Control Structures — Junior Level¶

Table of Contents¶

Introduction
Prerequisites
Glossary
Core Concepts
Conditional Statements
Loops
Switch/Match Statements
Code Examples
Best Practices
Common Mistakes
Cheat Sheet
Summary
Further Reading

Introduction¶

Focus: "What are control structures?" and "How to use them?"

Control structures determine the order in which statements are executed. Without them, code runs top to bottom — every line, every time. Control structures let you skip lines (conditionals), repeat lines (loops), and choose between paths (switch/match).

Every algorithm is built from just three building blocks: 1. Sequence — execute one statement after another 2. Selection — choose which statements to execute (if/else) 3. Iteration — repeat statements (for/while)

Prerequisites¶

Required: Variables and data types in Go/Java/Python
Required: Boolean expressions (true/false, ==, !=, <, >)
Helpful: Basic understanding of pseudo code

Glossary¶

Term	Definition
Conditional	A statement that executes code only if a condition is true
Loop	A statement that repeats code while a condition is true
Boolean Expression	An expression that evaluates to `true` or `false`
Iteration	One complete execution of a loop body
Infinite Loop	A loop that never terminates (usually a bug)
Break	A statement that exits the current loop immediately
Continue	A statement that skips the rest of the current iteration
Short-circuit	Evaluation stops as soon as the result is determined

Core Concepts¶

Concept 1: Boolean Expressions¶

Every control structure depends on a boolean expression — something that is either true or false.

Go¶

x := 10
fmt.Println(x > 5)      // true
fmt.Println(x == 10)     // true
fmt.Println(x > 5 && x < 20) // true (AND)
fmt.Println(x > 5 || x < 3)  // true (OR)
fmt.Println(!(x > 5))        // false (NOT)

Java¶

int x = 10;
System.out.println(x > 5);          // true
System.out.println(x == 10);        // true
System.out.println(x > 5 && x < 20); // true
System.out.println(x > 5 || x < 3);  // true
System.out.println(!(x > 5));        // false

Python¶

x = 10
print(x > 5)           # True
print(x == 10)          # True
print(x > 5 and x < 20) # True
print(x > 5 or x < 3)   # True
print(not (x > 5))       # False

Concept 2: Truthy and Falsy¶

Python treats some values as "falsy" even though they're not literally False:

# Falsy values in Python:
if not 0:       print("0 is falsy")
if not "":      print("empty string is falsy")
if not []:      print("empty list is falsy")
if not None:    print("None is falsy")
if not {}:      print("empty dict is falsy")

# Everything else is truthy
if 42:          print("non-zero is truthy")
if "hello":     print("non-empty string is truthy")
if [1, 2]:      print("non-empty list is truthy")

Go and Java don't have truthy/falsy — conditions must be explicit booleans:

// Go: if x {} — COMPILE ERROR (x is int, not bool)
if x != 0 { } // Correct

// Java: if (x) {} — COMPILE ERROR (x is int, not boolean)
if (x != 0) { } // Correct

Conditional Statements¶

if / else if / else¶

Go¶

score := 85

if score >= 90 {
    fmt.Println("A")
} else if score >= 80 {
    fmt.Println("B")
} else if score >= 70 {
    fmt.Println("C")
} else {
    fmt.Println("F")
}
// Output: B

Java¶

int score = 85;

if (score >= 90) {
    System.out.println("A");
} else if (score >= 80) {
    System.out.println("B");
} else if (score >= 70) {
    System.out.println("C");
} else {
    System.out.println("F");
}

Python¶

score = 85

if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
else:
    print("F")

Ternary / Conditional Expression¶

Go¶

// Go has NO ternary operator
// Use if-else instead
var result string
if x > 0 {
    result = "positive"
} else {
    result = "non-positive"
}

Java¶

String result = (x > 0) ? "positive" : "non-positive";

Python¶

result = "positive" if x > 0 else "non-positive"

Loops¶

for Loop¶

Go¶

// Classic for loop
for i := 0; i < 5; i++ {
    fmt.Println(i) // 0 1 2 3 4
}

// Range-based (for-each)
nums := []int{10, 20, 30}
for index, value := range nums {
    fmt.Printf("index=%d value=%d\n", index, value)
}

// While-style loop (Go only has 'for')
n := 10
for n > 0 {
    fmt.Println(n)
    n--
}

