Control Flow

Overview

Charl provides three main control flow constructs:

if/else Statements

The if statement executes a block of code only if a condition is true.

Basic if Statement

let x = 10

if x > 5 {
    print("x is greater than 5")
}

// With multiple statements
if x > 0 {
    print("x is positive")
    let doubled = x * 2
    print("doubled: " + str(doubled))
}

if/else Statement

let age = 18

if age >= 18 {
    print("You are an adult")
} else {
    print("You are a minor")
}

// With calculations
let temperature = 25.0
if temperature > 30.0 {
    print("It's hot outside")
} else {
    print("It's not too hot")
}

else if Chains

let score = 85

if score >= 90 {
    print("Grade: A")
} else if score >= 80 {
    print("Grade: B")
} else if score >= 70 {
    print("Grade: C")
} else if score >= 60 {
    print("Grade: D")
} else {
    print("Grade: F")
}

// Multiple conditions in else if
let x = 15
if x < 0 {
    print("Negative")
} else if x >= 0 && x < 10 {
    print("Small positive")
} else if x >= 10 && x < 100 {
    print("Medium positive")
} else {
    print("Large positive")
}

Nested if Statements

let is_weekend = true
let has_homework = false

if is_weekend {
    if has_homework {
        print("Do homework first, then relax")
    } else {
        print("Enjoy your free weekend!")
    }
} else {
    print("It's a weekday, time for school")
}

// Complex nesting
let age = 25
let has_license = true
let has_car = false

if age >= 18 {
    if has_license {
        if has_car {
            print("You can drive your own car")
        } else {
            print("You can drive, but need to rent a car")
        }
    } else {
        print("You need to get a license first")
    }
} else {
    print("You're too young to drive")
}

while Loops

The while loop repeats a block of code as long as a condition remains true.

Basic while Loop

// Count from 0 to 4
let i = 0
while i < 5 {
    print("Count: " + str(i))
    let i = i + 1  // Shadowing creates new variable
}

// Sum numbers 1 to 10
let sum = 0
let n = 1
while n <= 10 {
    let sum = sum + n
    let n = n + 1
}
print("Sum: " + str(sum))  // 55

Condition-Based Loops

// Loop until condition met
let x = 1.0
while x < 100.0 {
    print("x: " + str(x))
    let x = x * 2.0  // Double each iteration
}

// Search in array
let numbers = [1, 3, 5, 7, 9, 11]
let target = 7
let found = false
let index = 0

while index < len(numbers) && !found {
    if numbers[index] == target {
        print("Found at index: " + str(index))
        let found = true
    }
    let index = index + 1
}

Infinite Loops with Break Condition

// Training loop example
let epoch = 0
let max_epochs = 1000
let loss = 1.0
let min_loss = 0.01

while epoch < max_epochs {
    // Simulate training step
    let loss = loss * 0.95  // Loss decreases

    print("Epoch " + str(epoch) + ", Loss: " + str(loss))

    // Check convergence
    if loss < min_loss {
        print("Converged!")
        // In practice, you'd break here
        // Charl doesn't have break, so use condition
        let epoch = max_epochs  // Force loop to end
    }

    let epoch = epoch + 1
}

match Expressions

The match expression provides pattern matching for elegant conditional logic.

Basic Pattern Matching

let number = 2

let result = match number {
    1 => "one"
    2 => "two"
    3 => "three"
    _ => "other"  // Default case (wildcard)
}
print(result)  // "two"

// With different types
let status_code = 200
let message = match status_code {
    200 => "OK"
    404 => "Not Found"
    500 => "Internal Server Error"
    _ => "Unknown Status"
}

Matching with Expressions

let x = 42

let category = match x {
    0 => "zero"
    1 => "one"
    _ => if x < 0 {
        "negative"
    } else {
        "positive"
    }
}

// Multiple statements in match arms
let grade = 85
match grade {
    _ => if grade >= 90 {
        print("Excellent!")
        print("Grade: A")
    } else if grade >= 80 {
        print("Good job!")
        print("Grade: B")
    } else {
        print("Keep trying!")
        print("Grade: C or below")
    }
}

