Questions tagged [pattern-matching]
Questions on expression testing and manipulation through pattern matching and constructing efficient patterns.
1,264
questions
6
votes
0answers
36 views
Why does Longest not work on OptionsPattern?
When having a function with optional arguments and options, I frequently find myself writing something like this:
...
2
votes
1answer
43 views
Is there a way to extract a sub pattern from a repeated PatternSequence?
Consider the following Pattern expression:
...
8
votes
2answers
195 views
Pattern matching product with arbitrary number of terms
I am trying to write a function to convert an expression of the form
$$\alpha x_1^{n_1} x_2^{n_2} ... x_N^{n_N}$$
(with some coefficient $\alpha\in\mathbb{R}$) into a list of indices and powers
$$\{\{...
3
votes
1answer
70 views
Pattern matching problems involving FullForm and Plus and Times
I am trying to match an expression that will be a sum of more than one terms, with the last one being a product. For example, the objecta+b+x*y. I am using the ...
3
votes
1answer
106 views
Alternatives in pure function construction
When I declare a function f in the following form:
f[a|b] := 1
then I can use it like:
...
0
votes
0answers
25 views
Issues in expanding expressions with formal sums
I have a very long expression involving formal sums to infinity, unfixed functions and several variables.
In this very long expression, I need a typical term, like
$\frac{6 A \sigma \sum _{n=0}^{\...
0
votes
0answers
20 views
How to rearrange terms in an expression to a particular form (e.g. A/w-> A*(1/w)
I have a huge expression for which the numerators are a big mess, involving numbers, variables, formal sums and unevaluted function, let's call the numerators Ai, with i=1,2,...n
This "beast"...
3
votes
1answer
38 views
Type formation rules for 1-position expressions
I made a summary of the rules for formation of types for simple 1-position expressions, as I understand them.
Here it is in textual form:
A symbol, say, α, is a discrete entity, represented by a ...
2
votes
0answers
36 views
What sorts of expressions can be Solve'd for?
We can Solve for expressions that match _Symbol, as in Solve[x^2+1 == 0, x]—but we can also ...
1
vote
1answer
49 views
Defining a two-variable function with a parity property
I would like to define a function that has the following property $f(-x,y)=f(x,-y)$.
An example is $f(x,y)=x y$. For simple inputs, I can impose this easily enough as:
...
3
votes
1answer
61 views
How to Map over a BlankSequence? [closed]
I would like to replace any function f[x, y, ...] in a certain expression with the same function with all its arguments squared, i.e. ...
3
votes
1answer
67 views
How to do context-sensitive matching with the pattern system?
In Perl RegEx, we have the look-ahead/behind feature; it there a similar functionality available for Wolfram Language patterns?
Re @mikado:
Suppose we have a random expression in ...
4
votes
1answer
75 views
SequenceSplit around empty sequences is inconsistent
Bug introduced in 11.3 and persisting through 12.3
Only the first one recognizes a zero-length sequence between non-X's.
...
0
votes
1answer
50 views
Finding a mapping between two types of (generalized) hypergeometric series
I am given two functions, one is of the form $2F1(a,b,c;z)$, where $2F1$ is a hypergeometric series. The other one is a generalized hypergeometric series $3F2(d,e,f;g,h;w)$, where the characters are ...
1
vote
3answers
36 views
How can I let StringPosition consolidate overlapping matches?
Consider an example like this one:
StringPosition[" -aa.ef -ag.hi -aa.ei",
RegularExpression["\\s+-(?=a+\\.)"]]
which returns
...
3
votes
1answer
48 views
How can I let RegularExpression remove all colors from its regex-input string before processing it?
Regular expressions are a very powerful tool, but such expressions are often very cryptic. If you want to understand them, it is best to have colored pencils at hand to color-code the parts that ...
2
votes
0answers
37 views
1
vote
0answers
40 views
Behaviour of PatternTest [duplicate]
I was surprised by the following behavior of PatternTest and I think that this may be instructive for other people too.
Consider ...
1
vote
1answer
24 views
Why can't I get the argument for my base function from the transformed function?
Consider the following transformed function as an example:
...
2
votes
1answer
90 views
Why does this DeleteCases not work? [duplicate]
I believe this is simple but I couldn't figure out why this doesn't work.
Everything looks good to me.
...
1
vote
1answer
48 views
Comparing parts of patterns by name
Let's say I wanted to pick out all the rows of a table for which columns 3 and 5 are equal. The following doesn't work, as the second x is treated as an independent ...
3
votes
0answers
62 views
What's the time / space complexity of pattern matching?
For example, in ReplaceAll[expression, pattern], what are the main algorithms used internally? How does the asymptotic time / space complexity scale with the length ...
0
votes
2answers
53 views
Extract subscript of variable in product
My input is a product of the variable x with index:
x[1]^2 x[5] x[7]
How do I extract the indices of the above product to get ...
3
votes
0answers
32 views
Unexpected behaviour with Coefficient
If I run
expr = (CF G[0, zb])/(1 - zb) + (CF zb^2 G[0, zb])/(1 - zb) + (2 CF G[1, zb])/(1 - zb) + (2 CF zb^2 G[1, zb])/(1 - zb);
Coefficient[expr, zb, -1]
in ...
2
votes
0answers
56 views
How to produce an error message for each argument?
I want to print a helpful message for every wrong input. How do you compactly implement a function with custom pattern constraints, default values and error messages for each argument separately? For ...
