how to use indexes to copy a substring out of a string
Hi Roelof You may have a look at my StringExtensions package at https://github.com/hernanmd/StringExtensions I wrote several algorithms for working with substrings like: https://github.com/hernanmd/StringExtensions/blob/master/repository/StringEx... https://github.com/hernanmd/StringExtensions/blob/master/repository/StringEx... Some of them used in https://github.com/hernanmd/BioSmalltalk where you can find also algorithms for k-mer counting and clump finding: https://github.com/hernanmd/BioSmalltalk/blob/master/repository/BioTools.pac... https://github.com/hernanmd/BioSmalltalk/blob/master/repository/BioTools.pac... for the Bio repo just consider a Sequence like a more advanced String. Cheers, Hernán El sáb., 29 dic. 2018 a las 13:29, Roelof Wobben (<r.wobben@home.nl>) escribió:
Hello,
Still working on AdventOfCode
Im struggeling to see how this can be solved.
Now, a nice string is one with all of the following properties:
It contains a pair of any two letters that appears at least twice in the string without overlapping, like xyxy (xy) or aabcdefgaa (aa), but not like aaa (aa, but it overlaps). It contains at least one letter which repeats with exactly one letter between them, like xyx, abcdefeghi (efe), or even aaa.
my game plan was this
1) group the characters and filter everything out which has a count not equal to two
2) find the indexes of the characters and with the indexes use a copy method so I have a substring out of it
3) do the same with the substring so again 1 and 2
but how can I make 2 work.
Roelof
On Sun, 30 Dec 2018 at 00:29, Roelof Wobben <r.wobben@home.nl> wrote:
Hello,
Still working on AdventOfCode
I'm struggling to see how this can be solved.
Now, a nice string is one with all of the following properties:
- It contains a pair of any two letters that appears at least twice in the string without overlapping, like xyxy (xy) or aabcdefgaa (aa), but not like aaa (aa, but it overlaps).
To exclude overlaps, one approach could be to subtract each candidate pair from the string and then check the remainder-string for matches. I'm not sure, but suspect that as the candidate pair progresses through the string, only the trailing remainder-string needs matching since the preceding remainder-string has already been checked.
So a Pharo 7 Spotter search for 'pairs' finds 28 implementors where SequenceableCollection>>overlappingPairsCollect: looks quite close to what is required... 'abcdefg' asArray overlappingPairsCollect: [ :a :b | { a.b } ]. "#( #($a $b) #($b $c) #($c $d) #($d $e) #($e $f) #($f $g))" Reviewing its implementation... SequenceableCollection>>overlappingPairsCollect: aBlock | retval | retval := self species ofSize: self size - 1. 1 to: self size - 1 do: [:i | retval at: i put: (aBlock value: (self at: i) value: (self at: i + 1)) ]. ^retval it can be adapted to provide also a remainder... SequenceableCollection>>overlappingPairsRemainderCollect: aBlock | retval | retval := self species ofSize: self size - 1. 1 to: self size - 1 do: [:i | retval at: i put: (aBlock value: (self at: i) value: (self at: i + 1) value: (self allButFirst: i + 1)) ]. ^retval for use like this... 'abcdefg' asArray overlappingPairsRemainderCollect: [ :a :b :rem | { a.b.rem } ] "#( #($a $b #($c $d $e $f $g)) #($b $c #($d $e $f $g)) #($c $d #($e $f $g)) #($d $e #($f $g)) #($e $f #($g)) #($f $g #()))"
- It contains at least one letter which repeats with exactly one letter between them, like xyx, abcdefeghi (efe), or even aaa.
You might adapt "overlappingPairsCollect: aBlock" into "overlappingPairsSkip: n collect: aBlock" but changing "size -1" and "i + 1" into "size - n" and "i + n"
HTH cheers -ben P.S. If #overlappingPairsSkip:collect: seemed generically useful to push into Pharo, then the following would be redefined to reuse it... SequenceableCollection>>overlappingPairsCollect: aBlock ^self overlappingPairsSkip: 1 collect: aBlock The extra layer of function call doesn't add much overhead. On the other hand #overlappingPairsRemainderCollect: does lots of extra copying through #allButFirst: so would not be so good for reuse by #overlappingPairsCollect:
Let us approach this as simply as we can. (1) There are two numbers 1 <= i <= n-3, i+2 <= j <= n-1 such that (s at: i) = (s at: j) and (s at: i+1) = (s at: j+1) Brute force will get us where we want to go: String hasTwoCharacterNonoverlappingRepeat 1 to: self size - 3 do: [:i | i + 2 to: self size - 1 do: [:j | ((self at: i ) = (self at: j ) and: [(self at: i+1) = (self at: j+1)]) ifTrue: [^true]]]. ^false This is pretty much how you would do it in C or Java. This takes O(n**2) time where n is the size of the string. If you want to be *really* clever, you can do something with suffix arrays, but if you just want to be a little bit clever, you can sort the two-character substrings and their positions by just sorting the positions. Make an array a of the integers 1..n-1. Sort it by viewing each i in the array as the triple (s[i],s[i+1],i). You don't *make* the triples, just do the comparisons as if they existed. Now a linear scan of the array will find a match if there is one. The whole process is O(n.lg n). It's only worth doing when n is large. (2) Finding xyx. You did not say whether the middle letter can be the same as the outer ones or must be different. I'm going to suppose that it does not matter. hasXYX 3 to: self size do: [:i | (self at: i-2) = (self at: i) ifTrue: [^true]]. ^false Again, this is just what you would do in Java or C: public static boolean hasXYX(String s) { final int n = s.length(); for (int i = 2; i < n; i++) if (s.charAt(i-2) == s.charAt(i)) return true; } return false; } For what it's worth, in my system this scans a Scrabble dictionary with 146522 words (1,220,068 bytes), finding 2779 "nice" words, in 0.18 seconds, so there's not a lot of point in improving it. In both cases, DON'T copy. DON'T make substrings. Just state the problem simply and convert it to obvious code. Simple boring code is GOOD. On Sun, 30 Dec 2018 at 05:29, Roelof Wobben <r.wobben@home.nl> wrote:
Hello,
Still working on AdventOfCode
Im struggeling to see how this can be solved.
Now, a nice string is one with all of the following properties:
- It contains a pair of any two letters that appears at least twice in the string without overlapping, like xyxy (xy) or aabcdefgaa (aa), but not like aaa (aa, but it overlaps). - It contains at least one letter which repeats with exactly one letter between them, like xyx, abcdefeghi (efe), or even aaa.
my game plan was this
1) group the characters and filter everything out which has a count not equal to two
2) find the indexes of the characters and with the indexes use a copy method so I have a substring out of it
3) do the same with the substring so again 1 and 2
but how can I make 2 work.
Roelof
participants (4)
-
Ben Coman -
Hernán Morales Durand -
Richard O'Keefe -
Roelof Wobben