Dictionaries

Dictionaries are data structures that hold key-value pairs, written as key:value.

See also

• Python docs for dict

• Dictionaries in Python

• Dictionary keys as list

You can define a dictionary like this:

german_english_dictionary = {'Vorlesung':'Lecture', 'Gleichung':'Equation'}

For readability reasons, consider writing it like this:

german_english_dictionary = {
    'Vorlesung':'Lecture', 
    'Gleichung':'Equation'
}

german_english_dictionary

{'Vorlesung': 'Lecture', 'Gleichung': 'Equation'}

If you want to access a given entry in the dictionary, you can address it using square brackets [] and the key:

german_english_dictionary['Vorlesung']

'Lecture'

You can add elements to the dictionary:

german_english_dictionary['Tag'] = 'Day'

german_english_dictionary

{'Vorlesung': 'Lecture', 'Gleichung': 'Equation', 'Tag': 'Day'}

keys = list(german_english_dictionary.keys())

keys

['Vorlesung', 'Gleichung', 'Tag']

You can retrieve all keys from the dictionary as a list:

keys = list(german_english_dictionary.keys())

keys

['Vorlesung', 'Gleichung', 'Tag']

keys[1]

'Gleichung'

You can retrieve all values from the dictionary as a list:

values = list(german_english_dictionary.values())

values

['Lecture', 'Equation', 'Day']

Tables

We can use dictionaries to build table-like structures. Here, the value of a key:value pair is not a single value but a list of values, i.e., a column.

measurements_week = {
    'Monday':   [2.3, 3.1, 5.6],
    'Tuesday':  [1.8, 7.0, 4.3],
    'Wednesday':[4.5, 1.5, 3.2],
    'Thursday': [1.9, 2.0, 6.4],
    'Friday':   [4.4, 2.3, 5.4]
}

measurements_week

{'Monday': [2.3, 3.1, 5.6],
 'Tuesday': [1.8, 7.0, 4.3],
 'Wednesday': [4.5, 1.5, 3.2],
 'Thursday': [1.9, 2.0, 6.4],
 'Friday': [4.4, 2.3, 5.4]}

measurements_week['Monday']

[2.3, 3.1, 5.6]

We can also store variables in such tables:

w1 = 5
h1 = 3
area1 = w1 * h1

w2 = 2
h2 = 4
area2 = w2 * h2

rectangles = {
    "width": [w1, w2],
    "height": [h1, h2],
    "area": [area1, area2]
}

rectangles

{'width': [5, 2], 'height': [3, 4], 'area': [15, 8]}

Exercise

You just measured the radius of three circles. Write them into a table and add a column with corresponding circle area measurements.

r1 = 12
r2 = 8
r3 = 15