2.1 Control Flow

Control Flow refers to the order in which individual statements, instructions, functions, … are executed/evaluated in a program. Control Flow is essential for managing the program’s behavior and making decisions. Computers, and therefore also programs, are completely deterministic. This means you can always determine what will happen next when going through code.

Summary

Control Flow determines the execution order of code in a program.

• Line by Line: Code executes sequentially, top to bottom.
• Selection: Conditional statements likeif-else allow decisions based on conditions.
• Iteration: Loops like while or for repeat code while a condition is true or for a number of iterations.
• Jumps: break exits loops, continue skips to the next iteration
• Subroutines: Functions encapsulate reusable code, called with arguments and return values.
• Errors & Exceptions: Interrupt the normal execution; Exceptions can be recovered using try-except.

Line By Line

By default, Python executes code line by line. Each line of code is executed in the order in which it appears, from top to bottom.

print(1)
print(2)
print(1+2)

1
2
3

Selection (Conditional Statements)

Selection allows the program to make decisions. The program chooses what code to execute next based on a condition. The basis for the decisions is if a boolean is True, or whether a pattern matches.

x = 6
if(x > 5):
	print("x is greater than 5")          # executed if condition is true
else:
	print("x is less than or equal to 5") # executed if condition is false
print("End of the example")

x is greater than 5
End of the example

Repetition/Iteration

This allows you to repeat a block of code multiple times. The block is repeatedly executed while a boolean condition is true or for a fixed number of iterations.

x = 0
while x < 3:
    print("x is", x)
    x += 1
 
for i in range(5):
	print("Iteration number", i)

x is 0
x is 1
x is 2
Iteration number 0
Iteration number 1
Iteration number 2
Iteration number 3
Iteration number 4

Jumps

Jumps allow the program to exit the normal flow of execution and skip part of the code or go to a completely different position. Python allows you to exit a loop using break or go to the next iteration of a loop using continue.

for i in range(99):
	if i > 20:
		break
	if i%2 == 1:
		continue
	print(i)

0
2
4
6
8
10
12
14

Subroutines

A subroutine is a reusable block of code that performs a specific task. The most common type of subroutine is a function. When a function is called, the program jumps to the definition of the function, executes the code inside, and then returns the position where it was called. Additionally, you can give values to a function using arguments and receive values using return.

def greet():
    print("Hello!")
 
greet()  # Function call
print("Done")

Errors

Errors and Exceptions are ways in which Control Flow is interrupted when something goes wrong. A well written program should not contain errors on the programmers side. It should also prevent or properly handle exceptions caused by the user.

(Syntax-)Errors are usually unrecoverable issues disrupting the program’s flow, leading to failure. These are caused by the programmer not writing code in the correct Python syntax.

If something has correct syntax, but causes an error when executing it, it is called an Exception. These are recoverable issues that allow for controlled interruption. So the program can recover or provide feedback to the user. These can be caused both by the programmer (e.g. incorrect variable/function/… name; incorrect datatype; incorrect import; …) and the user (incorrect input of any kind; file not found; internet connection error; …). In Python you can prevent exceptions or use try-except blocks to handle them to not let your program crash.

# Error because of incorrect function name and string never closed
Print("incorrect string)
while True # Error because no colon ":"
print(1)   # Error because block inside while not indented

Questions

• What is the concept of Control Flow in programming?
• How does Python execute code by default? Explain with an example.
• Explain selection in your own words.
• Explain repetition/iteration in your own words.
• Explain break and continue in your own words.
• Explain subroutines in your own words.
• Explain errors and exceptions in your own words.
• What are the differences between a Syntax Error and an Exception? Explain with examples.