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Dictionaries

Dictionaries. Alexandra Stefan CSE1310 – University of Texas at Arlington. Dictionary. A sequence of key-value pairs: { key:value , key:value ,… key: value } Optimized for efficient search based on the key May not always print the elements in the same order

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Dictionaries

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  1. Dictionaries Alexandra Stefan CSE1310 – University of Texas at Arlington

  2. Dictionary • A sequence of key-value pairs: {key:value, key:value,… key: value} • Optimized for efficient search based on the key • May not always print the elements in the same order • As you make modifications (insert/delete elements) the order of the pairs may change to enable fast searching

  3. Legal Keys • No duplicate keys • Cannot have multiple entries for a key • Keys must be immutable types • Strings • Integers • Tuples • Mix and match is okay Slide by Chris Conly

  4. Working with dictionaries • {key:value, key:value,… key: value} • What operations would you like? What operations do you know from the other collections?

  5. Working with dictionaries • They are mutable • Shallow copies • d = {key:value,key:value,… key: value} • Functions: • d[new_key] = new_value • del d[key] • d.items(), d.keys(), d.values() • list(d.keys()), sorted(d.keys()) • len(d): number of key-value pairs • in: verifies that a key is in the dictionary • min(__), max(__)

  6. keys(), values(), items() • They are called view objects • They reflect changes made to the dictionary • If you add or delete items in the dictionary, the views will reflect that • “If keys, values and items views are iterated over with no intervening modifications to the dictionary, the order of items will directly correspond.” – Python Standard Library • To get a list of them use the list constructor: • list(d.keys()), list(d.values()), list(d.items()) • How connected are the lists and the views they are based on? • d = {1:[1,2]}, lv = list(d.values()), d[1][0] = 10, d, lv, lv[0][1] = 200, lv, d

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