Nested selection and iteration

Selection: IF, ELSE IF, and ELSE

Before looking at nested structures, it is important to understand the full form of selection. An IF statement runs a block of code only if a condition is true. Adding ELSE IF (written elif in Python) chains additional conditions that are only checked if the ones above were false. ELSE provides a final default branch that runs when no condition above was true.

IF score >= 70 THEN
    OUTPUT "Distinction"
ELSE IF score >= 50 THEN
    OUTPUT "Pass"
ELSE
    OUTPUT "Fail"
ENDIF

Each branch is mutually exclusive - only one branch executes per run. ELSE IF saves you from writing separate, overlapping IF statements, and ELSE ensures no case is missed. This single-level selection is the foundation; nesting places one complete IF structure inside another.

Nesting

Nesting means placing one structure entirely inside another of the same or different type. The inner structure only executes when the outer condition allows it. Each level of nesting is indented further - indentation is essential for reading nested code correctly and avoiding misplaced closing keywords such as ENDIF or ENDFOR.

• Nested selection (IF inside IF)
• Nested iteration (loop inside loop)

IF GameWon THEN
    IF Score > HighScore THEN
        OUTPUT "New high score!"
    ENDIF
    OUTPUT "Well done!"
ENDIF

# Python — nested selection
game_won = True
score = 950
high_score = 800

if game_won:                           # outer IF — checked first
    if score > high_score:             # inner IF — only reached if game_won is True
        print("New high score!")
    print("Well done!")                # runs whenever game_won is True

' VB.NET — nested selection
Dim gameWon As Boolean = True
Dim score As Integer = 950
Dim highScore As Integer = 800

If gameWon Then                        ' outer IF — checked first
    If score > highScore Then          ' inner IF — only reached if gameWon is True
        Console.WriteLine("New high score!")
    End If
    Console.WriteLine("Well done!")   ' runs whenever gameWon is True
End If

// C# — nested selection
bool gameWon = true;
int score = 950;
int highScore = 800;

if (gameWon)                           // outer IF — checked first
{
    if (score > highScore)             // inner IF — only reached if gameWon is true
    {
        Console.WriteLine("New high score!");
    }
    Console.WriteLine("Well done!");  // runs whenever gameWon is true
}

lives ← 3
WHILE lives > 0
    FOR i ← 1 TO 3
        OUTPUT "Round " + i
    ENDFOR
    lives ← lives - 1
ENDWHILE

# Python — nested iteration (WHILE containing FOR)
lives = 3

while lives > 0:                       # outer WHILE loop — condition-controlled
    for i in range(1, 4):             # inner FOR loop — runs completely each outer iteration
        print("Round", i)
    lives = lives - 1

# Total lines printed: 3 outer iterations x 3 inner = 9 lines

' VB.NET — nested iteration (WHILE containing FOR)
Dim lives As Integer = 3

Do While lives > 0                     ' outer WHILE loop — condition-controlled
    For i As Integer = 1 To 3         ' inner FOR loop — runs completely each outer iteration
        Console.WriteLine("Round " & i)
    Next i
    lives = lives - 1
Loop

' Total lines printed: 3 outer iterations x 3 inner = 9 lines

// C# — nested iteration (WHILE containing FOR)
int lives = 3;

while (lives > 0)                      // outer WHILE loop — condition-controlled
{
    for (int i = 1; i <= 3; i++)      // inner FOR loop — runs completely each outer iteration
    {
        Console.WriteLine("Round " + i);
    }
    lives = lives - 1;
}

// Total lines printed: 3 outer iterations x 3 inner = 9 lines