Basic Programming Constructs

Basic Programming Constructs

Understanding the three fundamental constructs of programming is essential for controlling the flow of a program. These constructs are sequence, selection, and iteration.

Sequence

• Definition: The execution of statements one after another in the order they are written.

• Example:

  print("Hello")
  print("World")
  

  This will output:

  Hello
  World
  

Selection

• Definition: Making decisions in a program based on conditions. This is usually implemented using if, else if, and else statements.

• Example:

  age = 18
  if age >= 18:
      print("You are an adult.")
  else:
      print("You are a minor.")
  

  This will output:

  You are an adult.
  

Iteration

• Definition: Repeating a block of code multiple times. There are two main types of iteration:

  • Count-controlled loops: Repeat a specific number of times, typically using for loops.
  • Condition-controlled loops: Repeat until a condition is met, typically using while loops.

• Count-controlled Example:

  for i in range(5):
      print(i)
  

  This will output:

  0
  1
  2
  3
  4
  

• Condition-controlled Example:

  i = 0
  while i < 5:
      print(i)
      i += 1
  

  This will output:

  0
  1
  2
  3
  4
  

Understanding these constructs allows for the development of structured and efficient programs that can handle complex tasks by controlling the flow of execution.

Inifinite loops

A condition controlled loop needs to have its condition set before entry and then reset or it will run forever.

This example here will never see i reach 5 and so will loop forever

  i = 0
  while i < 5:
      print(i)

Loop not entered

This example never see's entry into the loop as i has not been set beforehand

  while i < 5:
      print(i)
      i=i+1

Loops for Validation

Validation is when you check if a data entered is sensible. A loop can be used to do a validation check. If the data isn't valid then the program will loop continuously until a valid piece of data is entered.

password = input("enter your password: ")

while len(password) < 8:
  print("Error: Your password must be at least 8 characters.")
  password = input("Please enter your password: ")

print("Password is the correct length")

This program keeps asking for user input until the password is greater than 8.