Practical Match Examples

// Day of week
let day_num = 3
let day_name = match day_num {
    1 => "Monday"
    2 => "Tuesday"
    3 => "Wednesday"
    4 => "Thursday"
    5 => "Friday"
    6 => "Saturday"
    7 => "Sunday"
    _ => "Invalid day"
}

// Activation function selection
let activation = "relu"
let output = match activation {
    "relu" => if x > 0.0 { x } else { 0.0 }
    "sigmoid" => 1.0 / (1.0 + tensor_exp(tensor([-x])))
    "tanh" => tensor_tanh(tensor([x]))
    _ => x  // Linear (identity)
}

Combining Control Flow

You can combine different control flow constructs to create complex logic.

if inside while

// Process array elements conditionally
let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
let i = 0
let even_sum = 0
let odd_sum = 0

while i < len(numbers) {
    if numbers[i] % 2 == 0 {
        let even_sum = even_sum + numbers[i]
    } else {
        let odd_sum = odd_sum + numbers[i]
    }
    let i = i + 1
}

print("Even sum: " + str(even_sum))  // 30
print("Odd sum: " + str(odd_sum))    // 25

Nested Loops

// Matrix-like iteration
let rows = 3
let cols = 3
let i = 0

while i < rows {
    let j = 0
    while j < cols {
        let value = i * cols + j
        print("(" + str(i) + "," + str(j) + ") = " + str(value))
        let j = j + 1
    }
    let i = i + 1
}

// Multiplication table
let i = 1
while i <= 5 {
    let j = 1
    while j <= 5 {
        let product = i * j
        print(str(i) + " × " + str(j) + " = " + str(product))
        let j = j + 1
    }
    let i = i + 1
}

match inside Loops

// Process commands
let commands = ["start", "process", "stop", "restart"]
let i = 0

while i < len(commands) {
    let cmd = commands[i]
    match cmd {
        "start" => print("Starting system...")
        "stop" => print("Stopping system...")
        "restart" => print("Restarting system...")
        "process" => print("Processing data...")
        _ => print("Unknown command: " + cmd)
    }
    let i = i + 1
}

Complete Examples

Example 1: Factorial Calculation

// Calculate factorial using while loop
let n = 5
let result = 1
let i = 1

while i <= n {
    let result = result * i
    let i = i + 1
}

print("Factorial of " + str(n) + " is " + str(result))  // 120

Example 2: Fibonacci Sequence

// Generate first 10 Fibonacci numbers
let n = 10
let a = 0
let b = 1
let count = 0

print("Fibonacci sequence:")
while count < n {
    print(str(a))
    let temp = a + b
    let a = b
    let b = temp
    let count = count + 1
}

Example 3: Simple Training Loop

// Simplified gradient descent
let learning_rate = 0.1
let x = 10.0  // Starting point
let target = 5.0
let max_iterations = 100
let tolerance = 0.001
let iteration = 0

while iteration < max_iterations {
    // Calculate error (loss)
    let error = x - target

    // Check if converged
    if error < tolerance && error > -tolerance {
        print("Converged at iteration " + str(iteration))
        let iteration = max_iterations  // Exit loop
    } else {
        // Update x (gradient descent step)
        let x = x - learning_rate * error

        if iteration % 10 == 0 {
            print("Iteration " + str(iteration) + ": x = " + str(x))
        }

        let iteration = iteration + 1
    }
}

print("Final value: " + str(x))

Example 4: Data Validation

// Validate dataset
let data = [1.0, 2.0, 3.0, -1.0, 5.0]
let i = 0
let valid = true
let errors = 0

while i < len(data) {
    let value = data[i]

    if value < 0.0 {
        print("Error at index " + str(i) + ": negative value")
        let errors = errors + 1
        let valid = false
    } else if value > 100.0 {
        print("Error at index " + str(i) + ": value too large")
        let errors = errors + 1
        let valid = false
    }

    let i = i + 1
}

if valid {
    print("All data valid!")
} else {
    print("Found " + str(errors) + " errors")
}

Best Practices