Control Structures

Control structures are the decision‑making parts of a program. They let your code ask questions, choose between different paths, and decide what to do next. Think of them like traffic signals or signposts that guide your program’s flow.

“If I finish my homework, then I can play. Otherwise, I must study.”

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Why Are They Important?

Without control structures, a program would simply run every line in order, with no ability to adapt or react. In real‑world tasks we often need to:

• Make decisions: “If the user is logged in, show the dashboard; otherwise, show the login screen.”
• Handle multiple choices: “Choose a difficulty level from Easy, Medium, or Hard.”
• Build complex logic: “If both A and B are true, do X; if only one is true, do Y.”

Control structures turn a static list of commands into a dynamic, intelligent process.

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Control Flow Diagram

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ASCII Diagram – IF Decision

+-------------------+
|      Start        |
+-------------------+
         |
         v
  [Check Condition]
    /         \
 True        False
  |             |
Action 1     Action 2
  |             |
  +------+------+ 
         |
        End

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1. If Statement

Use an if statement to run a block of code only when a condition is true.

How it works

1. Evaluate the condition.
2. If the condition is true, execute the code inside the if block.
3. If the condition is false, skip the block and continue.

Syntax

if condition then
    // code to run when condition is true
end if

Examples

Without parentheses

set temperature to 30

if temperature > 25 then
    show("It's hot today")
end if

With parentheses

set temperature to 20

if (temperature <= 25) then
    show("It's cool today")
end if

Note: You can use set without a prior declare.

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2. If‑Else

An if-else statement lets you choose between two blocks:

• The if block runs when the condition is true.
• The else block runs when the condition is false.

Syntax

if condition then
    // code if true
else
    // code if false
end if

Example

set age to 16

if age >= 18 then
    show("Adult")
else
    show("Minor")
end if

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3. Else If

Use else if when you have multiple conditions to check in order:

1. Check the first if.
2. If that is false, check each else if in sequence.
3. If none match, an optional else block can handle all other cases.

Syntax

if condition1 then
    // code for condition1
else if condition2 then
    // code for condition2
else
    // fallback code
end if

Example

set score to 70

if score >= 90 then
    show("Excellent")
else if score >= 80 then
    show("Very Good")
else if score >= 70 then
    show("Good")
else
    show("Needs Improvement")
end if

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4. Nested If

A nested if places one if statement inside another. Use this when you need to make a second decision only if the first condition is true.

Syntax

if conditionA then
    if conditionB then
        // code when both A and B are true
    else
        // code when A is true but B is false
    end if
else
    // code when A is false
end if

Example

set userType to "admin"
set status to "active"

if userType == "admin" then
    if status == "active" then
        show("Admin access granted")
    else
        show("Admin account inactive")
    end if
else
    show("Standard user access")
end if

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5. Choose / When / Otherwise

choose is a multi‑branch structure (similar to switch‑case) for matching one variable against many possible values. Use when for each case and otherwise for the default. Place a colon (:) after each when or otherwise.

Syntax

choose variable
    when value1:
        // code for value1
    when value2:
        // code for value2
    otherwise:
        // code if no cases match
end choose

Example

set color to "green"

choose color
    when "red":
        show("Stop")
    when "yellow":
        show("Caution")
    when "green":
        show("Go")
    otherwise:
        show("Unknown signal")
end choose

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6. Comparison Operators

Comparison operators test relationships between values and return true or false.

• Equality: == or is
• Inequality: != or is not
• Greater / Less: >, <, >=, <=
• Membership: is in

Example

set role to "admin"

if role == "admin" then
    show("Welcome, Admin")
end if

if role is not "guest" then
    show("Access granted")
end if

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Summary Table

Structure	Syntax Example	Purpose
if	if cond then … end if	Run block if condition is true
if with ()	if (cond) then … end if	Alternative, explicit style
if-else	if … else … end if	Two‑way decision
else if	if … else if … else … end if	Multi‑way decision
Nested if	if … then if … end if end if	Decision inside another decision
choose	choose var when val: … otherwise: … end	Clean multi‑case selection
== / is	if x == y or if x is y	Equality comparison
!= / is not	if x != y or if x is not y	Inequality comparison
> / < / >= / <=	if x > y, etc.	Relational comparisons
is in	"a" is in list	Membership test

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Control structures are the foundation for making your programs dynamic, responsive, and intelligent. Master these tools to write code that can think and adapt to any situation.