Basic Features

Algorithms¶

Algorithms are the commands that make up our programs.

There are 6 basic building blocks that an algorithm consists of. Each of them serve a particular purpose.

• Sequence: a number of instructions that are processed one after the other.

• Assignment: used to store the value of an expression into a variable.

• Condition: the way that a computer asks a question which can only generate two possible responses: True (yes) or False (no).

• Selection: a statement that uses a condition to select, or determine, whether the next line of the program is to be executed.

• Iteration: the repetition of a number of instructions. These are generally referred to as loops and refer to a section of code that is repeated.

• Modularisation: refers to the process of breaking a large, unwieldy programming task into separate, smaller, more manageable subtasks or modules. Individual modules can then be cobbled together like building blocks to create a larger application.

In designing your algorithm, you will need to decide how to arrange these building block to achieve you outcome. To do this it is wise to follow good programming practices.

Variables¶

Variables are the names you give to computer memory locations which are used to store values in a computer program. Variables can store data of different types, and different variable types can do different things.

Variable Types¶

Variable can be of different types. The type of a variable indicates what kind of data is stored at that memory location.

The Python data types that we will be using are:

• Strings:

  • strings are sequences of character data

  • the string type in Python is called str

  • string literals may be delimited using either single or double quotes

  • Strings refresher

• Integers:

  • integers are whole numbers

  • the integer type in Python is called int

  • there is effectively no limit to how long an integer value can be, but it is constrained by the amount of memory your system has.

  • Integer refresher

• Floating-Point Numbers:

  • float values are numbers specified with a decimal point

  • the float type in Python designates a floating-point number

  • optionally, the character e or E followed by a positive or negative integer may be appended to specify scientific notation

  • Float refresher

• Boolean Type:

  • objects of Boolean type may have one of two values, True or False

  • the Boolean type in Python is called bool

  • a value that is true in Boolean context is sometimes said to be “truthy” and one that is false in Boolean context is said to be “falsie”

  • the “truthiness” of an object of Boolean type is self-evident: Boolean objects that are equal to True are truthy (true), and those equal to False are falsy (false)

  • Boolean refresher

Truthiness and Falseness of Variables¶

In Python, variables have an associated truthiness (evaluated as True) or falseness (evaluated as False) when used in a Boolean context like an if statement.

Falsy values in Python

For example, the following code:

if []:
    print("Truthy")
else:
    print("Falsy")

outputs:

Falsy

Scope: Local and Global Variables¶

When writing code, scope means where in your program a variable can be used.

• Global variables are made outside of functions and can be used anywhere in your code.

• Local variables are made inside functions and can only be used inside those functions.

Older languages like BASIC didn’t use local variables, so every part of the program could change any variable at any time. This made debugging hard.

Modern languages like Python use scope to avoid problems like this. In Python, you can’t use a variable everywhere unless it’s global. Where you create the variable decides where you’re allowed to use it. This helps keep your code more organised and easier to manage.

Python Scope and the LEGB Rule

Python follows a rule called LEGB to figure out which variable to use when you refer to a name. LEGB stands for:

• Local – Variables made inside a function. Only that function can use them. Each time the function runs, a new local scope is created.

• Enclosing – Variables made in outer functions, when you have functions inside functions. The inner function can use these variables, but can’t change them unless declared nonlocal.

• Global – Variables made at the top of your program. They can be used anywhere, but only changed inside a function if declared global.

• Built-in – Variables and functions that come with Python, like print() or len(). These are always available.

Python checks for a name in this order: Local → Enclosing → Global → Built-in. If it finds the name, it stops there. If not, it gives an error.

What You Can Do in Different Scopes

Control Structures¶

Flow of control through any given function is implemented with three basic types of control structures:

Sequential¶

Sequential code runs line by line from top to bottom. The program starts with the first line, then moves to the next, and keeps going in order until it finishes.

Conditions¶

A condition is how a computer checks if something is true or false. It’s like asking a yes/no question. The answer will always be either True or False.

In Python, conditions are used in if and while statements. They follow this pattern: value operator value

There are six main comparison operators in Python:

Selection¶

Selection is when a program makes a choice between different paths based on a condition. Only one path is taken — the others are skipped. After the selected path runs, the program continues with the next line after the selection block. This is also called branching, like how a tree or river splits into different directions.

Python has two main types of branching:

statements

This type checks conditions in order and chooses one path to run.

W3Schools: if...elif...else refresher

statement

• Checks a variable against different fixed values

• Similar to switch in other languages

• Added in Python 3.10

• LearnPython: match case statement

Iteration¶

Iteration means repeating a block of code. This is also called a loop. There are two main types:

• Indefinite iteration – used when you don’t know how many times the loop will run

  • Pre-test loop (while loop): checks the condition before running the code

  • Post-test loop (like do...while in other languages): runs the code then checks the condition — Python does not have this type

• Definite iteration – used when you know how many times to run

  • Counted loop (for loop): repeats for a fixed number of times or through each item in a sequence

Python has two loop types

Data Structures¶

Data structures are used to store data in an organized form. No matter what problem are you solving, in one way or another you have to deal with data — whether it’s an employee’s salary, stock prices, a grocery list, or even a simple telephone directory. ulhaq_2018_the

The easiest way to understand data structures is to see them as collections of related data — like a list of shopping items or a group of contacts with phone numbers. In fact they are frequently referred to as collections.

In Python, we commonly use these four data structures:

Syntax¶

Syntax is the set of rules a programming language follows to write commands properly. It decides which combinations of words, symbols, and punctuation are valid in that language.

Python syntax refresher

Libraries¶

A library is a collection of code — like functions, modules, or scripts — that you can use in your own programs. Python includes a standard library with many useful tools. You can also install extra libraries using pip, or even create your own.

Classes¶

In object-oriented programming, a class is a blueprint for creating objects. It defines the attributes (data) and methods (actions) that all objects of that type share.

For example, a Car class might include:

• attributes like colour and wheels

• methods like drive() and brake()

You can then create many car objects from that one class, like ferrari = Car().

A Truck class might share some features with Car but have differences. Both could inherit from a more general Vehicle class, which contains shared code. This avoids repeating code while still allowing for differences.

Inside a class:

• attributes store data, like speed or size

• methods perform actions, like drive() or turn_left()

To use a method, you create an object and call it like this: ferrari.drive(distance)

We covered classes and object-oriented programming in the Deepest Dungeon course.

Operations¶

Operators are symbols used to perform actions on variables or values in Python. These actions can include math, comparisons, logic checks, and more.

Python includes several types of operators:

• Arithmetic operators

• Assignment operators

• Comparison operators

• Logical operators

• Identity operators

• Membership operators

• Bitwise operators

Python operations refresher

Arithmetic Operators¶

Used to do basic maths with numbers.

Note: brackets () work just like in maths to control order.

Assignment Operators¶

Used to assign or update the value of a variable.

Comparison Operators¶

Used to compare values and return True or False.

Logical Operators¶

Used to combine or modify condition results.

Identity Operators¶

Check whether two variables are actually the same object in memory.

Membership Operators¶

Check if a value exists in a sequence like a list or string.

Bitwise Operators¶

Used with binary numbers. Rarely needed in basic Python, but good to know they exist.