Programming
and databases

Joern Ploennigs

Branching

Midjourney: Red or Blue Pill, ref. Leonardo da Vinci

Process

Conditional Branching

• We can use control statements that, based on a condition, branch to a particular statement.

• These conditions are statements whose output is a boolean value (True or False):

  • True - Boolean values
  • a - Variable values, if a has been assigned a boolean, i.e., the boolean equivalent of a
  • 2 < 5 - All comparison operators

If … Then …

• The basic form of such branching is:

  "If the condition is true, then execute the following statements."

• There is often also an "else" that executes the following statement instead.

• Python has three branching statements:

  • if - if
  • else - else
  • elif - short for else-if

If Statement

• The if statement is needed to start a conditional branch.

• Important: The conditional code block must be indented

if Condition:
    # then execute the following block only if the condition is true
    Statement1
    Statement2
    # ...

If the condition is True, the indented statements will be executed.

Example: Using an if statement to skip code

activateOutput = True
a = 10
b = 13
c = math.sqrt(a**2 + b**2) # Pythagoras

if activateOutput:
    print("Seitenlänge:" + str(c))
    umfang = a + b + c
    # ...

Alternatives

• else follows only after an if
• Is executed when the condition specified in the if evaluates to false
• Also requires indentation of the conditional code block

if Condition:
    # then execute the following block only if the condition is true
    Statement1
    Statement2
    # ...
else:
    Statement1b
    # ...

Example: Catching Division by 0 (Simple Branching)

Implicit Version

a = 5
b = 0
if b:  # the truth value of an int is False for 0 and True for != 0
    c = a / b
else:
    print("Division by Zero")
    c = None

Example: Catching division by zero (simple branching)

Explicit version

a = 5
b = 0
if b != 0: # Explicit truth value
    c = a / b
else:
    print("Division by Zero")
    c = None

Nested Conditionals

By indenting multiple levels, we can define nested conditionals:

if Condition1:
    StatementA
else:
    if Condition2:
        StatementB
    else:
        StatementC

Example: Checking the sign (Nested branching)

if a < 0:
    print("Number is negative")
else:
    if a > 0:
        print("Number is positive")
    else:
        print("Number is zero")

Multiple branching: ELIF statement

• Instead of an else statement, we can use the elif statement, which, like if, has a condition.

• An elif statement can be followed by:

  • Another elif statement
  • else statement - is executed when none of the previous statements were true
  • No statement – second variant of the optional code

Example: Testing the sign (multi-way branching)

if a < 0:
    print("Number is negative")
elif a > 0:
    print("Number is positive")
else:
    print("Number is zero")

Much cleaner and more readable than nested if-else statements!

Many Alternatives

In Python 3.10, the match statement was introduced, which already exists in many other languages. It allows you to compare a variable against several possible values.

Traditionally (many elifs):

if Variable == ValueA:
    StatementA
elif Variable == ValueB:
    StatementB
else:
    StatementC

New (match-case):

match Variable:
    case ValueA:
        StatementA
    case ValueB:
        StatementB
    case _:
        StatementC

Lesson Learned

Midjourney: Six Sense

What learning styles are there?

Lesson Learned - Notes

• You should always take notes (because we remember written material better, both by touch and visually).
• You should not copy the slides, but rather capture key information, terms, and questions.
• You should take the time to review your notes, clarify questions, and structure the content yourself.

Follow-Up

• Survey (Gain an overview): Skim through a topic to gain an overview
• Question (Questions): Note down questions about points you don’t understand
• Read (Read): Read through your notes; highlight keywords; research the questions
• Recite (Recall): Recapitulate each section (content, keywords, relationships). Organize the content, e.g., with mind maps. Repeat what you’ve learned multiple times (flashcards)
• Review (Recap): Mentally review the key points and answer your questions

Questions?

programmierung
und datenbanken