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

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15
votes
2answers
193 views

Unexpected behaviour when pattern matching with Longest

I came across some unexpected behaviour today when using the Longest function when trying to do some pattern matching. ...
15
votes
2answers
522 views

Successful match in Replace but not in Cases?

I have a little example that exhibits a successful pattern match in ReplaceAll that Cases misses and I wonder if the assembled ...
15
votes
3answers
428 views

Highlighting pattern strings in a large text

I would like to apply Style to strings that are matched, for example, with one or more specified patterns or regular expressions, a string or a list of both, ...
15
votes
2answers
1k views

How to use subscript in pattern names?

For example I want to define a function with parameters σx and σy, that is, the function will be declared as: f[σx_,σy_] := . . ....
14
votes
6answers
3k views

Counting negative values in list

I would like to count the negative values of a list. My approach was Count[data, -_] which doesn't work. How can I tell Mathematica to count all numbers with ...
14
votes
3answers
399 views

Make mathematica treat $e_i^2$ as numeric

With NumericQ[symbol] = True, I can declare that a symbol is numeric. I want the expressions matching: $$e_{\text{i$\_$}?\text{IntegerQ}}^2$$ to be treated as ...
14
votes
3answers
268 views

What's wrong with this pattern of the form Except[Repeated[…]]?

I'm trying to build a head that recognizes molecules. Here's the code ...
14
votes
3answers
1k views

Passing down arguments

In R, one can use ... to pass arguments down to another function. For example ...
14
votes
2answers
685 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
2answers
354 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
3answers
207 views

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

In this answer, what does the : Hold[$IterationLimit] part of the following construct do? ...
14
votes
3answers
633 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
140 views

Unexpected pattern matching behaviour: PatternSequence vs. Optional

fixed in 10.1 (windows) Consider the following pattern-matching expressions: ...
13
votes
10answers
738 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. ...
13
votes
6answers
914 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
5answers
726 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
4answers
388 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
506 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
2answers
308 views

Named patterns in Except

Bug introduced in 8 or earlier and fixed in 10.4.0 Bug isn't present in version 5.2 I don't think I quite understand how Except works. I want to define $f(a,b) ...
13
votes
6answers
336 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
3answers
404 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
507 views

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

Both of the following return True: ...
13
votes
1answer
372 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
107 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
4answers
944 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 ...
12
votes
3answers
913 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
638 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
3answers
264 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 ...
12
votes
2answers
407 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
292 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
864 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
2answers
195 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): ...
12
votes
1answer
308 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
238 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: ...
12
votes
2answers
202 views

Opposite of Alternatives in a Rule

Mathematica provides Alternatives, to match one of several patterns. In a rule, this is used as p1|p2|p3:>replacement. I ...
12
votes
1answer
219 views

Longest common substring for multiple strings?

I'm looking for a way to get the longest common substring for multiple strings such as {home/dir1/dir2/jmoasd.txt,home/dir1/dir2/ivbnoxcihv.txt,home/dir1/dir2/siuhgiuchv.txt} should yield ...
12
votes
0answers
125 views

Logical AND of multiple patterns

Mathematica provides an equivalent for "logical OR" when pattern matching, using Alternatives or the infix | operator. For ...
11
votes
4answers
670 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
193 views

Evaluation of Alternatives

I recently stumbled upon a few examples that suggest patterns containing Alternatives do not evaluate for individual alternatives. For example: ...
11
votes
4answers
479 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
2answers
304 views

How can one represent a string pattern with prohibited substrings?

Say I have the following string: "<hello>something WRONG</hello><hello>something else</hello>" I am trying to find code that works like the ...
11
votes
2answers
342 views

Using Except with strings

Suppose that I have the following sample list, called list: list = {"bBb", "aAa", "aaa"}; Now suppose I want to select the ...
11
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?
11
votes
4answers
296 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 $f(a,b,c;...
11
votes
3answers
962 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
396 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
307 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
5answers
425 views

Repeating Digits

I have written ...
10
votes
4answers
434 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 , ...