1 Example top

• link | changes

  ;; Suppose you have a set of descriptions of the weights of animals:
  
  user=> (def weights #{ {:name 'betsy :weight 1000}
                         {:name 'jake :weight 756}
                         {:name 'shyq :weight 1000} })
  
  
  ;; You want the names of all the animals that weight 1000. One way to do 
  ;; that uses `index`. First, you can group the set elements (the maps) so
  ;; that those with the same weights are in the same group.
  
  user=> (def by-weight (index weights [:weight]))
  #'user/by-weight
  
  user=> by-weight
  {{:weight 756} #{{:name jake, :weight 756}}, 
   {:weight 1000} #{{:name shyq, :weight 1000} 
                    {:name betsy, :weight 1000}}}
  
  
  ;; That structure's a little complicated, so let's first use it in simple 
  ;; ways. To get all the maps that describe animals that weigh 756, you'd 
  ;; do this:
  
  user=> (get by-weight {:weight 756})
  #{{:name jake, :weight 756}}
  
  
  ;; That's a set containing a single element. To confirm that there are two
  ;; animals with weight 1000, this:
  
  user=> (count (get by-weight {:weight 1000}))
  2
  
  
  ;; To get the names of those two animals we can map a name-extracting function
  ;; over the set of two maps. Since a keyword in a map is also a function that
  ;; returns its corresponding value, we can just use `:name` as our function:
  
  user=> (map :name (get by-weight {:weight 1000}))
  (shyq betsy)
  

  ;; Suppose you have a set of descriptions of the weights of animals: user=> (def weights #{ {:name 'betsy :weight 1000} {:name 'jake :weight 756} {:name 'shyq :weight 1000} }) ;; You want the names of all the animals that weight 1000. One way to do ;; that uses `index`. First, you can group the set elements (the maps) so ;; that those with the same weights are in the same group. user=> (def by-weight (index weights [:weight])) #'user/by-weight user=> by-weight {{:weight 756} #{{:name jake, :weight 756}}, {:weight 1000} #{{:name shyq, :weight 1000} {:name betsy, :weight 1000}}} ;; That structure's a little complicated, so let's first use it in simple ;; ways. To get all the maps that describe animals that weigh 756, you'd ;; do this: user=> (get by-weight {:weight 756}) #{{:name jake, :weight 756}} ;; That's a set containing a single element. To confirm that there are two ;; animals with weight 1000, this: user=> (count (get by-weight {:weight 1000})) 2 ;; To get the names of those two animals we can map a name-extracting function ;; over the set of two maps. Since a keyword in a map is also a function that ;; returns its corresponding value, we can just use `:name` as our function: user=> (map :name (get by-weight {:weight 1000})) (shyq betsy)