8.9. Idiom Patterns

Python for loop is equivalent to forEach in other languages
Other languages for loop is Python's while (sic!)

Code Complexity vs. Programmer Experience:

../../_images/iterator-patterns-programmer-exp.png

Time Complexity: https://wiki.python.org/moin/TimeComplexity

8.9.1. Foreach Element

Problem:

>>> data = ['a', 'b', 'c']
>>>
>>> i = 0
>>> while i < len(data):
...     x = data[i]
...     i += 1
...     print(x)
a
b
c

Solution:

>>> data = ['a', 'b', 'c']
>>>
>>> for x in data:
...     print(x)
a
b
c

8.9.2. Foreach Index

Problem:

>>> data = ['a', 'b', 'c']
>>>
>>> count = len(data)
>>> for i in range(count):
...     x = data[i]
...     print(x)
a
b
c

Solution:

>>> data = ['a', 'b', 'c']
>>>
>>> for x in data:
...     print(x)
a
b
c

8.9.3. Range

Do something multiple times

Problem:

>>> i = 0
>>> while i < 3:
...     print('hello')
...     i += 1
hello
hello
hello

Solution:

>>> for _ in range(3):
...     print('hello')
hello
hello
hello

8.9.4. Sum

Problem:

>>> data = [1, 2, 3]
>>>
>>> result = 0
>>> for value in data:
...     result += value

Solution:

>>> data = [1, 2, 3]
>>>
>>> result = sum(data)

8.9.5. Enumerate

Problem:

>>> data = ['a', 'b', 'c']
>>>
>>> index = 0
>>> for value in data:
...     print(index, value)
...     index += 1
0 a
1 b
2 c

Solution:

>>> data = ['a', 'b', 'c']
>>>
>>> for index, value in enumerate(data):
...     print(index, value)
0 a
1 b
2 c

8.9.6. Zip

Problem:

>>> data1 = ['a', 'b', 'c']
>>> data2 = [1, 2, 3]
>>>
>>> count = min(len(data1), len(data2))
>>> result = []
>>> for i in range(count):
...     a = data1[i]
...     b = data2[i]
...     result.append((a, b))

Solution:

>>> data1 = ['a', 'b', 'c']
>>> data2 = [1, 2, 3]
>>>
>>> result = zip(data1, data2)

8.9.7. Any

Problem:

>>> users = [
...     {'firstname': 'Mark', 'lastname': 'Watney', 'age':40},
...     {'firstname': 'Melissa', 'lastname': 'Lewis', 'age':41},
...     {'firstname': 'Rick', 'lastname': 'Martinez', 'age':39},
...     {'firstname': 'Alex', 'lastname': 'Vogel', 'age':15},
... ]
>>>
>>> result = True
>>> for user in users:
...     if not user['age']>=18:
...         result = False
>>>
>>> if result:
...     print('All users are adult')
... else:
...     print('We have at least one child')
We have at least one child

Solution:

>>> users = [
...     {'firstname': 'Mark', 'lastname': 'Watney', 'age':40},
...     {'firstname': 'Melissa', 'lastname': 'Lewis', 'age':41},
...     {'firstname': 'Rick', 'lastname': 'Martinez', 'age':39},
...     {'firstname': 'Alex', 'lastname': 'Vogel', 'age':15},
... ]
>>>
>>> if all(user['age']>=18 for user in users):
...     print('All users are adult')
... else:
...     print('We have at least one child')
We have at least one child

8.9.8. List Comprehension

Problem:

>>> data = ['a', 'b', 'c']
>>>
>>> result = []
>>> for x in data:
...     result.append(x)

Solution:

>>> data = ['a', 'b', 'c']
>>>
>>> result = [x for x in data]

8.9.9. Set Comprehension

Problem:

>>> data = ['a', 'b', 'c']
>>>
>>> result = set()
>>> for x in data:
...     result.add(x)

Solution:

>>> data = ['a', 'b', 'c']
>>>
>>> result = {x for x in data}

8.9.10. Dict Comprehension

Problem:

>>> data = {'a': 1, 'b': 2, 'c': 3}
>>>
>>> result = {}
>>> for key, value in data.items():
...     result[key] = value

Solution:

>>> data = {'a': 1, 'b': 2, 'c': 3}
>>>
>>> result = {k:v for k,v in data.items()}

8.9.11. For Else

x in data

Problem:

>>> data = [1, 2, 3]
>>> x = 10
>>>
>>> found = False
>>> for value in data:
...     if value == x:
...         print('Found')
...         found = True
...         break
>>> if not found:
...     print('Not Found')
Not Found

