Lecture11-Dictionaries-3pp

Cmput174 dic2onaries 15 5 131029 keys and values

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Unformatted text preview: 12 4 13 ­10 ­29 Adding key/value pairs to a Dic$onary • To add a new value, assign it to a new key • Dic2onaries do not need to use append() or insert() like lists do. 13 CMPUT174  ­ ­ Dic2onaries Upda$ng a Value in a Dic$onary • To update a value, re ­assign it • You can change the value associated with a key by re ­ assigning • For example: my_dict = {“greeting”:”Hello”}
 print(my_dict[“greeting”]) # outputs Hello
 my_dict[“greeting”] = “Hi”
 print(my_dict[“greeting”]) # outputs Hi" 14 CMPUT174  ­ ­ Dic2onaries Keys and values • A run2me error occurs if you try to access a dic2onary with a key which is not in it: • The get(key) method handles this error nicely: default parameter! CMPUT174  ­ ­ Dic2onaries 15 5 13 ­10 ­29 Keys and values • get(key) returns the value OR a default parameter: • To get all keys or all values in a dic2onary: CMPUT174  ­ ­ Dic2onaries 16 Membership tests • Checking if a key is in the dic2onary • To check if a value is in the dic2onary, we need to get the list of values first: CMPUT174  ­ ­ Dic2onaries 17 Dele$ng Pairs from a Dic$onary • There are two ways of removing pairs from a dic2onary CMPUT174  ­ ­ Dic2onaries 18 6 13 ­10 ­29 Dele$ng Pairs from a Dic$onary  Using pop() on a list removes and returns the last element • Dic2onaries don’t have a “last” element, so popitem() removes and returns some key/value pair from a dic2onary • Using pop(key) on a dic2onary requires an argument which is the key to remove. If the key exists, it is removed and the value is returned. If it does not, an error occurs. • Using pop(key, default) on a dic2o...
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