Python Interactive Guide - Step 2 Control Flow (1) - Conditional Statements (1): if Statements

In the previous lessons under “Step 1 Basic Syntax and Data Types,” we learned about data types and their operations.
From this lesson onwards, we’ll explore “Step 2 Control Flow” - methods to control the flow of program execution.

Step 2 Control Flow

Programs typically execute from top to bottom in sequence.
However, in real applications, there are processes we want to execute only when specific conditions are met, or processes we want to repeat.
The mechanisms that control such “program flow” are called control flow.

2.1. Conditional Statements

Conditional statements are control flow structures that execute specific processes only when certain conditions are met.

2.1.1. if Statement

The if statement executes the code block only when the specified condition is True.

• The indented block (above: print("Adult")) is executed only when the condition expression (above: age >= 18) evaluates to True
• If the condition expression evaluates to False, that block is skipped

flowchart TD
    START[Start] --> IF{"if age >= 18"}

    %% Arrows from IF
    IF -->|False| NEXT[Next process]
    IF -->|True| IF_BLK["if block process\n(print('Adult'))"]

    IF_BLK --> NEXT
    
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2.1.2. if-else Statement

Using the else clause, you can specify processes to execute when the condition is not met.

• When the condition expression is True, the if block is executed; when False, the else block is executed
• Either if or else is always executed

flowchart TD
    START[Start] --> IF{"if age >= 18"}

    %% Arrows from IF
    IF -->|False| ELSE{"else"}
    IF -->|True| IF_BLK["if block process\n(print('Adult'))"]

    %% Arrows from ELSE
    ELSE --> ELSE_BLK["else block process\n(print('Minor'))"]


    ELSE_BLK --> NEXT[Next process]
    IF_BLK --> NEXT
    
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2.1.3. if-elif-else Statement

When you want to check multiple conditions sequentially, use elif (short for else if).

• Conditions are evaluated from top to bottom, and only the block of the first True condition is executed
• Even if multiple conditions become True, only the first True condition is adopted
• The else block is executed only when all conditions are False
• The else clause is optional (if omitted, nothing is executed when all conditions are False)

flowchart TD
    START["Start"] --> IF{"if [condition]"}

    %% Arrows from IF
    IF -->|False| ELIF{"elif [condition]"}
    IF -->|True| IF_BLK["if block process"]
  
    %% Arrows from ELIF
    ELIF -->|False| ELIF2{"elif [condition2]"}
    ELIF -->|True| ELIF_BLK["elif block process"]

    %% Arrows from ELIF2
    ELIF2 -->|False| ELSE{else}
    ELIF2 -->|True| ELIF2_BLK["elif block(2) process"]

    %% Arrows from ELSE
    ELSE --> ELSE_BLK["else block process"]

    %% Arrows to Next process
    ELSE_BLK --> NEXT["Next process"]
    ELIF2_BLK --> NEXT
    ELIF_BLK --> NEXT
    IF_BLK --> NEXT

    %% Arrow color specification
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    linkStyle 2 stroke:green,stroke-width:2px
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    linkStyle 6 stroke:green,stroke-width:2px
    linkStyle 7 stroke:green,stroke-width:2px

2.1.4. Nested Conditional Statements

You can nest conditional statements inside other conditional statements.

• Inner conditional statements are evaluated only when the outer condition is True
• It’s important that indentation at each level is set correctly

flowchart TD
    START[Start] --> IF{"if age >= 18"}
    
    %% Outer if statement
    IF -->|False| ELSE{"else"}
    IF -->|True| IF_BLK["if block process\n(print('Adult'))"]

    ELSE --> ELSE_OUTER["else block process\n(print('Minors cannot enter'))"]
    
    %% Inner if statement
    IF_BLK --> INNER_IF{"if has_id"}
    INNER_IF -->|False| ELSE_INNER{"else"}
    INNER_IF -->|True| INNER_TRUE["inner if block process\n(print('ID verified. Entry allowed'))"]

    ELSE_INNER --> INNER_FALSE["inner else block process\n(print('No ID. Entry denied'))"]
    
    %% To next process
    ELSE_OUTER --> NEXT[Next process]
    INNER_FALSE --> NEXT
    INNER_TRUE --> NEXT
    
    %% Arrow color specification
    linkStyle 1 stroke:red,stroke-width:2px
    linkStyle 2 stroke:green,stroke-width:2px
    linkStyle 3 stroke:green,stroke-width:2px
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