Days 1#

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How to use this textbook: Read at a desk with Python open beside you. Every code example in this book should be typed by you, run by you, and then modified by you. Reading without typing builds nothing. Understanding happens in your fingers as much as in your mind.

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Chapter 1: What Is Computation?#

1.1 What Is a Computer?#

Most people think of a computer as the physical device sitting on a desk or in a pocket — the screen, the keyboard, the chip. But when a computer scientist says "computer," they mean something more precise: a machine that can read a sequence of instructions and follow them.

The key insight is that this sequence of instructions (the program) is itself data. The program lives in memory, just like the numbers and text the program works on. This means a computer is, in a sense, a universal machine: by changing the program, you change what the machine does. The same silicon chip can run a word processor, a game, a weather simulation, or a video call.

A modern computer has two essential components we care about right now:

• Memory (RAM): A huge grid of locations, each able to hold a small amount of information. Think of it as a very long list of numbered cubby holes. Every value your program uses, be it a number, a word, a result, lives in one of these locations.

• Processor (CPU): A chip that reads instructions from memory and executes them. The processor can do arithmetic, compare values, read from memory, write to memory, and decide which instruction to execute next. It does this billions of times per second.

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1.2 What Is a Program?#

A program is a sequence of precisely written instructions that tells a computer exactly what to do, step by step.

"Exactly" is the key word here. Computers do not guess at meaning. They do not ask "did you mean...?" They execute instructions with perfect literalness. If you tell a computer to add 5 and 3, it adds 5 and 3. If you accidentally tell it to add 5 and 4, it adds 5 and 4, without complaint, without correction.

This precision is both the power and the difficulty of programming.

• The power: a program runs exactly the same way every time, instantly, reliably.
• The difficulty: you must be precise. Every detail matters. A single misplaced character can crash a program or, worse, make it silently produce the wrong answer.

Syntax and Semantics#

Every programming language has two aspects:

• Syntax: The grammatical rules of the language. In English, "Dog the bit man" is syntactically wrong — the word order violates English grammar rules. In Python, 5 + * 3 is syntactically wrong, you can't put two operators next to each other without a value in between. Python will immediately tell you about syntax errors.

• Semantics: The meaning of a program. Syntax tells you whether a sentence is grammatically legal. Semantics tells you what it means. "Colorless green ideas sleep furiously" is syntactically fine in English but semantically nonsensical. In Python, x = "5" + 3 is syntactically valid Python but semantically broken at runtime (you can't add a string to a number this way).

Most beginners struggle most with semantic errors — programs that run without crashing but produce the wrong answer. These are also called bugs, and finding and fixing them is called debugging.

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1.3 What Is Python?#

Python is a programming language — a formal notation for writing programs. Python programs are plain text files. You could, in principle, write a Python program in a text editor and then hand the file to the Python interpreter, which reads it and executes it.

The Python interpreter is itself a program that reads Python code and carries out its instructions. When you type:

print("Hello, world!")

...you are writing a Python instruction. The interpreter reads that instruction and knows it means: "call the built-in print function with the string "Hello, world!" as its argument, and display that string on screen."

Interpreted vs. Compiled Languages#

Python is an interpreted language. This means the interpreter reads your code line by line and executes it as it goes. There is no separate compilation step.

Languages like C and Java are compiled — you must first translate the entire program into machine code, then run the result. Compilation catches more errors up front, and compiled programs run faster. But interpreted languages like Python are more flexible and much faster to experiment with — you can try something in the Python shell and see the result immediately.

For learning, the interactive nature of Python is ideal. You get instant feedback.

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1.4 The Python REPL#

REPL stands for Read-Evaluate-Print Loop. It's the interactive Python shell: you type an expression, Python evaluates it, prints the result, and waits for more.

Open a terminal (or Anaconda Prompt on Windows) and type python. You'll see:

Python 3.10.x ...
>>> 

The >>> is the prompt — Python is waiting for you.

Try these:

>>> 2 + 3
5
>>> 10 - 4
6
>>> 3 * 7
21
>>> 10 / 3
3.3333333333333335
>>> 10 // 3
3
>>> 10 % 3
1
>>> 2 ** 8
256

Each line: Python reads your expression, evaluates it to a value, and prints the value. The loop continues until you type exit() or press Ctrl+D.