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

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14
votes
2answers
620 views

How to detect loops in a vector field?

Given a vector field and I'd like to detect if there are streamlines in closed patterns. For example, if the input is a vector field like this: ...
14
votes
5answers
801 views

Getting number of binary digits combinations without “forbidden” patterns

I need to get the number of all combinations of binary digits in an 8-digit binary number, but not including those that follow some "forbidden" patterns like these: ...
14
votes
2answers
351 views

Pattern to match L, but not L[_]

I have an expression where I want to replace L but not L[_] For example, the following doesn't work and ...
14
votes
2answers
1k views

Determine whether some expression contains a given symbol

Given a symbol t and an expression expr, how can I determine whether or not the symbol t ...
14
votes
3answers
201 views

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

In this answer, what does the : Hold[$IterationLimit] part of the following construct do? ...
14
votes
3answers
590 views

How to match a cyclically repeating sequence?

I have run across several times recently where I must match a sequence that is cyclically repeating. For example, with the sequence, Sequence[a, b, c] how would ...
14
votes
1answer
130 views

Unexpected pattern matching behaviour: PatternSequence vs. Optional

fixed in 10.1 (windows) Consider the following pattern-matching expressions: ...
13
votes
5answers
664 views

How to remove duplicate characters from a string?

How do you remove duplicate characters in a string without converting the string to a list. I want to delete all duplicate characters in a string (i.e. get the unsorted union of characters). For ...
13
votes
3answers
1k views

Passing down arguments

In R, one can use ... to pass arguments down to another function. For example ...
13
votes
5answers
841 views

Calling Table with custom iterator

I often find myself in situations where I, for example, need to build a table for some expression, but want to set the number of points rather then the step size, so the code ends up looking like this ...
13
votes
4answers
378 views

The gap between MapAt and ReplacePart for 2D data tables with headers

Consider a relational table derived from survey data, where each column ("001-01" ...) represents a responder and each row (...
13
votes
2answers
474 views

How can I separate a separable function

I have a separable function $f[x,y]$, and I would like to find two functions $g[x]$ and $h[y]$ with $f[x,y]=g[x] h[y]$ where $g[x]$ doesn't depend on $y$ and $h[y]$ doesn't depend on $x$. Ideally, ...
13
votes
6answers
333 views

How can I mix the behaviors of StringReplace and ReplaceAll in a single rule?

I'm trying to take a list of the form { {"65 + 3 months", 75}, {"65 + 4 months", 75.1} }, and transform the string part to a number such as 65.25 or 65.333, ...
13
votes
2answers
273 views

Named patterns in Except

Bug introduced in 8 or earlier and persisting through 10.3 or later Bug isn't present in version 5.2 I don't think I quite understand how Except works. I want ...
13
votes
3answers
378 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 ...
13
votes
2answers
477 views

Should I use \\\“ or \” to match " in regex?

Both of the following return True: ...
13
votes
1answer
362 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. ...
13
votes
1answer
106 views

Report: IgnoringInactive: a bug?

Bug introduced in 10.0.0 and fixed in 10.0.1 Just to report. IgnoringInactiveshould work with Cases. In agreement with the ...
12
votes
10answers
725 views

Counting sublists containing 0

Given a list: data = {{0, 2}, {0, 0}, {1, 0}, {1, 2}, {2, 3}}. I would like to count the number of sublists which contain zero. ...
12
votes
4answers
432 views

How to enumerate all possible binary associations?

Suppose I have a list of symbols like: {a,b,c,d} I would like to enumerate all possible binary associations (combining symbols and/or sublists pairwise): ...
12
votes
3answers
839 views

Specifying string patterns in DeleteCases

It seems that DeleteCases is not compatible with string patterns, at least directly (see, for example, this question). Is this true? If so, why is this the case? ...
12
votes
2answers
599 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, ...
12
votes
2answers
396 views

Split a Unicode string maintaining uppercase characters

I want to split a string according to a predefined set of substrings (lowercase), though the actual text can contain uppercase characters anyplace. The task is to find the matches, longer preferred ...
12
votes
2answers
256 views

why MatchQ is fast and Replace slow?

Consider a large expression, say a polynomial with 25 terms expr=Product[Unique["a"],{i,1,25}]; to which you apply the following replacement rule: ...
12
votes
3answers
767 views

Delete duplicates in a list, depending on the sequence of numbers

Below, list is a representative sample of my list, which contains lists of integers. I would like to be able to input: ...
12
votes
1answer
289 views

Orderless pattern matching

The MMA docs state that "In matching patterns with Orderless functions, all possible orders of arguments are tried". Why then does the following not work? ...
12
votes
1answer
228 views

Why doesn't repeated PatternSequence work with explicit pattern names?

