Loops (for / while / for-in)

Free PDF of this lesson — save it, print it, revise offline. The video is the same lesson on the Code Scope YouTube channel.

Introduction

Nobody writes print a hundred times. When work repeats — a table of 2, a countdown, every name in a list — programmers write a loop: a block of code Dart runs again and again until a condition says stop. This lesson gives you Dart's three loops, the rule that keeps a loop from running forever, and the two escape words, break and continue. (Roman Urdu: Loop wohi kaam baar baar karta hai — aap aik dafa likhte hain, Dart sau dafa chalata hai.)

Key points

The for loop is for a known count of rounds. Its head has three parts: start (int i = 1), condition (i <= 10 — the loop runs while this is true), update (i++ — add 1 after every round).(Roman Urdu: for loop ki teen cheezein — kahan se shuru, kab tak, har dafa kitna barhe.)
The while loop is for an unknown count: it checks the condition first, then runs, again and again. If the condition never becomes false, the loop never stops — this is the famous infinite loop, and it freezes your program.
The golden rule of while: something inside the loop must move the condition towards false (like i-- or i++). Forget the update and the loop runs forever.
for-in walks through a list item by item — no counting, no indexes: for (String name in names) { ... }. Use it whenever you have a list (Lesson D.7) and want every item once.
break leaves the loop completely, right now. continue skips only the rest of this round and jumps to the next one. Break = exit the room; continue = skip this turn, stay in the room.
Loops are where interpolation shines: print('2 x $i = ${2 * i}') prints a fresh line every round because i changes every round.

Code examples

Example 1 — The for loop prints the table of 2

void main() {
  for (int i = 1; i <= 10; i++) {
    print('2 x $i = ${2 * i}');
  }
  print('Table khatam!');
}

What it does: i starts at 1. Dart checks i <= 10 (true), prints the line, then i++ makes it 2. This repeats until i becomes 11 — the check fails, the loop ends, and the line after the loop prints.

What you will see: ten lines — 2 x 1 = 2 up to 2 x 10 = 20 — then Table khatam!. You wrote one print line; the loop produced ten. That is the whole idea.

Example 2 — while: a countdown that cannot run forever

void main() {
  int i = 5;

  while (i > 0) {
    print('Counting: $i');
    i--; // yeh line bhool gaye to loop kabhi nahi rukega
  }
  print('Start!');
}

What it does: The condition i > 0 is checked before every round. Inside, i-- makes the number smaller each time (5, 4, 3, 2, 1) until i becomes 0 — the condition turns false and the loop stops by itself.

What you will see: Counting: 5 down to Counting: 1, then Start!. Delete the i--; line and run it in DartPad to feel the infinite loop once — then bring the line back. Every programmer learns this lesson exactly this way.

Example 3 — for-in with break and continue

void main() {
  List<String> names = ['Ali', 'Sara', 'Bilal', 'Zara'];

  for (String name in names) {
    if (name == 'Bilal') {
      continue; // Bilal ko skip — agle naam par jao
    }
    if (name == 'Zara') {
      break; // Zara aate hi loop band
    }
    print(name);
  }
  print('Loop khatam');
}

What it does: The loop picks each name in order. Ali prints. Sara prints. Bilal hits continue, so his print is skipped. Zara hits break before her print line, so she never prints and the loop ends at once.

What you will see: three lines — Ali, Sara, Loop khatam. No Bilal (skipped by continue), no Zara (stopped by break). The position of break and continue inside the loop decides everything.

Quick recap

for = known rounds (start / condition / update in one head).
while = unknown rounds; you MUST update the condition's value inside, or it never stops.
for-in = every item of a list, one by one, no indexes.
break leaves the loop; continue skips only this round.

Common mistakes

Forgetting the update in a while loop — i-- or i++ missing means the condition stays true forever and the program hangs. In DartPad, use the stop/reset and fix the line.
Off-by-one in for — i < 10 runs 1 to 9 (nine rounds), i <= 10 runs 1 to 10. When the count looks one short, this is the first place to look.
Putting break where continue belongs — if you want to skip one item and keep going, break is wrong; it kills the whole loop. Ask: "rukna hai ya sirf skip karna hai?" — skip means continue.

Practice

Use a for loop to print the table of 7 (7 x 1 to 7 x 10). Then print the same table backwards (10 down to 1) by changing the start, condition and update.
Make a list of 5 student names and print only the names longer than 4 letters, using for-in and an if inside (hint: name.length tells you the length).