0
votes
1answer
68 views
How to convert decimals to integers and joint them to a string for exporting
There's a list such as {0.1,2,0.15,5}. I want to know how to convert the decimal to integer( but begin with zero 0), and don't ...
1
vote
1answer
50 views
4
votes
1answer
77 views
Simplifying the Summation
I have to sum:
$$\frac{1}{2} x_{i,k} x_{j,\nu } g_{i,j,k,\mu }+\frac{1}{2} x_{i,k} x_{j,\mu } g_{i,j,k,\nu }+x_{i,l} x_{j,\nu } g_{i,j,l,\mu }+x_{i,\mu } x_{k,l} g_{i,k,l,\nu }$$
g and x are real ...
0
votes
0answers
70 views
Commutation relation
I have defined:
COM[a_, b_] := a ** b - b ** a
for commutator bracket for the operators. We also have:
...
2
votes
1answer
125 views
Operator products summing
Is there a way to perform the below summation in Mathematica?
$$\frac{\left(\hat{X}_{ij}+\mathbb i\delta_{ij}\right)}{\mathbb{i}}\frac{\left(\hat{X}_{kl}+\mathbb i\delta_{kl}\right)}{\mathbb{i}}$$
I ...
1
vote
1answer
96 views
Summation without writing term by term
https://mathematica.stackexchange.com/a/222410/73364
I have obtained the following code from the above link
...
0
votes
2answers
54 views
What pattern will distinguish an element of a list from a list when used in a function?
In Mathematica several versions of a function can have the same name with a pattern distinguishing which version of the function should be used for which type of input. Here is a minimum working ...
2
votes
0answers
30 views
recursively find StringCases
Can I recursively extract StringCases[] that match a specific pattern?
Here is a minimalist example (not my actual problem):
input
...
5
votes
1answer
143 views
Detecting patterns in a graph network
If we create a random graph
Chop[LowerTriangularize[RandomReal[{-1, 1}, {10, 10}], -1], 0.6] /.0 -> \[Infinity] // MatrixForm
WeightedAdjacencyGraph[%]
How can ...
0
votes
1answer
62 views
2
votes
0answers
25 views
VertexShapeFunction with string pattern matching [duplicate]
I am trying to visualize a heterogeneous graph which nodes are categorical. Here is a dummy graph for implementation:
Input is
...
2
votes
2answers
63 views
Checking if a polynomial contains a certain monomial
I want to check if a given polynomial contains a certain monomial while ignoring ordering of the variables. For example:
$$P(a,b,c)=abc + bca$$
And then I want to check if $P$ contains $bac$ (should ...
1
vote
2answers
58 views
WhenEvent inside Ndsolve how to evaluate actions numerically every time the event happens
I need to force a whenevent action to be evaluated numerically all the times the event happens.
The fact is that I use very complicated matrices and I need to invert them so the system requires a lot ...
5
votes
4answers
237 views
Selecting rows from two multi-column lists, where values are the same in a chosen column
If I have two multi-column lists, for example:
...
2
votes
2answers
189 views
How do you get substitution to recognize a derivative?
I have a simple expression involving a derivative:
x[t_] = Q/T[t];
Simplify[Derivative[1][x][t] /. Q -> x T[t]]
That produces:$$-\frac{x T'[t]}{T[t]}$$I have ...
3
votes
2answers
89 views
Comparison of Coefficients involving complex exponentials
I'm trying to solve a couple (independently) of equations of the following type
$ a_0 x + 3 a_1 y(1 + xe^{i3q} + e^{-iq + 2ip}) + ... = 0, \forall x, y, q, p$.
Using SolveAlways doesn't work well. So ...
4
votes
3answers
94 views
Cases in list with varying length in sublists
Given
t1 = {{14, {{1, 2, 3}}}, {15, {}}, {16, {{0, 0, 4}}}, {17, {{0, 1, 4}, {2, 2, 3}}}, {18, {{0, 3, 3}, {1, 1, 4}}}, {u, {{1, 0, 2}, {2, 3, 0}, {2, 3, 4}}}}
I ...
2
votes
1answer
51 views
How to use IntegerQ on list?
t1 ={0, 1, 2, Log[6]/Log[2], 3, Log[12]/Log[2], 4, Log[24]/Log[2]}
I want to get the Position of the Integers. I can do it by
<...
2
votes
0answers
57 views
How does Mathematica choose which function definition to use?
I was wondering what the process of matching function arguments arguments against different function definitions actually looks like for Mathematica.
(Just to be clear, I'm not simply looking for an ...
0
votes
1answer
45 views
1
vote
0answers
48 views
“Compile-time” patterns
If I define a function like
f[x : {_, _Integer}] := (* function body *)
then, as I understand it, every time f[expr] is ...
3
votes
1answer
49 views
warning in Position + IntersectingQ
Why there is a warning message (with correct output) for the following code ?
...
3
votes
1answer
88 views
BlankSequence with Plus
When I execute f[g[a, b]] /. f[g[l__]] :> f[l] I get f[a, b] as expected.
When I execute ...
3
votes
1answer
40 views
Generate replacement rule from user-defined pattern
I want to generate a replacement rule from a user-defined pattern. For example, the user provides a list of patterns {A[i_][j_], BB[k_]} and a user-defined function ...
6
votes
0answers
92 views
Exceeding recursion limit with no obvious recursion: Optional pattern-matching causing nonlocal behavior
So, I've come across something strange, and wanted to get a deeper understanding of what was going on. Consider the following code:
...
