Flow Control for-loop
The goal
For what reason…
Questions to David Rotermund
Logic blocks need to be indented. Preferable with 4 spaces!
The for statement
The for statement is used to iterate over the elements of a sequence (such as a string, tuple or list) or other iterable object
With range()
for i in range(0, 3):
print(i)
Output:
0
1
2
range()
class range(stop)
class range(start, stop[, step])
With a list
for i in [0, "A", 7, "nom num"]:
print(i)
Output:
0
A
7
nom num
The full statement
for_stmt ::= "for" target_list "in" starred_list ":" suite
["else" ":" suite]
for loop (the truth)
This is an optional topic!
The for loop is not counting up itself. It uses the __iter__ & __next__ combo of a instance of a class or a generator via yield.
Class
class ClassIterExample:
counter: int
counter_max: int
def __init__(self):
self.counter = 8
self.counter_max = 12
def __iter__(self):
return self
def __next__(self):
if self.counter < self.counter_max:
self.counter += 1
else:
raise StopIteration
important_result: int = self.counter**2
return f"Element: {self.counter}", important_result
test_instance = ClassIterExample()
for i in test_instance:
print(i)
Output:
('Element: 9', 81)
('Element: 10', 100)
('Element: 11', 121)
('Element: 12', 144)
Generator yield
from typing import Generator
def do_more(min_value: int, max_value: int) -> Generator:
for index in range(min_value, max_value):
yield index**2
for i in do_more(min_value=3, max_value=8):
print(i)
Output:
9
16
25
36
49
for loop – enumerate
enumerate(iterable, start=0)
If you need the index of the element then you can use enumerate:
for i in enumerate(range(10, 13)):
print(i)
Output:
(0, 10)
(1, 11)
(2, 12)
This is an optional topic!
for i in enumerate(range(10, 13)):
print(i)
else:
print("*")
print(i)
Output:
(0, 10)
(1, 11)
(2, 12)
*
(2, 12)
This is an optional topic!
class ClassIterExample:
counter: int
counter_max: int
def __init__(self):
self.counter = 8
self.counter_max = 12
def __iter__(self):
return self
def __next__(self):
if self.counter < self.counter_max:
self.counter += 1
else:
raise StopIteration
important_result: int = self.counter**2
return f"Element: {self.counter}", important_result
test_instance = ClassIterExample()
for i in enumerate(test_instance):
print(i)
Output:
(0, ('Element: 9', 81))
(1, ('Element: 10', 100))
(2, ('Element: 11', 121))
(3, ('Element: 12', 144))
You see a tuple with the enumerate id as a first component and a second tuple as result from the iterator.
break
break may only occur syntactically nested in a for or while loop, but not nested in a function or class definition within that loop.
It terminates the nearest enclosing loop, skipping the optional else clause if the loop has one.
If a for loop is terminated by break, the loop control target keeps its current value.
break_stmt ::= "break"
for i in range(0, 5):
if i == 2:
break
print(i)
Output:
0
1
continue
continue may only occur syntactically nested in a for or while loop, but not nested in a function or class definition within that loop. It continues with the next cycle of the nearest enclosing loop.
continue_stmt ::= "continue"
for i in range(0, 5):
if i == 2:
continue
print(i)
Output:
0
1
3
4
The source code is Open Source and can be found on GitHub.