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Questions tagged [replacement]

1636 questions
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Is there a difference between Replace with parameter "All" and ReplaceAll

Here is my extremly basic understanding of Replace and ReplaceAll. This post is also a way for me to check if I understood the mechanism behind, if you see mistakes in my explanation don't hesitate to correct me ! Replace is a function that will…
StarBucK
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Replace custom functions, leave built in functions untouched?

I have three expressions a[x, y], b[x, y], c[x, y] that act as placeholders for functions of two variables x,y. Consider the following substitution: a[x, y]/(b[x, y] c[x, y]) /. f_[x1_, y1_] :> f[2 x1, 3 y1] a[2 x, 3 y]/(64 b[x, y]^3 c[x,…
Kagaratsch
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Why Infinity /. _?(# < 6 &) -> 0 replaces Infinity with ComplexInfinity?

Suppose I define a matrix: mat = { {Infinity, 4, 9}, {6, Infinity, 10}, {8, 9, Infinity} }; Now I want to replace all values smaller than 6 with 0, mat /. _?(# < 6 &) -> 0 However, that ends up replacing Infinity with…
a06e
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How to ReplaceAll except within specified heads

ReplaceAll[expr, rule] and ReplaceRepeated[expr, rule] search through all subexpressions of expr applying rule where they match. Are there built-in Mathematica functions that do the same thing, except not make any replacements inside specified Heads…
QuantumDot
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How to assign a list of numbers into letter grades

Here is my list: collection = {76.6256, 51.9264, 50.238, 14.4203, 80.9205, 12.2036, 2.39568, 38.2747, 12.4422, 29.9621} collection /. {x_ /; x < 50 -> "F"} and this gives me {76.6256, 51.9264, 50.238, "F", 80.9205, "F", "F", "F",…
Sultan of Swing
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substitution rules excluding subscripted variables

Suppose I have an expression containing p,Subscript[p,1],Subscript[p,2],etc. I want to replace p with a numerical value while keeping the other two variables intact. How can I do it? If I naively use /.{p->0.5} for example, I will get…
wdg
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Using PatternSequence on its own in ReplaceAll?

Say I have this list: list = {0, 1, 2, 0, 1, 3, 0, 1, 4} and say I want to remove consecutive sequences of 0 and 1. I'd normally go for this: list //. {before___, 0, 1, after___} :> {before, after} {2,3,4} But a more concise way would be the…
Teake Nutma
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“Strange” behavior of Rule

According to the Help lhs->rhs evaluates rhs immediately. How to understand the output of the following code? ClearAll@x; {1, 3.5} /. x_?IntegerQ -> {x} Output is {{1},3.5} Since Mathematica evaluate the rhs immediately, I think the “correct”…
luyuwuli
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Indexed replacement

Is there some equivalence of MapIndexed for rules? Consider a substitution on 3 a + 4 b + 21 c + .... The first integer I want to replace by 1, the second integer by 4, and the i'th integer by i^2. 1 a + 4 b + 9 c + .... How would you do…
sjdh
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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 codes to specify which sets of rules need actually be scanned for a particular input…
matheorem
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Is there a way to replace only once if possible by trying the rules sequentially in order?

Is there a ReplaceOnce which does only one replacement if possible by trying the rules sequentially in order. Consider the following as an example: ReplaceOnce[{"May","5","May","5"},{"May"->1,"5"->2}] should…
user13892
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Piping and Replacement

I haven't been able to figure out how to use piping with certain functions. Specifically, I would like to apply a replacement at the end of a pipe, like the following: (x^2 + y + Log[z])^2 /. {y -> x/2} // Expand /. {z -> E^x} Also, is there a…
jmbejara
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Replace expressions with symbols

First of all: I'm new to Mathematica, so I would appreciate it if the answers are quite complete. I have the result of calculation that is expressed in $\sin$ and $\cos$. Now, all of these can be rewritten in terms of the values $T_j = \frac\pi{j}…
JT_NL
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Safely nesting RuleDelayed

Say I have the following, straightforward-seeming functions: makeARuleDelayed[a_, b_] := With[{anotherRule = makeAnotherRuleDelayed[b]}, v : a[b] :> ((v == 1 - v) /. anotherRule)]; makeAnotherRuleDelayed[x_] := v : h_[x] :> foo[h, x]; If I…
Pillsy
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A replacement question

I have: {{θ -> 0}, {θ -> (2 π)/ 3}, {θ -> π}, {θ -> (4 π)/3}} I would like to create a list of points $(\cos\theta,\sin\theta)$ using each of the values in this list. That is, I want a simple way to convert to: $$\{(\cos 0,\sin 0),…
David
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