Process¶
Loops¶
Loops are an important approach in programming to implement repetitive or incremental solutions.
In Python, the following are available:
for(equivalent to a for-each loop)while(equivalent to a while loop)
Important properties:
- They work exactly like all other control statements, using indentation for the block that follows the header line
- Unlike in functions, loop variables are also accessible outside the loop!
- The other loop variants are not explicitly present, but can be functionally emulated
For Loops - FOR Loop¶
The for loop repeats a block of statements for all elements in a sequence. Element is a variable that is always bound to the current element from the sequence.
for Element in Sequenz:
# Statement
Loops - For-each Loop Example¶
The for loop in Python is a for-each loop. It always iterates through a sequence of values, such as a list. The iterator variable e contains the current value from the list seq.
seq = ['a','b','c']
for e in seq:
print(e)
Output:
a
b
c
Loops - FOR-Loop Example¶
The for loop in Python can also be used as a traditional for loop, in which you repeat something n times. To do this, you generate a sequence of numbers with the range(n) function, through which the for-each loop then iterates.
n = 3
seq = range(n)
for e in seq:
print(e)
Output:
0
1
2
Loops - FOR Loop Example¶
This can be used, for example, to convert a list of measurements from feet in the imperial system to metres.
measurements_feet = [4.2, 2.3, 6.2, 10.5] # Input
measurements_meter = [] # Results
for feets in measurements_feet:
measurements_meter.append(0.3048 * feets)
print(measurements_meter)
Output:
[1.2801600000000002, 0.70104, 1.88976, 3.2004]
Loops - WHILE Loop¶
The while loop runs as long as a condition is true. Since this is evaluated at the start and can be false from the outset, the body may not be executed.
while condition:
# statement
As with an if statement, the condition is evaluated for its truth value:
- If this is
True, the loop runs once, after which it is checked again - If this is
False, the loop ends and the code is not (re)executed
Loops - While Loop vs. Do-While Loop vs. Repeat-Until¶
While loop
In a while loop, the condition can be false right from the start. So the loop does not have to be executed.
Bedingung = True/False
while Bedingung:
# Statement
Bedingung = False
Do-While loop
In a do-while loop, the condition is checked only at the end. The loop is therefore always executed at least once.
Bedingung = True
while Bedingung:
# Statement
Bedingung = False
Repeat-Until loop
In a Repeat-Until loop, the condition is also checked only at the end. The loop is repeated until the condition becomes true.
Bedingung = False
while not Bedingung:
# Statement
Bedingung = True
Program Flow Control¶
We now know two tools:
Conditional execution:
if,else,elif
Looping:
while,for
Every program that has ever run on a computer could be written with these tools.
