Topic 2.8 — for Loops

Goal: write and trace for loops (including common counting patterns), and understand how they compare to while loops.

The big idea

A for loop is the go-to loop when you know how many times you want to repeat (or you’re counting through a range). It bundles the loop’s initialization, condition, and update in one line.

Structure

for (initialization; condition; update) {
  // repeat while condition is true
}

Initialization happens once.
Condition is checked before each iteration.
Update happens after each iteration.

Counting up (most common)

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

Runs with i = 0, 1, 2, 3, 4 (5 iterations).

Counting down

for (int i = 10; i >= 1; i--) {
  // countdown
}

Great for reverse traversals.

for vs while (when to use which?)

Use this…	When…
`for`	you’re counting / you know (or can compute) the number of iterations
`while`	you loop until something happens (sentinel/unknown number of repeats)

Tracing order (exam favorite)

The loop follows this exact cycle:

Initialization (once)
Check condition
Run body
Run update
Repeat from step 2

for (int i = 1; i <= 3; i++) {
  x++;
}
// body runs 3 times

Common mistakes

Off-by-one errors: < vs <= changes the number of iterations.
Wrong update direction: using i++ when counting down (infinite loop risk).
Changing the loop variable in the body accidentally.

// Risky: i changes twice per loop
for (int i = 0; i < 10; i++) {
  i++; // extra change
}

Exam mindset

Identify start value, end condition, and step.
List the exact sequence of i values the loop will take.
Watch for off-by-one errors.
Remember: update happens after the body.

Quick self-check

How many times does this run? for(int i=0;i<4;i++)
What are the values of i for for(int i=2;i<=6;i+=2)?
What happens if your update never changes the condition?
When is a for loop usually better than a while loop?
Where does the update run: before or after the loop body?

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