Questions on expression testing and manipulation through pattern matching and constructing efficient patterns.

learn more… | top users | synonyms (1)

4
votes
2answers
165 views

How can I replace the general element of a geometric series with the sum of the series?

Consider the list mylist = {b(a/bc t)^r, a/b(b t)^r, a (c/d t)^r, ...} where a, b, c, t, etc., are real numbers between 0 and 1. I want to sum each term in the ...
14
votes
4answers
496 views

Returning an unevaluated expression with values substituted in

Let's say I have a function formula[x_List] := (x[[1]] - x[[2]]) + (x[[3]] - x[[4]]); and I want to pass it a variable and get back the unevaluated formula with ...
3
votes
2answers
105 views

More general pattern fails to match everything the more specific pattern does.

When I input this: MatchQ[x*SF[a,b] + y*SF[c,d], Plus[Times[x,SF[a,b]],Times[y,SF[c,d]]]] I get "True" as expected. But If I change it to: ...
3
votes
3answers
176 views

Write a function using pattern test to test whether the input is polynomial and print error message if not

I am doing this which is supposed to be an easy problem and which I think my code should be correct but for some reasons it is not working. May anyone help correcting my mistakes? How should I write ...
4
votes
2answers
217 views

Adapting “Find and Replace” functionality to work with Patterns

I was thinking if there is something like Find and Replace (Crtl+F) functionality adapted to work with patterns. Probably not, but it seems possible to create this with some effort and skills in ...
6
votes
1answer
203 views

Pattern definition for replacing plus and subtract

Suppose I have two functions $h_1(x)$ and $h_2(x)$, for which we know their sum and their difference: $$ h_1(x)+h_2(x) = d(x) \qquad h_1(x)-h_2(x) = i f(x) $$ I have a complicated expression, in ...
2
votes
1answer
60 views

Findings powers within strings - Cases function

Any idea which Cases statement will work for both A^2 and CA^4X^36 to find the powers? If I set sym to these two expressions, one requires a curly brackets around the first argument and the other ...
6
votes
1answer
203 views

Match sequence of zero or more characters except a given character

This looks like it should be simple: I want to match an "i" followed by an "o" with an arbitrary (possibly zero-length) sequence of characters in between, unless that sequence also contains an "i". ...
1
vote
1answer
131 views

Searching a string that may contain Null

I have an array of triplets that can be approximated by the following code: ...
8
votes
1answer
175 views

How to tell when a job is too big for the pattern matcher?

Pattern matching is great but it has its limitations. Consider sorting a list of numbers from smallest to largest: ...
-1
votes
2answers
172 views

RotateRight ith row of a matrix by i - 1

first time stack exchange poster here! So I was working on a problem, the purpose of which was to write a function in the procedural style to rotate right the ith row of a matrix by i - 1 spaces. Ok, ...
3
votes
2answers
575 views

Classifying poker hands by pattern matching

I am working on this question posed in a Mathematica course revised in 1998. Here are the instructions: Poker Hands You are to define and show working examples of a function "...
15
votes
3answers
491 views

Position of a pattern-matched part of an expression

What you have A pattern, with certain part of it labeled As an example — Label: wally ...
4
votes
1answer
321 views

Extracting text from nested HTML lists <ul> <li>

We wish to analyze the tree structure of HTML documents that consist of nested lists, ie, using the HTML tags <ul> and ...
3
votes
2answers
101 views

How to apply multiple/complicated requirements for a pattern in a function input [duplicate]

In this question. I suggested that to test whether the input of a function was a matrix of numbers one could create the following test: ...
27
votes
0answers
438 views

Why is StringExpression faster than RegularExpression?

Edit: as noted by Albert Retey the performance difference is only seen when sub expression extraction is performed. If this test is used below the timings are similar: ...
6
votes
2answers
196 views

Variable scoping problem when mapping over delayed replacement

This is something that got me curious while I was playing around with Mathematica. Consider the following (contrived) example: ...
2
votes
1answer
110 views

Flat function with repeated sequence

There are functions like Join[ ] that accept a variable number of arguments. Join is a flat function, as the order of arguments ...
2
votes
2answers
317 views

Write a function that returns the logarithmic derivative

How can we write a function that if we input an expression f, it returns the log derivative $\frac{1}{f} \frac{df}{dx}$. We have to use a conditional or pattern test so that the function accepts any ...
3
votes
0answers
139 views

I am missing a function like HoldPattern

Temporary message: I am planning to edit this question, making the examples less confusing and by moving the things that are confusing to a separate question For any confusion about expressions that ...
14
votes
3answers
165 views

What does `: Hold[$IterationLimit]` mean?

In this answer, what does the : Hold[$IterationLimit] part of the following construct do? ...
10
votes
1answer
503 views

Mathematica equivalent of regex '\w{2,3}'

It beats me to write the Mathematica pattern equivalent to the regex: \w{2,3} Any ideas?
5
votes
4answers
183 views

Extracting sequences from a list?

