The for Loop (EK 2.8.A.1)

A for loop packages all three loop-control elements into a single header, making it the preferred choice when the number of iterations is known before the loop begins. The CED defines three mandatory parts: initialization, boolean expression, and update.

for (initialization; booleanExpression; update) {
    // body: executes each time condition is true
}
// execution continues here when condition becomes false

// Most common form:
for (int i = 0; i < n; i++) {
    // executes n times, i goes 0, 1, 2, ..., n-1
}

Execution order — every iteration

int sum = 0;
for (int i = 1; i <= 5; i++) {
    sum += i;
}
System.out.println(sum);  // 15

// Trace:
// i=1: sum=1  | i=2: sum=3  | i=3: sum=6  | i=4: sum=10  | i=5: sum=15
// i=6: 6<=5 false → loop ends, sum=15

for (int i = 10; i >= 1; i--) {
    System.out.print(i + " ");
}
// Output: 10 9 8 7 6 5 4 3 2 1
// i starts at 10, decrements each iteration, stops when i < 1

for (int i = 0; i <= 10; i += 2) {
    System.out.print(i + " ");
}
// Output: 0 2 4 6 8 10
// Update of i+=2 means only even values

for-while Equivalence (EK 2.8.A.3)

Every for loop can be mechanically rewritten as an equivalent while loop by moving the initialization before the loop, keeping the condition, and moving the update to the end of the body. The AP exam tests this equivalence directly.

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

// Exact equivalent while loop
int i = 0;        // initialization moved before loop
while (i < 5) {   // same condition
    System.out.println(i);
    i++;           // update moved to end of body
}

Choosing while vs. for

for (int i = 0; i < 5; i++) {
    System.out.println(i);
    i++;   // BAD: modifies i again, prints only 0 2 4 (not 0 1 2 3 4)
}

Real-World Connection

For loops appear wherever a program processes a fixed-size collection: checking every character in a String, scanning every pixel in an image row, computing each month in a 12-month interest calculation, rendering each frame in a 60fps animation. The pattern is always the same — initialize a counter, check a bound, process, increment. Mastering the for loop unlocks all of Unit 4 (arrays and ArrayLists), where traversal is the foundational skill.

Summary

• Three parts: initialization (once), boolean expression (before each iteration), update (after each body execution). (EK 2.8.A.1, 2.8.A.2)
• Initialization runs exactly once before the first condition check.
• Update runs after the body, before the condition is re-evaluated.
• for ⇔ while: any for loop can be rewritten as a while loop and vice versa. (EK 2.8.A.3)
• Use for when iteration count is known; use while when it depends on runtime conditions.
• Off-by-one: i < n gives n iterations (0 to n-1); i <= n gives n+1 iterations.

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

int sum = 0;
for (int i = 1; i <= 5; i++) {
    sum += i * i;
}

for (int i = 10; i >= 1; i -= 3) {
    System.out.print(i + " ");
}

for (int k = 2; k <= 10; k += 2) {
    System.out.println(k);
}

int k = 2;
while (k < 10) {
    System.out.println(k);
    k += 2;
}

int k = 2;
while (k <= 10) {
    System.out.println(k);
    k += 2;
}

int k = 0;
while (k <= 10) {
    System.out.println(k);
    k += 2;
}

int k = 2;
while (k <= 10) {
    k += 2;
    System.out.println(k);
}

for (int i = 1; i <= 5; i++) {
    System.out.print(i + " ");
    i++;
}

for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 4; j++) {
        System.out.println(i + j);
    }
}

String result = "";
for (int i = 0; i < 5; i++) {
    result += i;
}

for (int i = 0; i < 10; i++)

for (int i = 1; i <= 10; i++)

for (int i = 0; i <= 10; i++)

for (int i = 10; i > 0; i--)

public static int mystery(int n) {
    int result = 1;
    for (int i = 2; i <= n; i++) {
        result *= i;
    }
    return result;
}

int total = 0;
for (int i = 1; i <= 4; i++) {
    for (int j = i; j <= 4; j++) {
        total++;
    }
}
System.out.println(total);