The for loop
- Dart course
- Beginner
- Lesson 42/116
- 4 min
Learn to write counted loops with for.
The for loop
To repeat a code block a fixed number of times you use a for loop. The form is for (init; condition; update) { ... }. The init runs once; the condition is checked before each iteration; the update runs after each iteration.
For example for (var i = 1; i <= 3; i++) { print(i); } prints 1, 2, 3. i++ increases i by one. When the condition becomes false the loop ends.
Init, condition, update
The for loop has three parts: init (var i = 1), condition (i <= 5) and update (i++). The init runs once before the loop; the condition is checked before each iteration; the update runs after each iteration.
The update is not always i++. With i += 2 you count by twos; with i-- you count backwards. When counting backwards adjust the condition: for (var i = 5; i >= 1; i--) { print(i); } counts from 5 down to 1.
Practice Count from 1 to 5
Build a for (var i = 1; i <= 5; i++) loop. In the body print each number with print(i);. The output must be 5 lines: 1, 2, 3, 4, 5 (one number per line).
Starter code
// Count from 1 to 5 with a for loop and print eachSample solution
for (var i = 1; i <= 5; i++) {
print(i);
}Expected output
1 2 3 4 5
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How many times does this loop run?
for (var i = 1; i <= 3; i++) {
print(i);
}- 3 times Correct answer
- 1 time
- Infinitely
- 0 times
Explanation: The condition is true for i=1, 2, 3; 3 iterations run.
2 Complete the for header
for (____) {
print(i);
}for (var i = 1; i <= 5; i++) {
print(i);
}Explanation: for (var i = 1; i <= 5; i++) has all three parts.
3 Match each part to its job
| Term | Meaning |
|---|---|
var i = 1 |
Init |
i <= 5 |
Condition |
i++ |
Update |
{ ... } |
Body |