3.5. Idiom Map¶
Map (convert) elements in sequence
Generator (lazy evaluated)
Built-in
3.5.1. Syntax¶
map(callable, *iterables)required
callable- Functionrequired
iterables- 1 or many sequence or iterator objects
>>> result = (float(x) for x in range(0,5))
>>> print(list(result))
[0.0, 1.0, 2.0, 3.0, 4.0]
>>> result = map(float, range(0,5))
>>> print(list(result))
[0.0, 1.0, 2.0, 3.0, 4.0]
3.5.2. Problem¶
>>> data = [1, 2, 3]
>>> result = []
>>>
>>> for x in data:
... result.append(float(x))
>>>
>>> print(result)
[1.0, 2.0, 3.0]
3.5.3. Solution¶
>>> data = [1, 2, 3]
>>> result = map(float, data)
>>>
>>> list(result)
[1.0, 2.0, 3.0]
3.5.4. Lazy Evaluation¶
>>> data = [1, 2, 3]
>>> result = map(float, data)
>>>
>>> next(result)
1.0
>>> next(result)
2.0
>>> next(result)
3.0
>>> next(result)
Traceback (most recent call last):
StopIteration
3.5.5. Multi Parameters¶
>>> def myfunc(x):
... return sum(x)
>>>
>>>
>>> DATA = [(1,2), (3,4)]
>>> result = map(myfunc, DATA)
>>> print(list(result))
[3, 7]
3.5.6. Starmap¶
>>> from itertools import starmap
>>>
>>>
>>> DATA = [
... (3.1415, 3),
... (2.71828, 2)]
>>>
>>> result = starmap(round, DATA) # round(number=3.1415, ndigits=2)
>>> print(list(result))
[3.142, 2.72]
3.5.7. Partial¶
>>> from functools import partial
>>>
>>>
>>> myround = partial(round, ndigits=1)
>>> DATA = [1.111, 2.222, 3.333]
>>>
>>> result = map(myround, DATA) # round(number=1.111, ndigits=1)
>>> print(list(result))
[1.1, 2.2, 3.3]
3.5.8. Use Case - 0x01¶
Built-in functions:
>>> DATA = [1, 2, 3]
>>> result = map(float, DATA)
>>>
>>> tuple(map(float, DATA))
(1.0, 2.0, 3.0)
>>> DATA = [1, 2, 3]
>>> result = map(float, DATA)
>>>
>>> set(map(float, DATA))
{1.0, 2.0, 3.0}
>>> DATA = [1, 2, 3]
>>> result = (float(x) for x in DATA)
>>>
>>> list(result)
[1.0, 2.0, 3.0]
>>> DATA = [1.1, 2.2, 3.3]
>>> result = map(round, DATA)
>>>
>>> list(result)
[1, 2, 3]
3.5.9. Use Case - 0x02¶
>>> def square(x):
... return x ** 2
>>>
>>>
>>> DATA = [1, 2, 3]
>>> result = map(square, DATA)
>>>
>>> list(result)
[1, 4, 9]
3.5.10. Use Case - 0x03¶
>>> def increment(x):
... return x + 1
>>>
>>>
>>> DATA = [1, 2, 3, 4]
>>> result = map(increment, DATA)
>>>
>>> list(result)
[2, 3, 4, 5]
3.5.11. Use Case - 0x04¶
>>> def translate(letter):
... return PL.get(letter, letter)
>>>
>>>
>>> DATA = 'zażółć gęślą jaźń'
>>> PL = {'ą': 'a', 'ć': 'c', 'ę': 'e',
... 'ł': 'l', 'ń': 'n', 'ó': 'o',
... 'ś': 's', 'ż': 'z', 'ź': 'z'}
>>>
>>> result = map(translate, DATA)
>>> ''.join(result)
'zazolc gesla jazn'
3.5.12. Use Case - 0x05¶
Standard input:
>>> import sys
>>>
>>>
... print(sum(map(int, sys.stdin)))
$ cat ~/.profile |grep addnum
alias addnum='python -c"import sys; print(sum(map(int, sys.stdin)))"'
3.5.13. Use Case - 0x06¶
>>> import requests
>>>
>>> url = 'https://python.astrotech.io/_static/iris-dirty.csv'
>>>
>>> data = requests.get(url).text
>>> header, *rows = data.splitlines()
>>> nrows, nfeatures, *class_labels = header.strip().split(',')
>>> label_encoder = dict(enumerate(class_labels))
>>> result = []
>>> for row in rows:
... *features, species = row.strip().split(',')
... features = map(float, features)
... species = label_encoder[int(species)]
... row = tuple(features) + (species,)
... result.append(row)
>>> def decode(row):
... *features, species = row.strip().split(',')
... features = map(float, features)
... species = label_encoder[int(species)]
... return tuple(features) + (species,)
>>>
>>> result = map(decode, rows)
>>> def decode(row):
... *features, species = row.strip().split(',')
... features = map(float, features)
... species = label_encoder[int(species)]
... return tuple(features) + (species,)
>>>
>>> with open('/tmp/myfile.csv') as file:
... header = file.readline()
... for line in map(decode, file):
... print(line)
3.5.14. Use Case - 0x07¶
>>>
... import pandas as pd
...
...
... DATA = 'https://python.astrotech.io/_static/phones-pl.csv'
...
... result = (
... pd
... .read_csv(DATA, parse_dates=['datetime'])
... .set_index('datetime', drop=True)
... .drop(columns=['id'])
... .loc['2000-01-01':'2000-03-01']
... .query('item == "sms"')
... .groupby(['period','item'])
... .agg(
... duration_count = ('duration', 'count'),
... duration_sum = ('duration', 'sum'),
... duration_median = ('duration', 'median'),
... duration_mean = ('duration', 'mean'),
... duration_std = ('duration', 'std'),
... duration_var = ('duration', 'var'),
... value = ('duration', lambda column: column.mean().astype(int))
... )
... )
3.5.15. Use Case - 0x08¶
>>> from functools import reduce
>>> from operator import add
>>>
>>>
>>> def even(x):
... return x % 2 == 0
>>>
>>> def positive(x):
... return x > 0
>>>
>>> def non_negative(x):
... return x >= 0
>>>
>>> def square(x):
... return x ** 2
>>>
>>> def add1(x):
... return x + 1
>>>
>>> def minus1(x):
... return x + 1
>>> data = range(0, 1024)
>>> data = filter(even, data)
>>> data = filter(positive, data)
>>> data = filter(non_negative, data)
>>> data = map(square, data)
>>> data = map(add1, data)
>>> data = map(minus1, data)
>>> result = reduce(add, data)
>>>
>>> result
178434046
>>> filters = [
... even,
... positive,
... non_negative,
... ]
>>>
>>> maps = [
... square,
... add1,
... minus1,
... ]
>>>
>>> def apply(data, fn):
... return map(fn, data)
>>>
>>>
>>> data = range(0, 1024)
>>> data = reduce(apply, filters, data)
>>> data = reduce(apply, maps, data)
>>> result = reduce(add, data)
>>>
>>> result
3072