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Conditionals -- IF/THEN/ELSE/ELSEIF, SELECT CASE

Making Decisions with Conditionals

So far, our programs have run from top to bottom. But what if we want our code to make choices? That's where conditionals come in. They allow your program to ask questions and perform different actions based on the answers. Because conditionals control the flow of logic, clarity is essential. Let's learn how by checking the status of a video game character.

Tip: Clear conditionals make code easier to read and maintain. Messy or deeply nested ones often lead to bugs, because they control the program's flow of logic. Always write conditionals so their purpose is obvious — future you (and other programmers) will thank you.

1. The Simple IF/THEN Statement

The most primitive conditional is the IF/THEN statement. It checks if a condition is true, and if it is, it runs a block of code. Let's check if our hero's health is critically low.

health = 9 if health < 10 then print "Health is critical! Use a potion!" end if

Because health is less than 10, the message is printed. If you changed health to 25, nothing would be printed.

2. Adding an ELSE for the Other Path

What if the condition is false? An ELSE statement lets you run a different block of code in that case.

health = 55 if health < 10 then print "Health is critical! Use a potion!" else print "You are healthy enough to continue." end if

Flowchart: How IF/ELSE Works

Start
↓
health < 10?
→ TRUE
Print "Critical!"
→ FALSE
Print "Healthy."
↓
End

3. Handling Multiple Conditions with ELSEIF

Sometimes you have more than two possibilities. You can chain conditions together using ELSEIF to create more complex logic.

health = 45 if health < 10 then print "Health is critical! Use a potion!" elseif health < 50 then print "Warning: Your health is getting low." else print "You are in good shape." end if

Important Sheerpower Rule!

In Sheerpower, a condition inside an IF must evaluate to exactly TRUE (1) or FALSE (0). There is no automatic true-or-false. You can't use just any non-zero number.

Wrong: x = 5
if x then ... (This will fail)

Correct: x = 5
if x <> 0 then ... (This works because x <> 0 evaluates to true)

4. Choosing a Value: the IF Expression

Sometimes the two paths differ only in the value they give. Instead of an IF block that assigns the same variable on both paths, write the choice where the value goes: if test then value else value.

in_rate = 12.50 out_rate = 18 units = 4 in_facility? = true charges = (if in_facility? then in_rate * units else out_rate * units) print 'Charges: '; charges

Output:

Charges: 50

An IF expression can stand wherever a value can: on the right of an assignment, in a print, inside a calculation or as a function's argument. ELSEIF chains more choices. Only the value that is chosen is worked out — so below, names$(1) is never looked at while the array is empty:

item_count = 1 print 'You have'; item_count; if item_count = 1 then 'item' else 'items' item_count = 3 print 'You have'; item_count; if item_count = 1 then 'item' else 'items' score = 84 grade$ = (if score >= 90 then 'A' elseif score >= 80 then 'B' elseif score >= 70 then 'C' else 'F') print 'Grade: '; grade$ dim names$(*) first_name$ = (if size(names$) > 0 then names$(1) else 'none') print 'First name: '; first_name$

Output:

You have 1 item You have 3 items Grade: B First name: none

The rules:

  • The ELSE value is required: an expression always has a value.
  • The test is TRUE (1) or FALSE (0), as in an IF statement.
  • The two values are both numbers or both strings.
  • The ELSE value reaches as far as an expression does: cost = (if member? then 0 else shipping + handling) adds handling only when member? is false. Parentheses end it sooner: cost = (if member? then 0 else shipping) + handling.
  • A routine called in function form, or an array expression, cannot be the THEN or ELSE value: work it out in a statement of its own first and use the result. In the test they are fine.
  • To choose element by element between two arrays, use where(test, a, b).

5. Handling Many States with SELECT CASE

When you need to check one value against many possibilities, a long IF/ELSEIF chain can quickly become hard to follow. The SELECT CASE statement offers a cleaner and more efficient way to handle this. In Sheerpower, SELECT CASE is more expressive than the switch/case statements found in many languages, making code easier to read and maintain.

// Using IF/ELSEIF IF score >= 90 THEN grade$ = "A" ELSEIF score >= 80 THEN grade$ = "B" ELSEIF score >= 70 THEN grade$ = "C" ELSE grade$ = "F" END IF
// Using SELECT CASE SELECT CASE score CASE is >= 90 : grade$ = "A" CASE is >= 80 : grade$ = "B" CASE is >= 70 : grade$ = "C" CASE ELSE : grade$ = "F" END SELECT

Flowchart: How SELECT CASE Works

Start
↓
Evaluate main_expr
→ "Poisoned"
Print "Taking damage"
→ 2 TO 6
Print "Range matched"
→ ELSE
Print "No match"
↓
End

Format

SELECT CASE main_expr 
  CASE expr1[, expr2,...] 
      block of code
  [CASE expr3[, expr4,...] 
      block of code
             ...] 
  [CASE IS {relational operator} exprN
      block of code
             ...] 
  [CASE ELSE
      block of code
             ...]
END SELECT
Important: A CASE block extends from its CASE line down to the next CASE, CASE ELSE, or END SELECT. There is no END CASE. Only the first matching CASE executes — there is no fall-through.

