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If statements#

Code Example

Runnable Example in Jac and JacLib

# If Statements

with entry {
    # Basic if statement
    x = 10;
    if x > 5 {
        print("x is greater than 5");
    }

    # If-else statement
    age = 18;
    if age >= 18 {
        print("adult");
    } else {
        print("minor");
    }

    # If-elif-else chain
    score = 85;
    if score >= 90 {
        print("A");
    } elif score >= 80 {
        print("B");
    } elif score >= 70 {
        print("C");
    } else {
        print("F");
    }

    # Multiple elif chains
    value = 15;
    if value < 5 {
        print("very low");
    } elif value < 10 {
        print("low");
    } elif value < 15 {
        print("medium");
    } elif value < 20 {
        print("high");
    } else {
        print("very high");
    }

    # Nested if statements
    a = 15;
    b = 20;
    if a > 10 {
        print("a > 10");
        if b > 15 {
            print("b > 15");
            if a + b > 30 {
                print("a + b > 30");
            }
        }
    }

    # Complex boolean expressions
    if a > 5 and b > 10 {
        print("both conditions true");
    }

    if a > 100 or b > 15 {
        print("at least one true");
    }

    if not (a > 50) {
        print("negation true");
    }

    if (a > 5 and b > 10) or (a < 20) {
        print("complex expression true");
    }

    # Chained comparisons
    temp = 25;
    if 20 <= temp <= 30 {
        print("comfortable temperature");
    }

    # Membership tests
    fruits = ["apple", "banana"];
    if "apple" in fruits {
        print("apple found");
    }

    if "grape" not in fruits {
        print("grape not found");
    }

    # Identity tests
    val = None;
    if val is None {
        print("val is None");
    }

    if val is not None {
        print("val is not None");
    } else {
        print("val is None");
    }
}

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# If Statements

with entry {
    # Basic if statement
    x = 10;
    if x > 5 {
        print("x is greater than 5");
    }

    # If-else statement
    age = 18;
    if age >= 18 {
        print("adult");
    } else {
        print("minor");
    }

    # If-elif-else chain
    score = 85;
    if score >= 90 {
        print("A");
    } elif score >= 80 {
        print("B");
    } elif score >= 70 {
        print("C");
    } else {
        print("F");
    }

    # Multiple elif chains
    value = 15;
    if value < 5 {
        print("very low");
    } elif value < 10 {
        print("low");
    } elif value < 15 {
        print("medium");
    } elif value < 20 {
        print("high");
    } else {
        print("very high");
    }

    # Nested if statements
    a = 15;
    b = 20;
    if a > 10 {
        print("a > 10");
        if b > 15 {
            print("b > 15");
            if a + b > 30 {
                print("a + b > 30");
            }
        }
    }

    # Complex boolean expressions
    if a > 5 and b > 10 {
        print("both conditions true");
    }

    if a > 100 or b > 15 {
        print("at least one true");
    }

    if not (a > 50) {
        print("negation true");
    }

    if (a > 5 and b > 10) or (a < 20) {
        print("complex expression true");
    }

    # Chained comparisons
    temp = 25;
    if 20 <= temp <= 30 {
        print("comfortable temperature");
    }

    # Membership tests
    fruits = ["apple", "banana"];
    if "apple" in fruits {
        print("apple found");
    }

    if "grape" not in fruits {
        print("grape not found");
    }

    # Identity tests
    val = None;
    if val is None {
        print("val is None");
    }

    if val is not None {
        print("val is not None");
    } else {
        print("val is None");
    }
}

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from __future__ import annotations
x = 10
if x > 5:
    print('x is greater than 5')
age = 18
if age >= 18:
    print('adult')
else:
    print('minor')
score = 85
if score >= 90:
    print('A')
elif score >= 80:
    print('B')
elif score >= 70:
    print('C')
else:
    print('F')
value = 15
if value < 5:
    print('very low')
elif value < 10:
    print('low')
elif value < 15:
    print('medium')
elif value < 20:
    print('high')
else:
    print('very high')
a = 15
b = 20
if a > 10:
    print('a > 10')
    if b > 15:
        print('b > 15')
        if a + b > 30:
            print('a + b > 30')
if a > 5 and b > 10:
    print('both conditions true')
if a > 100 or b > 15:
    print('at least one true')
if not a > 50:
    print('negation true')
if a > 5 and b > 10 or a < 20:
    print('complex expression true')
temp = 25
if 20 <= temp <= 30:
    print('comfortable temperature')
fruits = ['apple', 'banana']
if 'apple' in fruits:
    print('apple found')
if 'grape' not in fruits:
    print('grape not found')
val = None
if val is None:
    print('val is None')
if val is not None:
    print('val is not None')
else:
    print('val is None')
Jac Grammar Snippet 
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135
136
if_stmt: KW_IF expression code_block (elif_stmt | else_stmt)?
elif_stmt: KW_ELIF expression code_block (elif_stmt | else_stmt)?
else_stmt: KW_ELSE code_block

Description

If Statements in Jac

If statements provide conditional control flow through if, elif, and else keywords. They allow code to execute different paths based on boolean conditions.

