For loops
EasyRepeat a known number of times
Problem
A for loop repeats a block a known number of times. It has three parts: initialisation, a condition checked before each pass, and an update after each pass.
Repeat something a set number of times, like counting 1 to n.
The idea
A for loop bundles three things into one line: initialise a counter, test a condition before each pass, and update the counter after each pass. Use it when you know up front how many iterations you want — typically once per element of an array.
The trick
- `for (int i = 0; i < n; i++)` visits every index exactly once; `<=` visits one too many.
- The condition is checked before the first pass, so a loop can run zero times.
- Nested loops multiply: a loop inside a loop over n elements is n² work.
—
0
i1
n5
Step 1 of 7. The counter starts at 1 and the loop runs while i ≤ 5. Values: —. i 1, n 5.
1/7
Optimal
time—space—
1for (int i = 1; i <= n; i++) {2 // runs n times, i = 1, 2, ..., n3 print(i);4}Input
- array
- [—]
Memory
- i
- 1
- n
- 5
- printed
- —
Check yourself
3 quick questions about this walkthrough. A wrong answer costs nothing.
Examples
Example 1
- Input:
- for i in 1..5: print(i)
- Output:
- 1 2 3 4 5
- Explanation:
- Three parts: start at 1, keep going while i <= 5, add 1 each turn. Change any one and you change the sequence.
Example 2
- Input:
- sum = 0; for i in 1..5: sum += i
- Output:
- 15
- Explanation:
- The loop body runs five times, and sum carries its value between turns rather than resetting.
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