Suppose I have a list of arbitrary length like {1, 2, 3, "Open", 3, 2, "Close", 9, 3, 4, "Open", 1, 0, "Close", 3, 5} and I am trying to extract the sequences ...
3
votes
2answers
103 views

Select gives different answers in versions 8 and 9

Consider the code below: s = Solve[(3 - Cos[4*x])*(Sin[x] - Cos[x]) == 2, x, InverseFunctions -> True]; Select[s[[All, 1, 2]], Element[#, Reals] &] In MMA ...
34
votes
9answers
1k views

Injecting a sequence of expressions into a held expression

Consider the following toy example: Hold[{1, 2, x}] /. x -> Sequence[3, 4] It will give Hold[{1, 2, Sequence[3, 4]}] ...
4
votes
2answers
218 views

Unexpected behavior of rule matching a pattern

I am a beginner exploring the world of Mathematica. I expected the following code T[6, 5, 4, 1, 2, 3] /. {T[a___, 1, b___] -> Length[List[b]]} should return ...
11
votes
1answer
251 views

I can't use PatternSequence with Cases

I have a list of integers: s = {4, 5, 5, 6, 2, 5, 3, 2, 4, 2, 1, 1, 4, 3, 4, 3, 1, 6, 3, 4}; And if I do this, both occurrences of 4 after 3 are replaced. ...
16
votes
5answers
2k views

The difference between 0. and 0

I have a function for which 0 is a special case: f[A___, 0, B___] := 0 But since I am doing numerics, sometimes in the course ...
10
votes
1answer
231 views

Why is ReplaceAll behaving like this?

I'm learning to use the ReplaceAll function and I found the behavior of which is quite confusing. For Sqrt[f[x, y]] /. f[___] -> u Mathematica returns ...
4
votes
5answers
829 views

Removing zero values from ordered pairs

I have some processed data, which are basically a collection of ordered pairs in the form: {{x1, y1}, {x2, y2}, ..., {xn, yn}} Ordered pairs which have either a ...
3
votes
1answer
154 views

Problem with | operator in pattern matching

My first question here as I only got into Mathematica programming recently. Basically I have a large symbolic algebraic expression where I know certain variables with head pε appear with degree 2 in ...
1
vote
2answers
227 views

Deep Pattern matching with repeating arguments

I have a function that accepts a single argument that is described by the following grammar: ...
3
votes
1answer
443 views

Adjacency List representation of a graph

I am looking to draw a graph knowing a simple adjacency representation of the graph like: A = {{1, 3}, {2, 3}, {3, 4, 5}, {4, 5}, {5}} Where ...
2
votes
1answer
46 views

transformation patterns don't work with constants

I want to do the reverse of PowerExpand and convert 2*Log[x+y] to Log[(x+y)^2]. The rule: t_ Log[x_] -> Log[x^t] works for ...
5
votes
1answer
171 views

Why does this pattern for combining two lists require triple underscores (___) to work?

I understand that __ is a list of one or more elements, and that ___ is 0 or more elements, but when I try this rule with 2 ...
6
votes
2answers
279 views

Rule for orderless function Times

I have problems extending a replacement rule. Schematically, this is the situation: I have an expression and a rule, and they work perfectly together. ...
10
votes
3answers
765 views

Passing down arguments

In R, one can use ... to pass arguments down to another function. For example ...
1
vote
2answers
102 views

Named patterns as function argument descriptors

I have a function timeToMinutes that takes a 24-hour time as a string argument (in digital clock format, e.g., "15:47") and converts it to the number of minutes ...
13
votes
3answers
256 views

Question about using a pattern in ReplacePart

I noticed that "ReplacePart" can use pattern. For example: ReplacePart[{{0, 0, 0}, {0, 0, 0}, {0, 0, 0}}, {i_, i_} -> x] {{x, 0, 0}, {0, x, 0}, {0, 0, x}} So ...
5
votes
2answers
250 views

Keep Ordering of Bra Ket When Printing

I would like to keep the ordering of the bra ket-print but right now this doesn't happen. Any Idea? I tried playing with HoldForm but then i could evaluate the expression inside properly ...
5
votes
0answers
175 views

Convert logical relational expression to / from disjunctive and conjunctive forms?

I would like to convert logical relational expressions in disjunctive form, e.g., $$(x \lt -1) \lor (-1 \lt x \lt +1) \lor (x \gt +1)$$ into conjunctive form, e.g., $$(x \ne -1) \land (x \ne +1)$$ ...
11
votes
2answers
380 views

Soft-Match String Comparison

I have a list of names in which I would like to check for duplicates. However, within the list, the duplicates may not show up as exact duplicates of each other - for instance, ...
3
votes
1answer
146 views

Extending D for four-derivatives

I'm currently trying to consistently define rules for extending D[] to four-derivatives. As 'backend' I'm using the package TRACER (http://library.wolfram.com/infocenter/MathSource/2987/), which can ...
2
votes
1answer
150 views

Using transformation rules to manipulate expressions

I'm trying out Mathematica as a replacement for LaTeX for typesetting math. It works very well, but I would like to take advantage of Mathematica's computer algebra capabilities as well. Specifically, ...
3
votes
1answer
136 views

Downvalues vs. Scoping for Functions

Regarding my recent question on using a default value for a function argument when a pattern was not met yielded some interesting answers, but the general consensus was "Yes this can be done, but ...
6
votes
4answers
433 views

Function argument to default under certain condition

Inspired by this and this question (and how I handle this in practice), what is the best way to default a function value when a certain condition is met? For example, if a function is defined as: ...
4
votes
2answers
155 views

How to achieve Set+Part like behaviour in custom Set function?

So I've been toiling away on my DataFrame package. I've been trying to get Set to work with it, but it seems it either can't be done (ie via ...
8
votes
4answers
401 views

Best way to modify values in a list of rules?

Recently I had to solve a problem similar to this: Let's say I have a list of list of rules ...
3
votes
2answers
135 views

Get DownValues that do not involve patterns?

Say I have a function with a few definitions that also implements memoization: f[x_] := f[x] = x^2 f[x_, y_] := f[x, y] = x^2 + y^2 I then apply the function to ...
8
votes
3answers
302 views

Patternmatching sets

How to match set-patterns against sets? A set (in the mathematical sense) is a list of elements without repetition and order of elements does not matter. For example, we have a pattern set ...