReplaceAll seems not to reach inside Image, and I don't see why it shouldn't. Here is an example. Create a black and white image ...
1
vote
2answers
152 views

Replace a column in table with a list of numbers?

I have a Table of values e.g. {{x,y,z},{x,y,z},{x,y,z}…} How do I replace the the "z" column with a List of values?
4
votes
1answer
210 views

replace items in a list with items in a another list

Suppose I have a list A={x1,x2,x3}, a list B={3,4,5}, and a function f[x]=x1+x2+x3. Is there ...
2
votes
2answers
136 views

Generating an editable table that can be used as input

Say I have a number $n_{max}$. I want a command to generate a table like 1 1 2 2 3 3 4 4 ... nmax nmax where the second column is editable, and this table can be ...
1
vote
2answers
92 views

How to “reversely” replace definitions?

I have the following problem at hand. 1.) I have a list of definitions like ...
0
votes
2answers
105 views

Handling output that was returned unevaluated from a function

When evaluating functions, including those described in this post, how do Mathematica users handle values returned from a function that are unevaluated? For example (based on 1): ...
3
votes
1answer
38 views

Table doesn't replace inside conditionals (If, Which) [duplicate]

This: Table[If[cond, y], {y, 1, 3}] outputs this: {If[cond, y], If[cond, y], If[cond, y]} The ...
6
votes
3answers
380 views

How to make all numbers equal to one in a Series?

I have many outputs of one-dimensional Series expansions for which I am only interested in the general tending with the variable. For example, I would like to be able to transform something like ...
5
votes
2answers
186 views

How to “ReplaceAll” using a set of conditional expressions instead of a list of rules [closed]

Suppose that I have two linear functions f[x_] := f0 + f1 x g[x_] := g0 + g1 x and a (possibly rather complicated) set of conditional expressions, obtained ...
0
votes
2answers
76 views

How do I use a passed-in symbol as a tag to define UpValues?

ClearAll@foo; foo[tag_] := ( tag /: tag[i_] := tag[[i]] ); a = {7, 8, 9}; foo[a]; a[2] I wanted the above to output 8, ...
1
vote
1answer
61 views

How do I extend (prepend or append to) an existing TagSetDelayed rule? [duplicate]

I have a TagSetDelayed rule associated with, say, tag: ...
0
votes
1answer
393 views

How to properly substitute values into variables?

I have an array of functions in terms of a[0] through a[n] and I wish to replace the a[]'s with previously calculated values p[0] through p[n], as in below: ...
1
vote
1answer
47 views

How to substitue local variables into expression with free variables?

I'm wondering how to do the following: expr = L + R; foo[L_, R_] := expr; foo[1, 2] (* L + R *) and have it substitute the function variables ...
7
votes
3answers
158 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*) ...
1
vote
2answers
93 views

How to get a rule for, e.g., (x + y) to apply to (-x - y), etc.?

For example, the substitution (x + y) -> s fails here: In[1]:= (-x - y) /. (x + y) -> s Out[1]= -x - y Of course, I ...
3
votes
2answers
129 views

truncating terms in symbolic expression basing on their values

I need to simplify big symbolic solution of equations system by truncating terms that are orders of magnitude less then others in subexpressions like a + b. The solution is big fraction with sum of ...
3
votes
1answer
68 views

Have transformation rules to be explicitly set?

I think this problem is not the hardest one, but I can't hit on it. I want to NSolve an equation. For this equation I use a transformation rule "ampTimes". First I used it like this: ...
4
votes
1answer
118 views

Replace rule with function? Derivatives don't evaluate

Say I have an expression (call it expr) involving a function, f[x]. I'd like to be able to evaluate that for a particular choice of f[x] without setting that choice for the whole session. I thought to ...
3
votes
4answers
304 views

Replace symbols with values without evaluation

If I have assigned values to a, b and c: ...
3
votes
1answer
62 views

Replacing multiple instances of a symbol with different variables and assignments

I have an expression of the form: expr=x+y+n+Sin[n]+n^2 I would like to replace each of the variables n with a different ...
3
votes
1answer
89 views

some confusion about ReplaceRepeated

This is Right. NestWhile[Log, 100, # > 1 &] {100} //. {x_} /; x > 1 :> {Log[x]} 100. //. x_ /; x > 1 :> Log[x] This is wrong,why? ...
0
votes
1answer
87 views

How to convert the expression '{a->a0, b->b0}' to the expression 'a=a0; b=b0'? [duplicate]

How to convert the expression '{a->a0, b->b0}' to the expression 'a=a0; b=b0'? I think this is a common question and do not know if someone has asked it before. Any help or suggestion will be ...
1
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0answers
63 views

How to simplify expressions

I read several similar posts but I have not found a solution yet. However, I think that my problem should be quite easy. So here we are. Relations ...
0
votes
1answer
304 views

How to replace multiple variables at once

I would like to make a replacement "c1 C1 -> F1, c2 C2-> F2...." such that the expression like this "c1 C1 c2 C2 c3 C3 c4 C4" becomes "F1 F2 F3 F4". I used the standard replacement method: ...
0
votes
1answer
93 views

How to transform the polynomial in this way?

