Control Flow

Chapter Status: ✅ 100% Working (7/7 core examples)

Last updated: 2025-10-13 Ruchy version: ruchy v1.84.0

Core Control Flow (7/7) - 100% Pass Rate:

• Example 1: If/else ✅
• Example 2: If without else ✅
• Example 3: If/else if/else chains ✅
• Example 4: While loop (with let mut) ✅
• Example 5: For loop with range ✅
• Example 6: Match expression ✅
• Example 7: Break and continue ✅

Features Validated:

• ✅ let mut (mutable variables)
• ✅ while loops
• ✅ for..in with ranges (0..3)
• ✅ match expressions with wildcard
• ✅ break statement
• ✅ continue statement
• ✅ if/else conditionals

DataFrame Examples: Untested (require DataFrame, advanced iteration)

Chapter Status: ✅ 100% Test-Driven (7/7 examples passing) Ruchy Version: v1.84.0 Testing: All examples verified with make test-ch05 and 7-layer validation

The Problem

Programs need to make decisions and repeat actions. Control flow structures like conditionals, loops, and pattern matching allow programs to respond to different situations and process data efficiently.

Test-Driven Examples

Example 1: Basic If/Else

This example is tested in tests/ch05-control-flow/test_01_if_else.ruchy:

fun main() {
    let x = 10;
    if x > 5 {
        println("x is greater than 5");
    } else {
        println("x is not greater than 5");
    }
}

Output:

x is greater than 5

Example 2: If Without Else

This example is tested in tests/ch05-control-flow/test_02_if_only.ruchy:

fun main() {
    let score = 85;
    if score >= 80 {
        println("Great job!");
    }
    println("Score processed");
}

Output:

Great job!
Score processed

Example 3: If/Else If/Else Chains

This example is tested in tests/ch05-control-flow/test_03_if_else_if.ruchy:

fun main() {
    let grade = 75;
    if grade >= 90 {
        println("A grade");
    } else if grade >= 80 {
        println("B grade");
    } else if grade >= 70 {
        println("C grade");
    } else {
        println("Below C");
    }
}

Output:

C grade

Example 4: While Loop

This example is tested in tests/ch05-control-flow/test_04_while_loop.ruchy:

fun main() {
    let mut i = 0;
    while i < 3 {
        println(i);
        i = i + 1;
    }
    println("Done");
}

Output:

0
1
2
Done

Example 5: For Loop with Range

This example is tested in tests/ch05-control-flow/test_05_for_loop.ruchy:

fun main() {
    for i in 0..3 {
        println(i);
    }
    println("For loop done");
}

Output:

0
1
2
For loop done

Example 6: Match Expression

This example is tested in tests/ch05-control-flow/test_06_match.ruchy:

fun main() {
    let number = 2;
    match number {
        1 => println("One"),
        2 => println("Two"),
        3 => println("Three"),
        _ => println("Other")
    }
}

Output:

Two

Example 7: Break and Continue

This example is tested in tests/ch05-control-flow/test_07_break_continue.ruchy:

fun main() {
    let mut i = 0;
    while i < 10 {
        i = i + 1;
        if i == 3 {
            continue;
        }
        if i == 6 {
            break;
        }
        println(i);
    }
    println("Loop ended");
}

Output:

1
2
4
5
Loop ended

Core Concepts

Conditional Statements

• If statements: Execute code based on conditions
• Else clauses: Alternative execution path
• Else if: Chain multiple conditions
• Syntax: if condition { ... } else { ... }

Loop Constructs

• While loops: Repeat while condition is true
• For loops: Iterate over ranges or collections
• Break: Exit loop early
• Continue: Skip to next iteration

Pattern Matching

• Match expressions: Choose execution path based on value
• Arms: Each pattern => action pair
• Wildcard: _ matches any value
• Exhaustiveness: All possible values must be covered

Mutability

• Variables in loops often need mut keyword
• Allows modification of variable values
• Required for loop counters and accumulators

Key Syntax

Conditionals

fun main() {
    let x = 10;
    let y = 5;

    if x > y {
        println("x is greater");
    } else if x < y {
        println("y is greater");
    } else {
        println("they are equal");
    }
}

Loops

fun main() {
    // While loop
    let mut count = 0;
    while count < 3 {
        println("Count:", count);
        count = count + 1;
    }

    // For loop with range
    for i in 1..4 {
        println("Iteration:", i);
    }
}

Match

fun main() {
    let number = 2;
    match number {
        1 => println("One"),
        2 => println("Two"),
        3 => println("Three"),
        _ => println("Other")
    }
}

Testing Your Code

All examples in this chapter can be verified:

# Test all control flow examples
make test-ch05

# Test specific example
make test-file FILE=tests/ch05-control-flow/test_01_if_else.ruchy

Common Patterns

Decision Making

fun main() {
    let user_input = 75;
    let threshold = 50;

    if user_input > threshold {
        println("High value:", user_input);
    } else {
        println("Normal value:", user_input);
    }
}

Counting Loop

fun main() {
    let mut count = 0;
    while count < 10 {
        println("Count is:", count);
        count = count + 1;
    }
}

Range Processing

fun main() {
    for i in 1..5 {
        println("Processing item", i);
    }
}

Value Classification

fun main() {
    let status_code = 200;
    match status_code {
        200 => println("Success"),
        404 => println("Not Found"),
        500 => println("Server Error"),
        _ => println("Unknown Status")
    }
}

Performance Notes

• If statements: Very fast, compile to conditional jumps
• While loops: Efficient for unknown iteration counts
• For loops: Optimized for known ranges
• Match: Often optimized to jump tables

Summary

✅ What Works (Test-Verified in v1.10.0):

• If/else statements with boolean conditions
• If without else (optional else)
• If/else if/else chains
• While loops with mutable variables
• For loops with range syntax (start..end)
• Match expressions with multiple arms
• Break and continue statements
• Mutable variables with mut keyword

⏳ Not Yet Tested (Future Investigation):

• Match with complex patterns (structs, enums)
• Nested loops
• Loop labels
• For loops with collections (not ranges)
• Complex boolean logic (&&, ||)

Exercises

Based on our tested examples, try these:

1. Exercise 1: Create a grade calculator using if/else if chains
2. Exercise 2: Write a countdown loop using while
3. Exercise 3: Use match to handle different menu choices
4. Exercise 4: Create a number guessing game with loops and conditionals

Next Steps

Control flow gives you the tools to build interactive and dynamic programs. In the next chapter, we’ll explore data structures to organize and manipulate collections of data.

Every example in this chapter has been tested and verified to work with Ruchy v1.10.0