(guile.info.gz) Hash Table Examples

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(guile.info.gz) Hash Table Reference

 
 22.7.3.1 Hash Table Examples
 ............................
 
 For demonstration purposes, this section gives a few usage examples of
 some hash table procedures, together with some explanation what they do.
 
    First we start by creating a new hash table with 31 slots, and
 populate it with two key/value pairs.
 
      (define h (make-hash-table 31))
 
      (hashq-create-handle! h 'foo "bar")
      =>
      (foo . "bar")
 
      (hashq-create-handle! h 'braz "zonk")
      =>
      (braz . "zonk")
 
      (hashq-create-handle! h 'frob #f)
      =>
      (frob . #f)
 
    You can get the value for a given key with the procedure
 `hashq-ref', but the problem with this procedure is that you cannot
 reliably determine whether a key does exists in the table.  The reason
 is that the procedure returns `#f' if the key is not in the table, but
 it will return the same value if the key is in the table and just
 happens to have the value `#f', as you can see in the following
 examples.
 
      (hashq-ref h 'foo)
      =>
      "bar"
 
      (hashq-ref h 'frob)
      =>
      #f
 
      (hashq-ref h 'not-there)
      =>
      #f
 
    Better is to use the procedure `hashq-get-handle', which makes a
 distinction between the two cases.  Just like `assq', this procedure
 returns a key/value-pair on success, and `#f' if the key is not found.
 
      (hashq-get-handle h 'foo)
      =>
      (foo . "bar")
 
      (hashq-get-handle h 'not-there)
      =>
      #f
 
    There is no procedure for calculating the number of key/value-pairs
 in a hash table, but `hash-fold' can be used for doing exactly that.
 
      (hash-fold (lambda (key value seed) (+ 1 seed)) 0 h)
      =>
      3
 

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