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2 points by akkartik 2059 days ago | link | parent

Nice example. For exposition is it useful to simplify the problem by focusing on lists?

  (def first-difference(seq1 seq2)
    (if (or no.seq1 no.seq2
            (~iso car.seq1 car.seq2))
      0
      (+ 1 (first-difference cdr.seq1 cdr.seq2))))
Edit: no, that's not right, because it doesn't return nil on identical sequences. Here are two variants with and without the accumulator. Which is easier for noobs?

  (def first-difference(seq1 seq2 (o pos 0))
    (if
      (and no.seq1 no.seq2)
        nil
      (or no.seq1 no.seq2 (~iso car.seq1 car.seq2))
        pos
      'else
        (first-difference cdr.seq1 cdr.seq2 (+ pos 1))))

  (def first-difference(seq1 seq2)
    (if
      (and no.seq1 no.seq2)
        nil
      (or no.seq1 no.seq2 (~iso car.seq1 car.seq2))
        0
      'else
        (only.+ (first-difference cdr.seq1 cdr.seq2) 1)))
 
  (test-iso "" 0 (first-difference nil '(34)))
  (test-iso "" nil (first-difference '(1) '(1)))
  (test-iso "" 1 (first-difference '(1) '(1 2)))
  (test-iso "" 3 (first-difference '(1 2 3 4) '(1 2 3)))


1 point by jsgrahamus 2059 days ago | link

  arc> (def 1st-diff (str1 str2 i)
    (if (or (is str1 "") (is str2 ""))
      0
      (or (> i (- (len str1) 1)) (> i (- (len str2) 1)))
      i
      (no (is (str1 i) (str2 i)))
      i
      (1st-diff str1 str2 (+ i 1))))
  #<procedure: 1st-diff>
  arc> (1st-diff "" "" 0)
  0
  arc> (1st-diff "" "" 0)
  0
  arc> (= astring "abcde")
  "abcde"
  arc> (= bstring "abced")
  "abced"
  arc> (1st-diff "" "" 0)
  0
  arc> (1st-diff astring "" 0)
  0
  arc> (1st-diff "" astring 0)
  0
  arc> (1st-diff astring bstring 0)
  3
  arc> (1st-diff bstring astring 0)
  3
  arc> (1st-diff astring "abcdef" 0)
  5
  arc> (1st-diff "abcdef" astring 0)
  5
  arc>

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1 point by jsgrahamus 2058 days ago | link

It's interesting to compare this to the solution in my workday language. One of the reasons that M is shorter is because its functions are more tolerant. For arc, I had to make sure that the index to the string was not too large. In M, it simply returns an empty string if you attempt to access a string position beyond the length. And I used iteration with M. Perhaps the arc solution would have been shorter had I done the same there.

Interesting exercise for this newbie.

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2 points by akkartik 2058 days ago | link

Yeah you can index past the end in my arc variant (http://github.com/akkartik/wart). But it currently returns nil. What should it be?

I'd love to see the M version expanded into english.

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2 points by jsgrahamus 2058 days ago | link

Great.

I'm used to seeing it as the empty string

Here is the expanded M version (! = LF):

  IF X=Y WRITE !,0 ; No difference
  ELSE  FOR I=1:1 IF $EXTRACT(X,I)'=$EXTRACT(Y,I) WRITE !,I QUIT

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1 point by akkartik 2058 days ago | link

I still don't know enough MUMPS to follow it :)

Are you concatenating strings? Is that where returning "" is useful? In wart concatenating strings is tolerant of non-strings, so I think returning nil should be ok.

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2 points by jsgrahamus 2057 days ago | link

If the string X = the string Y, print 0 (to indicate there are no differences / MUMPS uses 1-based indexing of strings)

Otherwise, go through each character of the strings and at the point where they differ, print that index and quit looping.

MUMPS does not error out on attempting to extract a character beyond the string's length. So in that event, 1 string's extract will return the empty string, which will differ from the other string's extract and will cause you to print the index and quit. Not generating such an error, of course, cuts down on the code needed.

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2 points by zck 2057 days ago | link

So if we enter the for loop, we know there is a difference between the two sequences.

So we just need to find an index position i where (isnt seq1.i seq2.i) is true. If indexing off the end of a string causes an error, we need to put in code to prevent that. If indexing off the end of a string returns anything that can't be an element of the string, we can compare each index i until we find a difference, knowing that we'll eventually find one.

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1 point by jsgrahamus 2057 days ago | link

Correct.

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1 point by jsgrahamus 2059 days ago | link

test-iso?

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1 point by akkartik 2058 days ago | link

https://github.com/nex3/arc/blob/ec47676f99af11b6fa8935a3cb0...

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