I have two functions f and g and a polynomial ...
0
votes
2answers
206 views

Replace all elements of a list

I have a set that looks like: s = {{1, 2, 3} -> 9, {4, 1, 9} -> 9, {3, 7, 3} -> 1}; Now, I want to replace each element of the set by the lhs of "...
0
votes
0answers
112 views

Alternate method of conjugation using replaceall

Mathematica really struggles to simplify complex conjugates of complicated expressions - even given assumptions for every symbol in the expression being real. Often times I've found that I can ...
2
votes
1answer
79 views

Convert Integer to Numeric with Replacement rules

I have a long list of triples, each looking something like {AGO, 1988, 2345.23}. Some of these, however, have an integer in the third spot, like this: ...
11
votes
2answers
210 views

ReplaceRepeated seemingly omits some rules

Is there a simple way in Mathematica to prevent rule reordering? Let me give an example: ...
0
votes
0answers
56 views

How to make this particular replacement involving exp

I apologize if similar questions have already been asked, but I have some difficulties with replacements in Mathematica and I would like to make a substitution like ...
1
vote
2answers
126 views

Why does using ReplaceAll twice produce a different result?

Why does this: Solve[a x^4 + b x^3 + c x^2 + d x + e == 0, x] /. {2 c^3 - 9 b c d + 27 a d^2 + 27 b^2 e - 72 a c e -> F} produce a different result from ...
2
votes
3answers
184 views

Slow image processing using ReplaceAll

Can anyone explain me, why this line of simple code is so slow? And can it be improved somehow? ...
2
votes
2answers
109 views

Converting a date to a calendar quarter

I have a list that includes dates. I need to convert each date to be the last day of the quarter that it's in. So for example I want {2013,7,1,0,0,0} to become {2013,9,30,0,0,0}. Actually I don't ...
0
votes
1answer
91 views

Want to use column names as indices, but without quotes

The following works: ...
1
vote
1answer
45 views

Different ordering for KroneckerDelta?

I have an expression like this: Assuming[m [\Element] Integers && n [\Element] Integers, 1/2 I J n (4-m n+n^2) KroneckerDelta[m,-n]]//FullSimplify Now, ...
7
votes
1answer
176 views

What is the principle behind Dispatch?

I want to know what Dispatch actually does to rule the list. Why is it so fast? The documentation says Dispatch generates a dispatch table which uses hash ...
0
votes
1answer
111 views

replace is not happening for specific vector valued expression

Given x[t_] := {Subscript[x, 1][t], Subscript[x, 2][t]} following replace works fine x[t] /. x[t] -> 0 0 but following ...
1
vote
2answers
67 views

How do I avoid vector variables being treated as scalars in the course of replacements?

I often work with a mixture of symbolic and concrete values as I develop teaching materials. I introduce variables symbolically and then need to instantiate them to specific values while continuing ...
1
vote
2answers
104 views

How to define a rule to simplify the notation for derivatives of chain rule?

Suppose that $u(x,y)$ is a function, and $x=r \cos t$, $y= r \sin t$, then we can define a new function $v(r,t):=u(x,y)$ with $x,y$ replaced by the above equations. It is quite easy to compute $v$'s ...
0
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0answers
43 views

is there a way to speed up rule replace? [duplicate]

I am currently doing calculation with 7 parameters, each parameter set corresponding to a value. And I store the data in the format as a list of rule, like following ...
5
votes
4answers
227 views

How to split a list with respect to its nested sublists?

I have a problem with the following task: having some nested list list = {{k6},{k1,k3,k5},{k7}} I would like to replace all sublists by their permutations and ...
0
votes
2answers
149 views

Replace doesn't replace the denominator [duplicate]

I have the following expression in Mathematica -((E^(2 - 2 Sqrt[(x[1] - x[2])^2]) (x[1] - x[2]))/Sqrt[(x[1] - x[2])^2]) I want to replace anything of the ...
2
votes
1answer
137 views

Derivation after substitution not working

I have a code that will output an expression in form of a functions like this ...
3
votes
3answers
78 views

Operations on expressions within Hold [duplicate]

I want to convert {Hold[1 + 2], Hold[3 + 4], Hold[5*6]} into Hold[{1 + 2, 3 + 4, 5*6}]. I have tried ...
0
votes
1answer
143 views

How to return the rhs of a transformation rule from within Compile[]

I have defined the following function: ...
0
votes
1answer
93 views

Function generating a rule with a Patterns: e.g. toRule[ lhs, rhs] into lhs[x_] -> rhs [duplicate]

I want to create a rule as a function of z from a left and right hand side. My (non-working) code and usage is: ...