Why would this work: Clear[f] f[a : PatternSequence[b_, c_]] := {a}; f[1, 2] (* {1, 2} *) and this also works: ...
11
votes
4answers
646 views

Assessing argument type in set delayed function definitions

I'm wondering how to properly assess the type of arguments passed to functions defined with :=. I want my functions to be the most efficient, unambiguous and clear ...
11
votes
9answers
1k views

How can a power of 0 or 1 be replaced?

Consider this given example for ReplaceAll 1 + x^2 + x^4 /. x^p_ -> f[p] This returns ...
11
votes
4answers
887 views

Using Position (which requires a pattern) in the same manner as Select (which requires a criterion)

I am using Mathematica7. I have a list of lists (creatively named list in the following sample code) containing data. I would like to find the positions of the ...
11
votes
4answers
454 views

How to match expressions with a repeating pattern

The expression I want to match obey a simple pattern that repeats it self a number of times. f[a]@f[b]@f[c]@...@f[X] How would you match all expressions of this ...
11
votes
3answers
239 views

Pattern matching on Orderless functions inside Hold

MatchQ[Hold[1 + 2], Hold[2 + 1]] returns False. But in the presence of a Blank[] in the ...
11
votes
4answers
291 views

Recognizing special cases of a defined function for permuted arguments

I have a number of ugly ways to implement the following pattern recognition task, but I'm looking for something elegant to keep my notebook tidy. I need to define a function of six variables ...
11
votes
3answers
906 views

Pattern matching a pattern with patterns

Confusing title, I know. But the question is, if we have two patterns which have the same general structure but different names used in the patterns and different names: ...
11
votes
1answer
384 views

Selectively clear DownValues

How can I clear a subset of a symbol's DownValues ? For example, suppose I have created some DownValues for $f$ like this: ...
11
votes
1answer
304 views

Empty Blank regression in v10.1

Bug introduced in 10.1 and fixed in 10.2.0 In v10.1.0.0 on Linux, when Blank[] is used in Alternatives with pattern that can ...
10
votes
4answers
419 views

What is the correct way to use specific rule for specific pattern match when parsing an expression?

update: Thanks for the answers below. But I noticed they do not handle the following special cases: lis=1 and lis=z , ...
10
votes
3answers
797 views
10
votes
4answers
247 views

Is it possible to separately call the default value of a variable which isn't at the end of the sequence?

After reading this post I recalled an old question I once met. We know that we can set default values for variables of a function: ...
10
votes
2answers
520 views

Is there a way to identify a symbolic fraction?

Is there a way to use Head to detect a symbolic fraction? In particular I find, Head[a/b] Head[1/5] Times Rational ...
10
votes
4answers
558 views

How to use pattern matching to assign values to Subscript[f,x_]?

I want to define two subscripted functions Subscript[f,1] and Subscript[f,2]. To keep the assignments local, I would like to ...
10
votes
2answers
318 views

Why can I not replace a Dataset using a rule?

Simple replacement of a Dataset object does not seem to follow the expected behavior... Dataset[{}] /. _Dataset->1 Returns the empty Dataset not 1 as I would ...
10
votes
2answers
306 views

Why doesn't Cases match all instances of XMLElement, given Infinity levelspec?

Why doesn't Cases match all instances of XMLElement, given Infinity levelspec? ...
10
votes
4answers
492 views

Building a tree

Given a list of word characters, such as this one, I'd like to build a tree, similar to this makeTree function, but with the tree in a different format. So, for an input such as ...
10
votes
1answer
231 views

Question about Rules and Pattern (The operator patt/;test)

Maybe this question is so trivial but it has confused me. I'm studying the What the @#%^&*?! do all those funny signs mean? and in the Rules and patterns under ...
10
votes
5answers
628 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 ...
10
votes
2answers
174 views

Optional throws error when called from a package. Workaround?

Bug introduced in 8.0 or earlier and persisting through 10.3 or later. Case number: 305932. In the front-end notebook (Mathematica 9.0 and 10.2): ...
10
votes
3answers
770 views

Find subexpression to minimize leafcount after replacment with temporary variable

Given an expression expr, is there an automated way to find a subexpression subexpr such that replacing ...
10
votes
2answers
200 views

Unexpected result from StringReplace

The following works as expected. StringReplace["xxxyxz", "x" ~~ _ -> "ww"] "wwwwww" And so does this. ...
10
votes
1answer
1k 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?