Class 22 — Wednesday March 6

It’s a fact, look it up

Really need to know — What lies farther down the road — Need a future map

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Look both ways

• Back or ahead

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Do you hear

• Tips to be a better listener.

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Agenda

• Consider dict

• hacer 뭔가 frais oggi

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Maxims (among others)

• Read the problem in its entirety before problem solving

• Determine for yourself why the sample problem solutions are as they are

• Problem solve before you code

• Check whether your solution produces the same answers as the sample runs

• Check whether your solution matches problem specifications

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Huh?

• dónde está mi lápiz

• donde esta mi lapiz

• hvor 是 该 tumili maleri av Munch

• est la baguette fraîche aujourd'hui

• est la baguette fraiche aujourd'hui

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Dictionary

• The human brain is great with associations; e.g., we typically associate apple with red and January 1st with New Year's Day.

• A dict is Python's software equivallent of a collection of associations.

• In particular, a dict is unnordered collection of associations.

• The elements of dict are called mappings. A mapping associates one object (called the key) with another object (called the value).

• The literal for an empty dict is {}.

• With a dictionary and a key you can simply lookup its meaning.

• There is no simple way with a dictionary and a value to determine what key(s) map to the value.

• A literal mapping is written using the : to separate the key with its value. For example, the variable flora_to_color defined below is a dict that associates flora with colors.

flora_to_color = { 'spruce': 'green', 'apple': 'red', 'shamrock': 'green',

  'banana': 'yellow', 'potato': 'brown', 'eggplant': 'purple' }

• The len() function works on a dict. len( flora_to_color ) is 6, because the definition of flora_to_color included six mappings.

• The brackets [] operator in conjunction with assignment can be used to put a mapping into a dict. The below statement adds the mapping 'lemon' to 'yellow'.

flora_to_color[ 'lemon' ] = 'yellow'

• The keys of a dict are always distinct. If there had already been a mapping with 'lemon' as the key, the above assignment would replace the association.

• As our flora_to_color example shows, the values of a dict are not necessarily unique.

• Other information about dict objects are listed below. In the below, assume that d is a dict.

• d.get( k ) returns the value that key k maps to in d. If there is no such value, returns None.

• d[ k ] returns the value that key k maps to in d. If there is no such value, an error occurs.

• d[ k ] = v sets d to have a mapping from k to v. If there was a prior mapping from k in d, then that mapping is removed. In dictionary parlance, there is a now a mapping from key k to value v in d.

• del d[ k ] removes k’s mapping in d.

• ( k in d ) indicates whether k is a key value in d.

• ( k not in d ) indicates whether k is not a key value in d.

• d.clear() removes all mappings in d.

• d.keys() are the keys for d. There are no duplicates among the keys.

• d.values() are the values of the keys in d. There are duplicate values if more than one key maps to the same value.

• d.pop( k ) removes k’s mapping in d and returns the value of that mapping.

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Downloads

• Problem here_to_there.py

• Problem huh.py

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Examples

• Problem good_doggie.py

• Problem what_did_you_say.py

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To do

• Homework

• Look over artifacts

• Information on a dict is always available in the module documentation section of the website.

• Additional information on collections is in general is available in the collections epistile.

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