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# Flow Control for-loop
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## The goal
For what reason...
Questions to [David Rotermund ](mailto:davrot@uni-bremen.de )
**Logic blocks need to be indented.
## [The for statement](https://docs.python.org/3/reference/compound_stmts.html#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()
```python
for i in range(0, 3):
print(i)
```
Output:
```python
0
1
2
```
### [range()](https://docs.python.org/3/library/stdtypes.html#ranges)
```python
class range(stop)
class range(start, stop[, step])
```
### With a list
```python
for i in [0, "A", 7, "nom num"]:
print(i)
```
Output:
```python
0
A
7
nom num
```
## The full statement
```python
for_stmt ::= "for" target_list "in" starred_list ":" suite
["else" ":" suite]
```
## for loop (the truth)
{: .topic-optional}
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 ](https://docs.python.org/3/reference/simple_stmts.html#the-yield-statement ).
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### Class
```python
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:
```python
('Element: 9', 81)
('Element: 10', 100)
('Element: 11', 121)
('Element: 12', 144)
```
### Generator yield
```python
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:
```python
9
16
25
36
49
```
## for loop -- [enumerate](https://docs.python.org/3/library/functions.html#enumerate)
```python
enumerate(iterable, start=0)
```
If you need the index of the element then you can use enumerate:
```python
for i in enumerate(range(10, 13)):
print(i)
```
Output:
```python
(0, 10)
(1, 11)
(2, 12)
```
{: .topic-optional}
This is an optional topic!
```python
for i in enumerate(range(10, 13)):
print(i)
else:
print("*")
print(i)
```
Output:
```python
(0, 10)
(1, 11)
(2, 12)
*
(2, 12)
```
{: .topic-optional}
This is an optional topic!
```python
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:
```python
(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](https://docs.python.org/3/reference/simple_stmts.html#the-break-statement)
> 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.
```python
break_stmt ::= "break"
```
```python
for i in range(0, 5):
if i == 2:
break
print(i)
```
Output:
```python
0
1
```
## [continue](https://docs.python.org/3/reference/simple_stmts.html#the-continue-statement)
> 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.
```python
continue_stmt ::= "continue"
```
```python
for i in range(0, 5):
if i == 2:
continue
print(i)
```
Output:
```python
0
1
3
4
```
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