• Iterate over sequences (iterables)

• Repeat if until result is False

10.1.1. Syntax

• Continue execution when argument is True

• Stops if argument is False

Generic syntax:

while <condition>:
<do something>


Example:

>>>
... while True:
...     pass


10.1.2. Getitem with Const

>>> data = ['a', 'b', 'c']
>>>
>>> print(data[0])
a
>>> print(data[1])
b
>>> print(data[2])
c
>>> print(data[3])
Traceback (most recent call last):
IndexError: list index out of range


10.1.3. Getitem with Variable

>>> data = ['a', 'b', 'c']
>>>
>>> i = 0               # i = 0
>>> print(data[i])      # data[0]
a
>>>
>>> i += 1              # i = 1
>>> print(data[i])      # data[1]
b
>>>
>>> i += 1              # i = 2
>>> print(data[i])      # data[2]
c
>>>
>>> i += 1              # i = 3
>>> print(data[i])      # data[3]
Traceback (most recent call last):
IndexError: list index out of range


10.1.4. Getitem with Boundary Checking

>>> data = ['a', 'b', 'c']
>>> len(data)
3

>>> data = ['a', 'b', 'c']
>>> i = 0
>>>
>>> if i < len(data):
...     print(data[i])  # data[0]
...     i += 1          # i = 1
a
>>>
>>> if i < len(data):   # True
...     print(data[i])  # data[1]
...     i += 1          # i = 2
b
>>>
>>> if i < len(data):   # True
...     print(data[i])  # data[2]
...     i += 1          # i = 3
c
>>>
>>> if i < len(data):   # False
...     print(data[i])  # will not execute
...     i += 1          # will not execute


10.1.5. While Loop

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


10.1.6. Sequence Iteration

Better idea for this is to use for loop. for loop supports Iterators. len() must write all numbers to memory, to calculate its length:

>>> i = 0
>>> data = ['a', 'b', 'c']
>>>
>>> while i < 3:
...     print(i, data[i])
...     i += 1
0 a
1 b
2 c

>>> i = 0
>>> data = ['a', 'b', 'c']
>>> length = 3
>>>
>>> while i < length:
...     print(i, data[i])
...     i += 1
0 a
1 b
2 c

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


10.1.7. Good Practices

• The longer the loop scope, the longer the variable name should be

• Avoid one letters if scope is longer than one line

• Use i for loop counter (it is traditional name in almost all languages)

10.1.8. Assignments

"""
* Assignment: Loop While Range
* Type: class assignment
* Complexity: easy
* Lines of code: 5 lines
* Time: 3 min

English:
1. Generate result: list[int] with numbers from 0 to 5 (exclusive)
2. Do not use range()
3. Run doctests - all must succeed

Polish:
1. Wygeneruj result: list[int] z liczbami od 0 do 5 (rozłącznie)
2. Nie używaj range()
3. Uruchom doctesty - wszystkie muszą się powieść

Tests:
>>> import sys; sys.tracebacklimit = 0

>>> assert type(result) is list
>>> assert all(type(x) is int for x in result)

>>> result
[0, 1, 2, 3, 4]
"""

# List with numbers from 0 to 5 (exclusive)
# type: list[int]
result = ...


"""
* Assignment: Loop For Count
* Type: class assignment
* Complexity: easy
* Lines of code: 10 lines
* Time: 8 min

English:
1. Count occurrences of each color
2. Do not use list.count()
3. Run doctests - all must succeed

Polish:
1. Zlicz wystąpienia każdego z kolorów
2. Nie używaj list.count()
3. Uruchom doctesty - wszystkie muszą się powieść

Tests:
>>> import sys; sys.tracebacklimit = 0

>>> assert red is not Ellipsis, \
'Assign your result to variable red'
>>> assert green is not Ellipsis, \
'Assign your result to variable green'
>>> assert blue is not Ellipsis, \
'Assign your result to variable blue'

>>> assert type(red) is int, \
'Variable red has invalid type, should be list'
>>> assert type(green) is int, \
'Variable green has invalid type, should be list'
>>> assert type(blue) is int, \
'Variable blue has invalid type, should be list'

>>> red
3
>>> green
2
>>> blue
2
"""

DATA = ['red', 'green', 'blue', 'red', 'green', 'red', 'blue']

red = 0
green = 0
blue = 0

# If variable COLOR is:
# - 'red' then increment variable red by 1
# - 'green' then increment variable green by 1
# - 'blue' then increment variable blue by 1
...


"""
* Assignment: Loop While toFloat
* Type: class assignment
* Complexity: easy
* Lines of code: 5 lines
* Time: 5 min

English:
1. Create result: list[float]
2. Use while to iterate over DATA
3. Convert current elements of DATA to float
4. Converted value append to result
5. Run doctests - all must succeed

Polish:
1. Stwórz result: list[float]
2. Użyj while do iterowania po DATA
3. Przekonwertuj obecny element DATA do float
4. Przekonwertowaną wartość dodaj na koniec result
5. Uruchom doctesty - wszystkie muszą się powieść

Hints:
* Stop or Ctrl+C kills infinite loop

Tests:
>>> import sys; sys.tracebacklimit = 0

>>> type(result)
<class 'list'>

>>> assert all(type(x) is float for x in result)

>>> result
[2.0, 3.0, 3.5, 4.0, 4.5, 5.0]
"""

DATA = (2, 3, 3.5, 4, 4.5, 5)

# Values from DATA converted to float
# type: list[float]
result = ...


"""
* Assignment: Loop While toStr
* Type: class assignment
* Complexity: easy
* Lines of code: 4 lines
* Time: 5 min

English:
1. Create result: str
2. Use while to iterate over DATA
3. Add current element of DATA to result
4. Do not use str.join()
5. Run doctests - all must succeed

Polish:
1. Stwórz result: str
2. Użyj while do iterowania po DATA
3. Dodaj obecny element z DATA do result
4. Nie używaj str.join()
5. Uruchom doctesty - wszystkie muszą się powieść

Hints:
* Stop or Ctrl+C kills infinite loop

Tests:
>>> import sys; sys.tracebacklimit = 0

>>> type(result)
<class 'str'>
>>> result
'hello'
"""

DATA = ['h', 'e', 'l', 'l', 'o']

# Joined DATA values
# type: str
result = ...


"""
* Assignment: Loop While Translate
* Type: class assignment
* Complexity: medium
* Lines of code: 5 lines
* Time: 5 min

English:
1. Use while to iterate over DATA
2. If letter is in PL then use conversion value as letter
3. Add letter to result
4. Run doctests - all must succeed

Polish:
1. Użyj while do iteracji po DATA
2. Jeżeli litera jest w PL to użyj skonwertowanej wartości jako litera
3. Dodaj literę do result
4. Uruchom doctesty - wszystkie muszą się powieść

Hints:
* Stop or Ctrl+C kills infinite loop

Tests:
>>> import sys; sys.tracebacklimit = 0

>>> type(result)
<class 'str'>
>>> result
'zazolc gesla jazn'
"""

PL = {
'ą': 'a',
'ć': 'c',
'ę': 'e',
'ł': 'l',
'ń': 'n',
'ó': 'o',
'ś': 's',
'ż': 'z',
'ź': 'z',
}

DATA = 'zażółć gęślą jaźń'

# DATA with substituted PL diacritic chars to ASCII letters
# type: str
result = ...