124 lines
2.1 KiB
Markdown
124 lines
2.1 KiB
Markdown
---
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tags: [python, data-structures]
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---
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# Dictionaries in Python
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Dictionaries are basically the Python equivalent of objects in JS.
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Dictionaries:
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- Are ordered (in contrast to JS)
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- Are mutable
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- Are indexed by a key which references a value
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- Can be increased/decreased in length by adding/removing new members.
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## Basic usage
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Dictionaries are declared with `{...}`:
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```python
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cities = {
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'Wales': 'Cardiff',
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'England': 'London',
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'Scotland': 'Edinburgh',
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'Northern Ireland': 'Belfast',
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'Ireland': 'Dublin'
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}
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print(type(citites))
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# <class 'dict'>
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```
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## Accessing entries
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```python
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print(cities['Wales'])
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# Cardiff
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print(cities.get('Ireland'))
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# Dublin
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print(cities.values())
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# ['Cardiff', 'London', 'Edinburgh', 'Belfast', 'Dublin']
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print(cities.keys())
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# ['Wales', 'England', 'Scotland', 'Northern Ireland', 'Ireland']
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print(cities.items())
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# [('Wales', 'Cardiff'), ('England', 'London'), ('Scotland', 'Edinburgh'), ('Northern Ireland', 'Belfast'), ('Ireland', 'Dublin')]
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```
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## Predicates
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```py
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print('Wales' in cities)
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# True
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print('France' not in cities)
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# True
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```
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## Looping
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```py
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for country in cities:
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print(country, end=', ')
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print(cities[country])
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"""
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Wales, Cardiff
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England, London
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Scotland, Edinburgh
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Northern Ireland, Belfast
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Ireland, Dublin
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"""
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for e in country.values():
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print(e)
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```
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## Updating values
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```py
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cities['Wales'] = 'Swansea'
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print(cities)
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```
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## Removing values
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```py
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# Remove last item
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cities.popitem()
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print(cities)
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# {'Wales': 'Cardiff', 'England': 'London', 'Scotland': 'Edinburgh', 'Northern Ireland': 'Belfast'}
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# Remove specific entry by key
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cities.pop('Northern Ireland')
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print(cities)
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# {'Wales': 'Cardiff', 'England': 'London', 'Scotland': 'Edinburgh'}
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del cities['Scotland']
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print(cities)
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{'Wales': 'Cardiff', 'England': 'London'}
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```
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## Containers as values
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```py
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seasons = {
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'Spring': ('Mar', 'Apr', 'May'),
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'Summer': ('June', 'July', 'August'),
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'Autumn': ('September', 'October', 'November'),
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'Winter': ('December', 'January', 'February')}
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print(seasons['Spring'])
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print(seasons['Spring'][1])
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"""
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('Mar', 'Apr', 'May')
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Apr
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"""
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```
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