Solution:

>>> data = [1, 2, 3]
>>> x = 10
>>>
>>> for value in data:
...     if value == x:
...         print('Found')
...         break
... else:
...     print('Not Found')
Not Found

8.9.12. While Else

x in data

Problem:

>>> data = [1, 2, 3]
>>> x = 10
>>>
>>> found = False
>>> while i < len(data):
...     value = data[i]
...     i += 1
...     if value == x:
...         print('Found')
...         found = True
...         break
>>> if not found:
...     print('Not Found')
Not Found

Solution:

>>> data = [1, 2, 3]
>>> x = 10
>>>
>>> while i < len(data):
...     value = data[i]
...     i += 1
...     if value == x:
...         print('Found')
...         break
... else:
...     print('Not Found')
Not Found

8.9.13. Str Startswith One

Problem:

>>> data = 'mwatney@nasa.gov'
>>>
>>> data[:7] == 'mwatney'
True

Solution:

>>> data = 'mwatney@nasa.gov'
>>>
>>> data.startswith('mwatney')
True

8.9.14. Str Startswith Many

Problem:

>>> data = 'mwatney@nasa.gov'
>>>
>>> data[:7] == 'mwatney' or data[:6] == 'mlewis'
True

Solution:

>>> data = 'mwatney@nasa.gov'
>>>
>>> data.startswith(('mwatney', 'mlewis'))
True

8.9.15. Str Endswith One

Problem:

>>> data = 'mwatney@nasa.gov'
>>>
>>> data[-8:] == 'nasa.gov'
True

Solution:

>>> data = 'mwatney@nasa.gov'
>>>
>>> data.endswith('nasa.gov')
True

8.9.16. Str Endswith Many

Problem:

>>> data = 'mwatney@nasa.gov'
>>>
>>> data[-8:] == 'nasa.gov' or data[-7:] == 'esa.int'
True

Solution:

>>> data = 'mwatney@nasa.gov'
>>>
>>> data.endswith(('nasa.gov', 'esa.int'))
True

8.9.17. Join Strings with Character

Problem:

>>> data = ['Mark', 'Melissa', 'Rick']
>>>
>>> result = ''
>>> for user in data:
...     result += user + ','
>>> result = result.removesuffix(',')

Solution:

>>> data = ['Mark', 'Melissa', 'Rick']
>>>
>>> result = ','.join(data)

8.9.18. Join Lines with Newlines

Problem:

>>> data = ['line1', 'line2', 'line3']
>>>
>>> result = [line+'\n' for line in data]

Solution:

>>> data = ['line1', 'line2', 'line3']
>>>
>>> result = '\n'.join(data) + '\n'

8.9.19. Map

Problem:

>>> def transform(x):
...     return ...
...
>>> data = [1, 2, 3]
>>>
>>> result = [transform(x) for x in data]

Solution:

>>> def transform(x):
...     return ...
...
>>> data = [1, 2, 3]
>>>
>>> result = map(transform, data)

8.9.20. Filter

Problem:

>>> def predicate(x):
...     return ...
...
>>> data = [1, 2, 3]
>>>
>>> result = [x for x in data if predicate(x)]

Solution:

>>> def predicate(x):
...     return ...
...
>>> data = [1, 2, 3]
>>>
>>> result = filter(predicate, data)

8.9.21. Other Programming Languages

For:

for (int i = 0; i <= 10; i++)  # C/C++/Java
for (var i = 0; i <= 10; i++)  # JavaScript

i = 0
while i <= 10:  #  Python
    i += 1

ForEach Index:

for (let idx in collection)         # JavaScript
for idx in range(len(collection))   # Python

ForEach Element:

for (var element : collection)   # Java
for (let element of collection)  # JavaScript
for element in collection        # Python
for i in range(0,10)             # Python

Enumerate:

for (int i = 0; i <= collection.length; i++)  # C++ / JAVA / JavaScript
    element = collection[i]

for (let i in collection)  # JavaScript
    element = collection[i]

for i in range(len(collection)):  # Python (1to1 algorithm conversion)
    element = collection[i]

for i, element in enumerate(collection)  # Python (Pythonic way)

Zip:

for (int i = 0; i <= collection.length; i++)  # C++ / JAVA / JavaScript
    a = collection1[i]
    b = collection2[i]

for i in range(len(collection1)):  # Python (1to1 algorithm conversion)
    a = collection1[i]
    b = collection2[i]

for a, b in zip(collection1, collection2)  # Python (Pythonic way)