// Infinite loop
for {
    // runs forever until break
    break
}

Java¶

// Classic for loop
for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

// Enhanced for (for-each)
int[] nums = {10, 20, 30};
for (int value : nums) {
    System.out.println(value);
}

// While loop
int n = 10;
while (n > 0) {
    System.out.println(n);
    n--;
}

// Do-while (runs at least once)
do {
    System.out.println("runs once");
} while (false);

Python¶

# For loop (over range)
for i in range(5):
    print(i)  # 0 1 2 3 4

# For-each
nums = [10, 20, 30]
for value in nums:
    print(value)

# With index
for index, value in enumerate(nums):
    print(f"index={index} value={value}")

# While loop
n = 10
while n > 0:
    print(n)
    n -= 1

# Python has no do-while

break and continue¶

Go¶

// break — exit loop
for i := 0; i < 10; i++ {
    if i == 5 {
        break // stops at 5
    }
    fmt.Println(i) // 0 1 2 3 4
}

// continue — skip iteration
for i := 0; i < 10; i++ {
    if i%2 == 0 {
        continue // skip even numbers
    }
    fmt.Println(i) // 1 3 5 7 9
}

Java¶

for (int i = 0; i < 10; i++) {
    if (i == 5) break;
    System.out.println(i);
}

for (int i = 0; i < 10; i++) {
    if (i % 2 == 0) continue;
    System.out.println(i);
}

Python¶

for i in range(10):
    if i == 5:
        break
    print(i)

for i in range(10):
    if i % 2 == 0:
        continue
    print(i)

Nested Loops¶

Go¶

for i := 1; i <= 3; i++ {
    for j := 1; j <= 3; j++ {
        fmt.Printf("%d×%d=%d  ", i, j, i*j)
    }
    fmt.Println()
}
// 1×1=1  1×2=2  1×3=3
// 2×1=2  2×2=4  2×3=6
// 3×1=3  3×2=6  3×3=9

Java¶

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        System.out.printf("%d×%d=%d  ", i, j, i * j);
    }
    System.out.println();
}

Python¶

for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i}×{j}={i*j}", end="  ")
    print()

Switch/Match Statements¶

Go¶

day := "Tuesday"
switch day {
case "Monday":
    fmt.Println("Start of week")
case "Tuesday", "Wednesday", "Thursday":
    fmt.Println("Midweek")
case "Friday":
    fmt.Println("TGIF!")
default:
    fmt.Println("Weekend")
}
// Go: no fallthrough by default (opposite of Java/C)

Java¶

String day = "Tuesday";

// Classic switch (needs break!)
switch (day) {
    case "Monday":
        System.out.println("Start of week");
        break;
    case "Tuesday":
    case "Wednesday":
    case "Thursday":
        System.out.println("Midweek");
        break;
    case "Friday":
        System.out.println("TGIF!");
        break;
    default:
        System.out.println("Weekend");
}

// Switch expression (Java 14+, no break needed)
String msg = switch (day) {
    case "Monday" -> "Start of week";
    case "Tuesday", "Wednesday", "Thursday" -> "Midweek";
    case "Friday" -> "TGIF!";
    default -> "Weekend";
};

Python¶

day = "Tuesday"

# match/case (Python 3.10+)
match day:
    case "Monday":
        print("Start of week")
    case "Tuesday" | "Wednesday" | "Thursday":
        print("Midweek")
    case "Friday":
        print("TGIF!")
    case _:
        print("Weekend")

# Before 3.10: use if/elif/else or dict dispatch
messages = {
    "Monday": "Start of week",
    "Friday": "TGIF!",
}
print(messages.get(day, "Midweek"))

Code Examples¶

Example 1: FizzBuzz¶

Go¶

for i := 1; i <= 100; i++ {
    switch {
    case i%15 == 0:
        fmt.Println("FizzBuzz")
    case i%3 == 0:
        fmt.Println("Fizz")
    case i%5 == 0:
        fmt.Println("Buzz")
    default:
        fmt.Println(i)
    }
}

Java¶

for (int i = 1; i <= 100; i++) {
    if (i % 15 == 0) System.out.println("FizzBuzz");
    else if (i % 3 == 0) System.out.println("Fizz");
    else if (i % 5 == 0) System.out.println("Buzz");
    else System.out.println(i);
}

Python¶

for i in range(1, 101):
    if i % 15 == 0:
        print("FizzBuzz")
    elif i % 3 == 0:
        print("Fizz")
    elif i % 5 == 0:
        print("Buzz")
    else:
        print(i)

