Wiederholung: Hörsaalfrage¶
What are if, elif, and else used for?
Recap: IF, ELIF, ELSE¶
if, elif, else are commands for branching programs in Python
if Condition1:
print("Will run when Condition1 is true")
elif Condition2:
print("Will run when Condition1 is false and Condition2 is true")
else:
print("Will run when Condition1 is false and Condition2 is false")
Recap: In-Class Question¶
What loop types exist?
Recap: Loops¶
Loops are an important concept in programming for implementing repetitive or incremental solutions.
In Python, the following are available:
for(equivalent to a for-each loop)while(equivalent to a while loop)
Key properties:
- They work exactly like all other control statements—by indentation after a header line
- Unlike in functions, variables from loops are also available outside the loop!
- The other loop variants aren’t explicitly present either, but can be emulated functionally
Recap: While loops¶
While loop
In a While loop, the condition can be false right at the start. So the loop does not have to be executed.
Condition = True/False
while Condition:
# Statement
Condition = 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.
Condition = True
while Condition:
# Statement
Condition = False
Repeat-Until loop
In a Repeat-Until loop, the condition is also checked only at the end. The loop repeats until the condition becomes true.
Condition = False
while not Condition:
# Statement
Condition = True
Process¶
Functions – Mathematical¶
Functions in programming languages implement the mathematical concept of a function.
They represent a mapping from an input set to an output set.
Mapping: A function $f$ assigns to every element $x$ of a domain $D$ an element $y$ of a codomain $Z$.
$$ f : D \to Z, \quad x \mapsto y $$
Functions – Programming¶
Reusable code that performs a specific task.
- Take a tuple of input values (arguments)
- Execute a fixed sequence of expressions and assignments
- Return a tuple of output values (return values)
- Are only executed when they are called in an expression
Functions in Python¶
# Code block of the function definition
def functionname(arg1, arg2): # <- the function name is selectable, arg1 and arg2 are arguments (parameters)
statement1 # <- The code block starts here and is indented
statement2 # <- Still part of the indented code block
- Function definitions begin with
def - It is followed by a name, arguments and a
: - Arguments are in scope for the entire function body
- When called, the arguments are bound to input data
Functions with a Return Value¶
def functionName(arg1):
statement1
return outputValue
returnends execution and returns the output value(s)Returningmeans: The return value replaces the function call in the original expression- If no
returnis specified, the function returnsNone
Example – Doubling a Value¶
Multiplication
x = arg1 * 2 # 4
x = arg1 * 4 # 16
x = arg1 * 8 # 128
Bit shifting (faster)
def times2(arg1):
return arg1 << 1
x = arg1 << 1 # $2^1$
x = arg1 << 2 # $2^2$
x = arg1 << 3 # $2^3$
Example – Euclidean distance¶
from math import sqrt
def distance(a1, a2, b1, b2):
return sqrt((a1 - b1)**2 + (a2 - b2)**2)
a1, a2 = 2, 3
b1, b2 = 6, 6
x = distance(a1, a2, b1, b2) # 5.0
Default values for parameters¶
def funktionsname(arg1, arg2="default"):
return statement1
- Arguments can have default values
- They are optional when calling; provided values override the default
Example – Appending a unit to a number¶
def measurement(number, unit="meters"):
return str(number) + " " + unit
x = measurement(12) # "12 meters"
x = measurement(5.5, "kg") # "5.5 kg"
Passing Argument Values (Concepts)¶
Pass-by-value
- Only values are copied
- The passed-in variables are not accessible inside the function
- Safer, since there is no unexpected rebinding
Pass-by-reference
- Reference to the same data
- Values inside the function can be modified
- Saves time and memory
Passing Argument Values in Python¶
- Hybrid variant, often called "pass-by-assignment"
- Mutable data types: behave like pass-by-reference
- Immutable data types: behave like pass-by-value
- If a mutable variable is rebound, it has no effect outside its scope
Built-in Functions¶
Python has an extensive list of built-in functions.
| Function | Purpose |
|---|---|
print(), input() |
Output, input |
id(), type() |
Variable IDs, data types |
int(), str(), float() |
Data type conversion |
list(), tuple(), set(), dict() |
Complex data types |
len() |
Length of a complex data type |
abs(), max(), min() |
Mathematical basic functions |
exit() |
Exit the program |
sorted() |
Sorting |
| … |
What are functions used for?¶
- Readability, modularity, and reusability
"Write once, run anywhere" - Modern style: break programs down into as many core functions as possible
