Lecture 8 for Programming 1 Course

The lecture begins by introducing the concept of conditional statements, specifically 'if' statements. Unlike previous codes that executed sequentially, conditional statements allow for selective execution of code blocks based on whether a condition is true or false. This is crucial for creating programs that can make decisions and perform different actions based on various inputs or states.

The 'if' statement, derived from the English word 'if', is explained as a reserved keyword used to introduce a condition. If the condition evaluates to true, a specific block of code (statement) is executed; otherwise, it is skipped. The syntax involves 'if (condition):' followed by the indented statements to be executed. An example demonstrates checking if a temperature is above freezing.

A flowchart illustrates the logical flow of an 'if' statement: it checks a condition, and if true, executes a command block before continuing; if false, it skips the command block. It's clarified that an 'if' statement can execute multiple lines of code, forming a block that is entirely executed or entirely ignored based on the condition. The lecture also hints at nested 'if' statements for more complex conditions.

Key comparison operators are introduced: '==' (equal to), '!=' (not equal to), '<' (less than), '>' (greater than), '<=' (less than or equal to), and '>=' (greater than or equal to). The critical distinction between '=' (assignment) and '==' (comparison) is highlighted. Examples are provided to demonstrate how these operators evaluate to 'true' or 'false' in conditional expressions.

A practical example of using 'if-else' is presented: determining if a number is even or odd. The condition involves checking the remainder of a number divided by 2 using the '%' (modulo) operator. If the remainder is 0, the number is even; otherwise, it's odd. This demonstrates how 'else' provides an alternative action when the 'if' condition is false.

The lecture culminates in building a simple calculator using 'if-elif-else' statements. This extended conditional structure allows for multiple conditions to be checked sequentially. The calculator takes two numbers and an operator (+, -, *, /) as input. It then uses 'if-elif-else' to identify the operator and perform the corresponding arithmetic operation, demonstrating a comprehensive application of conditional logic.

The importance of indentation in Python is stressed. Indentation defines code blocks under conditional statements. Any line indented after an 'if' or 'elif' condition is considered part of that block. Returning to a lesser indentation level signifies the end of the block, allowing for new conditions or independent code to follow. This structural aspect is vital for correct program execution.