Key Features

  • Main expression: Any valid expression, not just a variable.
  • Multiple matches: Each CASE may hold multiple expressions and ranges, separated by commas.
  • Ranges: Use expr1 TO expr2 to cover numeric or string ranges.
  • CASE IS: Compare with a relational operator. All standard operators are supported: <, <=, =, >, >=.
  • CASE ELSE: Executes only if no other case matches.
  • Type safety: All expressions must be the same data type as main_expr. Mismatched types raise an exception.

Examples

// Example 1: Multiple expressions per CASE do input "Procedure (add, del, exit)": pro$ if _exit then exit do pro$ = ucase$(pro$) select case pro$ case "ADD" print "Adding..." case "DEL", "DELETE" print "Deleting..." case else print "Procedure must be: add, del or exit" end select loop end
// Example 2: Range checking a = 5 select case a case 1 print "one" case 2 to 6 print "range" case else print "else" end select b$ = "c" select case b$ case "a" print "a" case "b" to "e" print "range" case else print "else" end select // Output: range / range
// Example 3: CASE IS with relational operators do input "Your income per year": income if _back or _exit then exit do select case income case is < 0 print "A negative income? You are in debt!" case 0 print "No income?" case is >= 100000 print "Wow, high income!" case is > 0 print "A positive income." end select loop end
// Example 4: First match only (numeric) x = 5 select case x case 1 to 10 print "Matched range 1 to 10" case 5 print "Matched exactly 5" case else print "No match" end select // Output: Matched range 1 to 10 // Example 5: First match only (string) letter$ = "c" select case letter$ case "a" to "f" print "Matched range a to f" case "c" print "Matched exactly c" case else print "No match" end select // Output: Matched range a to f

Compact CASE Form — Single Statement

When each CASE needs to perform just one simple action, Sheerpower supports a compact form using a colon (:) on the CASE line. This is functionally identical to the block form, but it reads like a clean mapping table.

The compact form is ideal for small dispatch logic, menu handlers, and readable state messages.

number = 3 select case number case 1: print "One!" case 2: print "Two!" case 3: print "Three!" case else: print "Something else!" end select

Problem: A full block for each CASE can feel heavy when each case only needs a single statement.

Solution: Use CASE expr : statement to keep simple CASE logic compact and obvious.

Efficiency: This is purely a readability feature. It does not change behavior or performance. Only the first matching CASE executes, and there is still no fall-through.

Takeaway: Use the compact CASE form for one statement per case. When a case grows beyond one statement, switch back to the block form for clarity.

If a CASE needs more than one statement, prefer the standard block layout:

select case number case 3 print "Three!" print "Lucky number." case else print "Something else!" end select

Best Practice: Order your CASE statements from most specific to most general. Since Sheerpower stops at the first match, a broad range (like 1 TO 10) will prevent later, more specific cases (like 5) from ever running. Always place exact matches or narrower conditions before wider ranges.

❌ Bad Order

x = 5 select case x case 1 to 10 print "Matched range 1 to 10" case 5 print "Matched exactly 5" end select // Output: Matched range 1 to 10

✅ Good Order

x = 5 select case x case 5 print "Matched exactly 5" case 1 to 10 print "Matched range 1 to 10" end select // Output: Matched exactly 5

Advanced Pattern: SELECT CASE TRUE

This pattern allows complex conditional logic without deep nesting.

Since the SELECT CASE construct accepts any expression, you can use SELECT CASE TRUE as a clear alternative to a chain of IF/ELSEIF statements. Each CASE contains a Boolean expression. When that expression evaluates to TRUE, its block executes.

This pattern preserves the "first match only" behavior while avoiding deeply nested conditional blocks.

len = 12 a = 5 b = 10 select case true case a > b print "greater" case len = 12 print "len was 12" end select
Sheerpower's SELECT CASE is more expressive than traditional switch/case constructs. It supports:
  • Any expression — not just a single variable.
  • Multiple values per CASE, separated by commas.
  • Ranges with expr1 TO expr2, for numbers and strings.
  • Relational tests via CASE IS with <, <=, =, >, >=.
  • Boolean-driven patterns using SELECT CASE TRUE.
  • No fall-through — only the first match executes.
  • Type safety — all expressions must match the type of the main expression.
Each CASE block runs until the next CASE (or CASE ELSE / END SELECT), and only the first matching case is executed. This combination makes SELECT CASE both safer and more expressive than traditional switch/case statements in other languages.

Performance Note: In Sheerpower, comparisons — whether in IF, CASE, or any other expression — run at extremely high speed, millions per second. The order of CASE statements only affects which block executes first; the timing difference is so small (a few billionths of a second) that it will rarely impact performance. Prioritize clarity over micro-optimization — conditionals are almost never the bottleneck.
Summary: You now know how to control the flow of your program!
  • Use IF for a single condition.
  • Add ELSE to handle the "false" path.
  • Use ELSEIF for multiple, related conditions.
  • Use SELECT CASE for checking one main value against many different values. It's cleaner and more readable.
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