9.1. Loop While¶
9.1.1. Rationale¶
>>> data = ['a', 'b', 'c']
>>>
>>> data[0]
'a'
>>> data[1]
'b'
>>> data[2]
'c'
>>> data = ['a', 'b', 'c']
>>> i = 0
>>>
>>> if i < 3:
... print(data[i])
... i += 1
a
>>>
>>> if i < 3:
... print(data[i])
... i += 1
b
>>>
>>> if i < 3:
... print(data[i])
... i += 1
c
>>> data = ['a', 'b', 'c']
>>> i = 0
>>>
>>> while i < 3:
... print(data[i])
... i += 1
a
b
c
9.1.2. Syntax¶
Continue execution when argument is
TrueStops if argument is
False
while loop generic syntax:
while <condition>:
<do something>
>>>
... while True:
... pass
9.1.3. Convention¶
The longer the loop scope, the longer the variable name should be
Avoid one letters if scope is longer than one line
Generic names:
i- for loop counterabort- for abort flagsabort_flag- for abort flags
9.1.4. Infinite Loop¶
Never ending loop. Used in servers to wait forever for incoming connections. Used in games for game logic.
>>>
... while True:
... print('hello')
9.1.5. Until¶
Has stop conditions
>>> i = 0
>>>
>>> while i < 3:
... print(i)
... i += 1
0
1
2
9.1.6. Iterating Over Sequence¶
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
9.1.7. Exit flag¶
Exit flag pattern is useful if you have for example multi-threaded application:
>>> abort = False
>>> i = 10
>>>
>>> while not abort:
... print(i)
... i -= 1
...
... if i == 6:
... print('Fuel leak detected. Abort, Abort, Abort!')
... abort = True
10
9
8
7
Fuel leak detected. Abort, Abort, Abort!
9.1.8. Force Exit the Loop¶
Force exit the loop using break keyword:
>>> i = 10
>>>
>>> while True:
... print(i)
... i -= 1
...
... if i == 6:
... print('Fuel leak detected. Abort, Abort, Abort!')
... break
10
9
8
7
Fuel leak detected. Abort, Abort, Abort!
Exiting the loop using break keyword:
>>>
... while True:
... number = input('Type number: ')
...
... if not number:
... # if user hit enter
... # without typing a number
... break
9.1.9. Force Skip Iteration¶
if
continueis encountered, it will jump to next loop iteration
>>> TEXT = ['# "Moon Speech" by John F. Kennedy, Rice Stadium, Houston, TX, 1962-09-12',
... '# Source: http://er.jsc.nasa.gov/seh/ricetalk.htm',
... 'We choose to go to the Moon.',
... 'We choose to go to the Moon in this decade and do the other things.',
... 'Not because they are easy, but because they are hard.',
... 'Because that goal will serve to organize and measure the best of our energies a skills.',
... 'Because that challenge is one that we are willing to accept.',
... 'One we are unwilling to postpone.',
... 'And one we intend to win']
>>>
>>> i = 0
>>>
>>> while i < len(TEXT):
... line = TEXT[i]
... i += 1
...
... if line.startswith('#'):
... continue
...
... print(line)
We choose to go to the Moon.
We choose to go to the Moon in this decade and do the other things.
Not because they are easy, but because they are hard.
Because that goal will serve to organize and measure the best of our energies a skills.
Because that challenge is one that we are willing to accept.
One we are unwilling to postpone.
And one we intend to win
Force skip iteration using continue keyword:
>>> i = 0
>>>
>>> while i < 10:
... print(i, end=', ')
... i += 1
...
... if i % 3:
... continue
...
... print(end='\n')
0, 1, 2,
3, 4, 5,
6, 7, 8,
9,
9.1.10. Assignments¶
"""
* Assignment: Loop While to Float
* Required: yes
* 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)
# list[float]: values from DATA converted to float
result = ...
"""
* Assignment: Loop While to Str
* Required: yes
* 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']
# str: joined DATA values
result = ...
"""
* Assignment: Loop While Translate
* Required: yes
* 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'
# str: DATA with substituted PL diacritic chars to ASCII letters
result = ...
"""
* Assignment: Loop While Input
* Required: no
* Complexity: medium
* Lines of code: 14 lines
* Time: 13 min
English:
1. Define `grades: list[float]`
2. Using `input()` ask user about grade, one at a time
3. User will type only valid `int` or `float`
4. To iterate use only `while` loop
5. If grade is in `GRADE_SCALE` - add it to `grades`
6. If grade is not in `GRADE_SCALE`, skip this iteration
7. If user pressed Enter key, end inserting data
8. Define `result: float` with arithmetic mean of `grades`
9. Test case when report list is empty
10. Run doctests - all must succeed
Polish:
1. Zdefiniuj `grades: list[float]`
2. Do iterowania użyj tylko pętli `while`
3. Używając `input()` poproś użytkownika o ocenę, jedną na raz
4. Użytkownik poda tylko poprawne `int` lub `float`
5. Jeżeli ocena jest w `GRADE_SCALE` - dodaj ją do `grades`
6. Jeżeli oceny nie ma w `GRADE_SCALE`, pomiń tą iterację
7. Jeżeli użytkownik wcisnął Enter, zakończ wprowadzanie danych
8. Zdefiniuj `result: float` ze średnią arytmetyczą `grades`
9. Przetestuj przypadek, gdy dzienniczek jest pusty
10. Uruchom doctesty - wszystkie muszą się powieść
Hints:
* `Stop` or `Ctrl+C` kills infinite loop
* `mean = sum(...) / len(...)`
Tests:
>>> import sys; sys.tracebacklimit = 0
>>> from statistics import mean
>>> type(grades)
<class 'list'>
>>> type(result)
<class 'float'>
>>> assert all(type(x) is float for x in grades)
>>> mean(grades) == result # doctest: +SKIP
True
>>> result
3.5
"""
from unittest.mock import MagicMock
# Simulate user input (for test automation)
input = MagicMock(side_effect=['1', '2', '2.5', '3', '3.5', '4', '5', '6', ''])
GRADE_SCALE = (2.0, 3.0, 3.5, 4.0, 4.5, 5.0)
# list[float]: all user grades
grades = ...
# float: arithmetic mean of grades
result = ...