How while Loops Work (EK 2.7.B.1)

A while loop repeats a block of statements as long as a boolean condition remains true. The condition is evaluated before each iteration, including the very first one. If the condition is false on the first check, the body never executes at all. This is the defining characteristic that distinguishes while from other potential loop types.

while (booleanExpression) {
    // body: executes each time condition is true
    // MUST update the loop control variable here
}
// execution continues here when condition becomes false

Execution sequence — every iteration

int count = 1;            // initialize before loop
while (count <= 5) {       // check: 1<=5 true, 2<=5 true... 6<=5 false
    System.out.print(count + " ");  // prints: 1 2 3 4 5
    count++;                // update: prevents infinite loop
}
System.out.println("Done");  // always runs after loop

The Loop Body Executes Zero Times (EK 2.7.A.3)

Because the condition is checked before the body runs, a while loop may execute its body zero times if the initial condition is already false. This is not an error — it is expected behavior for a pre-test loop. Code after the loop always runs regardless.

int n = 10;
while (n < 5) {                    // 10 < 5 = false immediately
    System.out.println("Inside");  // NEVER executes
}
System.out.println("After");       // always runs → prints "After"

Common Patterns

int n = 5;
while (n > 0) {
    System.out.print(n + " ");
    n--;           // decrement toward stopping condition
}
// Output: 5 4 3 2 1

// Loop until user enters -1
int value = getInput();          // first read before loop
while (value != -1) {            // -1 is the sentinel
    process(value);
    value = getInput();          // read again inside body
}
// Terminates when sentinel is entered

Infinite Loops (EK 2.7.A.2)

An infinite loop occurs when the boolean expression never becomes false. This happens when the loop control variable is never updated, the update goes in the wrong direction, or the condition can never be reached. On the AP exam, infinite loop identification is a common spot-the-error question.

int count = 1;
while (count <= 5) {
    System.out.println(count);
    // forgot count++; → count stays 1 forever → infinite loop
}

// Intended: print 1 through 10 (10 iterations)
int i = 1;
while (i < 10) {   // BUG: stops at 9, prints 1-9 only (9 iterations)
    System.out.print(i + " ");
    i++;
}

// Fix: use <= to include the boundary
while (i <= 10) { /* ... */ }  // prints 1-10 (10 iterations)

int count = 0;
int total = 0;
while (count < 5) {
    total++;       // updates total, NOT count
    // count never changes → infinite loop
}

Real-World Connection

Every waiting mechanism in software is a while loop. A server waits for client requests while the server is running. A game loop runs while the game is not over. A download manager retries while the connection fails. A spell checker scans while characters remain in the document. The while loop is the foundational tool for any algorithm that must repeat until a condition changes — which is most real-world algorithms.

Summary

• Condition checked before each iteration, including the first. (EK 2.7.B.1)
• Zero iterations is valid when the initial condition is false. (EK 2.7.A.3)
• Infinite loop occurs when the condition never becomes false. (EK 2.7.A.2)
• Off-by-one error: < vs <= determines whether the boundary value is included. (EK 2.7.A.4)
• Three requirements: initialize the loop variable before the loop, test it in the condition, update it in the body.
• Sentinel pattern: read before the loop, test against the sentinel, re-read at end of body.

int n = 3;
while (n > 0) {
    System.out.print(n + " ");
    n--;
}

int count = 0;
while (count < 5) {
    count += 2;
}

int total = 0;
int i = 1;
while (i <= 4) {
    total += i;
    i++;
}

int x = 10;
while (x > 0) {
    x--;
}

int x = 1;
while (x != 10) {
    x += 3;
}

int x = 1;
while (x < 10) {
    x++;
}

int x = 0;
while (x < 10) {
    System.out.println(x);
}

int k = 6;
while (k > 0) {
    System.out.print(k + " ");
    k -= 2;
}

int i=1;
while(i<=10){System.out.println(i);i++;}

int i=1;
while(i<10){System.out.println(i);i++;}

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

int i=1;
while(i!=11){System.out.println(i);i++;}

int n = 15;
while (n > 0) {
    if (n % 3 == 0) {
        System.out.print(n + " ");
    }
    n -= 3;
}

int a = 1;
int b = 1;
while (a <= 20) {
    int temp = b;
    b = a + b;
    a = temp;
    System.out.print(a + " ");
}

int x = 5;
while (/* condition */) {
    x -= 2;
}

int result = 1;
int n = 5;
while (n > 1) {
    result *= n;
    n--;
}