Basic If Statement

Lines 4-8 demonstrate the simplest conditional. When the condition x > 5 evaluates to true, the block executes.

If-Else Statement

Lines 11-16 show binary choice logic. Exactly one block executes: the if block when true, otherwise the else block.

If-Elif-Else Chain

Lines 19-28 demonstrate multiple exclusive conditions. Evaluation is top-down. The first true condition executes, then the entire chain terminates.

Multiple Elif Branches

Lines 31-42 show extended conditional chains with many branches. Each elif provides an additional condition to check if previous conditions were false.

Nested If Statements

Lines 45-55 demonstrate if statements within if statements. Inner conditions only evaluate if outer conditions are true.

Complex Boolean Expressions

Lines 58-72 show combining conditions with logical operators:

AND operator (line 58-60):

Both conditions must be true.

OR operator (line 62-64):

At least one condition must be true.

NOT operator (line 66-68):

Negates the condition.

Combined operators (line 70-72):

Parentheses control precedence.

Chained Comparisons

Lines 75-78 demonstrate Python-style chained comparisons. Equivalent to 20 <= temp and temp <= 30, but evaluates temp only once.

Membership Tests

Lines 81-88 show in and not in operators. Works with any iterable: lists, tuples, sets, dictionaries (checks keys), strings.

Identity Tests

Lines 91-100 demonstrate is and is not for identity checking. Use is for None checks, not ==.

Comparison Operators

Operator Meaning Example Line
== Equal Throughout
!= Not equal Implicit
< Less than 32
<= Less than or equal 76
> Greater than 6
>= Greater than or equal 12, 20
is Identity 92
is not Not identity 96
in Membership 82
not in Non-membership 86

Logical Operators

Operator Meaning Precedence
not Negation Highest
and Both must be true Medium
or At least one must be true Lowest

Control Flow Diagram

flowchart TD
    Start([If Statement]) --> Cond1{If Condition<br/>True?}
    Cond1 -->|Yes| IfBlock[Execute If Block]
    Cond1 -->|No| Elif{Elif<br/>Present?}
    IfBlock --> Done([Continue])
    Elif -->|Yes| ElifCond{Elif Condition<br/>True?}
    Elif -->|No| Else{Else<br/>Present?}
    ElifCond -->|Yes| ElifBlock[Execute Elif Block]
    ElifCond -->|No| NextElif{More<br/>Elif?}
    ElifBlock --> Done
    NextElif -->|Yes| ElifCond
    NextElif -->|No| Else
    Else -->|Yes| ElseBlock[Execute Else Block]
    Else -->|No| Done
    ElseBlock --> Done

Truthy and Falsy Values

Jac follows Python's truthiness rules:

Falsy values: - False - None - 0 (numeric zero) - "" (empty string) - [] (empty list) - {} (empty dict)

Truthy values: - Everything else

Example:

Common Patterns

Guard pattern (early return):

Range checking:

None checking:

Membership filtering:

Multi-condition validation:

Short-Circuit Evaluation

and and or use short-circuit evaluation:

If x != 0 is false, y / x doesn't execute (preventing division by zero).

If Expression (Ternary)

Jac supports if expressions for value selection:

If expressions must have an else clause and return a value.

Block Syntax Rules

  1. Braces required: { } delimit blocks, not indentation
  2. Semicolons required: Each statement ends with ;
  3. Single-line allowed: if x > 5 { print("yes"); } is valid
  4. Multi-line preferred: For readability

If Statement vs If Expression

Feature If Statement If Expression
Syntax if cond { ... } else { ... } val1 if cond else val2
Returns value No Yes
Multiple statements Yes No (expressions only)
Else required No Yes
Use case Control flow Value selection

Key Differences from Python

  1. Braces required: Jac uses { }, not indentation
  2. Semicolons required: Each statement ends with ;
  3. Same truthiness: Empty collections are falsy
  4. Same operators: and, or, not, in, is
  5. Same chaining: a <= b <= c works identically