Computer Programming I SS2 Digital Technologies Lesson Note

What is a Program?

Computers are actually quite “stupid.” They cannot think for themselves. If you tell a computer to “make a sandwich,” it won’t know where to start.

A Program is simply a set of very clear, step-by-step instructions that tell a computer exactly what to do. Before we write those instructions in a language like Python or Java, we must first plan them out using Algorithms and Flowcharts.

Algorithms: The “Recipe” for Success

An Algorithm is a step-by-step list of instructions written in plain English to solve a problem.

Think of it as a recipe: If you miss a step in a cake recipe (like forgetting the eggs), the cake fails. If you miss a step in an algorithm, the computer crashes.

Example: Algorithm to Cross a Busy Road

1. Walk to the edge of the sidewalk.
2. Look to the left.
3. Look to the right.
4. If a car is coming, wait and go back to Step 2.
5. If no car is coming, walk across the road.

Characteristics of a Good Algorithm:

• Precision: Every step is clear.
• Uniqueness: There is no confusion about what to do next.
• Finiteness: It must eventually end (it shouldn’t go on forever).

Flowcharts: The “Map” of Logic

Sometimes, a list of words is hard to follow. A Flowchart is a diagram that uses different shapes to show the “flow” of an algorithm. It’s a visual way to see how a program makes decisions.

The Standard Symbols

In the world of computers, we can’t just use any shape. Each shape has a specific meaning:

Drawing a Simple Flowchart

Let’s take our “Crossing the Road” algorithm and turn it into a flowchart.

• We start with an Oval (Start).
• We use a Rectangle to “Look for cars.”
• We use a Diamond to ask: “Is a car coming?”
  1. If YES, the arrow loops back up to “Wait.”
  2. If NO, the arrow goes down to “Cross Road.”
• We end with an Oval (Finish).

Logic Gates: The “Switches” of Thinking

At the very basic level, computers think using “ON” and “OFF” (1 and 0). We use Logic Gates to combine these signals.

• AND Gate: Both signals must be “ON” for the result to be “ON.” (e.g., You can only enter the lab if you have a key AND the teacher is present).
• OR Gate: Only one signal needs to be “ON.” (e.g., You can pass the exam if you score high in the Test OR high in the Exam).
• NOT Gate: It flips the signal. If you put in “ON,” you get “OFF.”

Summary for Students

Class Practical Activity

1. Group Work: On a piece of paper, write an Algorithm for “Withdrawing money from an ATM.” (Don’t forget the PIN check!).
2. Individual Task: Draw a Flowchart that asks a user for their age. If they are 18 or older, display “You can Vote.” If they are younger, display “Too Young.”

Challenge: Try to combine two “Decision” diamonds in one flowchart!