I don't know it's intentional or a bug, but now object->string returns something different.
My understanding of length and fold is the same for deep lists as for hash table. It's great you have provided a patch so a constant time way to know no of key/value bindings in a hash table could be more accessible.. 在 2013-2-17，19:59，Daniel Hartwig <mand...@gmail.com> 写道： > On 17 February 2013 11:55, Hengqing Hu <hengqing...@gmail.com> wrote: >> Dear list, >> >> It seems the read syntax of a hash table has been silently changed. >> >> Now (make-hash-table) produces >> #<hash-table 172f1c0 0/31> >> instead what was in 1.8 >> #<hash-table 0/31> >> >> But the document is not updated yet? >> > > Hi > > That's how hash-table values are printed, true, but it is not _read > syntax_. Neither is it a documented format (although it does appear > in the example output of one command). You can not rely on the format > of that string to be anything, it is just a convenience. :-) > > The manual recommends to use hash-fold for counting the elements: > (hash-fold (lambda (key value count) (1+ count)) 0 table) > > You could also adapt your parser to be more flexible, looking only at > the tail of the string after the last whitespace. Although such hacks > work, it is not advisable. > > >> It would be more convinient if a hash-length procedure could be provided. > > Right. > > The two modules (rnrs hashtables) and (srfi srfi-69) both provide > *-size. Their interfaces are slightly different to the native > hash-tables, and the objects produced are not compatible with each > other. > > Not sure why the native tables don't have an equivalent, perhaps because: > - the terms ‘size’ and ‘length’ are not so good for hash tables [1]; > - it's very easy to do with hash-fold; > - usually more interesting to count particular items rather than everything; > and > - there may not be any guarantees on uniqueness of keys amongst the > elements stored in hash table, so what is meant by “number of > elements” can vary between applications [2]? > > Hence, hash-fold is always preferred to determine this. > > However, for quick and dirty case it's simple enough to write -count, > so I attach a patch for this with total count in constant time: > > -- Scheme Procedure: hash-count pred table > -- C Function: scm_hash_count (pred, table) > Return the number of elements in the given hash TABLE for > which `(PRED KEY VALUE)' returns true. If PRED itself is > `#f', return the total number of elements. > > > Regards > > > [1] Internally and in the documentation, ‘size’ is already associated > with the number of buckets. This is consistent with how some academic > literature uses the term also. > [2] For example, a vhash can store multiple elements with the same key > and provides access to them. > <0001-add-hash-count-for-native-tables.patch>
