Exercise 7: Look up a key

Aim: use a map and make a missing-key policy explicit. Save ex07.fib.

Type and predict

(ns main (:use fib.core fib.coll))
(defun main () -> i64
  (let ((stock {"tea" 3 "coffee" 0})
        (restocked (assoc stock "tea" 5)))
    (do
      (println (get stock "tea" -1))
      (println (get stock "coffee" -1))
      (println (get stock "water" -1))
      (println (get restocked "tea" -1))
      (println (get stock "tea" -1))
      0)))

Map entries alternate key and value. This map's keys are strings and its values are integers. The third argument to get supplies a default for an absent key. assoc returns an updated map. Do not depend on map iteration/printing order.

Check your prediction

3
0
-1
5
3
0

Change it

1. Add water with quantity 2 to a new map. Show that stock still lacks water.
2. Change the default to zero. Explain which two situations now look identical.
3. Find contains? in the library declarations and use it to distinguish absent water from coffee with zero stock.

Break and repair

In a copy, add a string quantity to this integer-valued map. Read the type diagnostic, then restore a number. A key's spelling can be wrong even when its type is right: try "Tea", predict the default and repair the spelling.

Checkpoint

Explain why a default of zero would be reasonable for a word-frequency counter but insufficient when you need to distinguish “unknown product” from “sold out.”

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