Example 2: Find All Pairs that Sum to Target¶

Go¶

func findPairs(arr []int, target int) [][2]int {
    var pairs [][2]int
    for i := 0; i < len(arr); i++ {
        for j := i + 1; j < len(arr); j++ {
            if arr[i]+arr[j] == target {
                pairs = append(pairs, [2]int{arr[i], arr[j]})
            }
        }
    }
    return pairs
}

Java¶

public static List<int[]> findPairs(int[] arr, int target) {
    List<int[]> pairs = new ArrayList<>();
    for (int i = 0; i < arr.length; i++) {
        for (int j = i + 1; j < arr.length; j++) {
            if (arr[i] + arr[j] == target) {
                pairs.add(new int[]{arr[i], arr[j]});
            }
        }
    }
    return pairs;
}

Python¶

def find_pairs(arr, target):
    pairs = []
    for i in range(len(arr)):
        for j in range(i + 1, len(arr)):
            if arr[i] + arr[j] == target:
                pairs.append((arr[i], arr[j]))
    return pairs

Best Practices¶

Keep conditions simple: Extract complex conditions into boolean variables

isAdult := age >= 18
hasPermission := role == "admin"
if isAdult && hasPermission { ... }

Avoid deep nesting: Use early return/continue to flatten code
Prefer for-each over index loops when you don't need the index
Always have an exit condition in while loops to avoid infinite loops
Use switch/match instead of long if-else chains for multiple values

Common Mistakes¶

Mistake	Language	Example	Fix
`=` instead of `==`	Java/Python	`if (x = 5)`	`if (x == 5)`
Missing `break` in switch	Java	Falls through all cases	Add `break` or use `->` syntax
Off-by-one in loop	All	`for i = 0 to n`	`for i = 0 to n-1`
Infinite loop	All	Forgetting to update loop variable	Ensure condition will eventually be false
Modifying collection in loop	All	Remove elements while iterating	Use separate filtered list
`!` vs `not`	Go/Java vs Python	`!condition` vs `not condition`	Know your language

Cheat Sheet¶

Conditionals¶

	Go	Java	Python
if	`if x > 0 {`	`if (x > 0) {`	`if x > 0:`
else if	`} else if {`	`} else if {`	`elif`
else	`} else {`	`} else {`	`else:`
ternary	N/A	`x > 0 ? "a" : "b"`	`"a" if x > 0 else "b"`
AND	`&&`	`&&`	`and`
OR	`\\|\\|`	`\\|\\|`	`or`
NOT	`!`	`!`	`not`

Loops¶

	Go	Java	Python
for	`for i := 0; i < n; i++ {`	`for (int i=0; i<n; i++) {`	`for i in range(n):`
for-each	`for _, v := range arr {`	`for (int v : arr) {`	`for v in arr:`
while	`for x > 0 {`	`while (x > 0) {`	`while x > 0:`
do-while	N/A	`do {...} while(cond);`	N/A
infinite	`for {`	`while (true) {`	`while True:`
break	`break`	`break;`	`break`
continue	`continue`	`continue;`	`continue`

Summary¶

Control structures are the backbone of every algorithm. Master if/else for decisions, for/while for repetition, and switch/match for multi-way branching. The logic is the same across Go, Java, and Python — only the syntax differs.

Control Structures — Junior Level¶

Table of Contents¶

Introduction¶

Prerequisites¶

Glossary¶

Core Concepts¶

Concept 1: Boolean Expressions¶

Go¶

Java¶

Python¶

Concept 2: Truthy and Falsy¶

Conditional Statements¶

if / else if / else¶

Go¶

Java¶

Python¶

Ternary / Conditional Expression¶

Go¶

Java¶

Python¶

Loops¶

for Loop¶

Go¶

Java¶

Python¶

break and continue¶

Go¶

Java¶

Python¶

Nested Loops¶

Go¶

Java¶

Python¶

Switch/Match Statements¶

Go¶

Java¶

Python¶

Code Examples¶

Example 1: FizzBuzz¶

Go¶

Java¶

Python¶

Example 2: Find All Pairs that Sum to Target¶

Go¶

Java¶

Python¶

Best Practices¶

Common Mistakes¶

Cheat Sheet¶

Conditionals¶

Loops¶

Summary¶

Further Reading¶