The for loop
Learn to write counted loops with for and use a counter.
The for loop
For tasks that repeat a known number of times you use the for loop. It has three parts: init, condition and update. for (int i = 0; i < 5; i++) { ... } runs the block 5 times; i starts at 0 and grows by one each step.
The counter usually starts at 0; this fits well with arrays. The condition is checked before each step; when it is false the loop ends. i++ runs after each step.
Counter and accumulation
Inside a for loop you can accumulate a variable to compute sums or products. The pattern int sum = 0; for (...) sum += i; gives the sum of numbers from 1 to N.
It is important to reset the accumulator BEFORE the loop; otherwise the old value stays. sum += i is a shortcut for sum = sum + i and adds i to the current value each step.
Exercise Print 1 through 5
Use a for loop to print the numbers 1 through 5 (inclusive) with Console.WriteLine. The counter must start at 1 and the condition must be i <= 5.
Show example solution
Write for (int i = 1; i <= 5; i++) { Console.WriteLine(i); }
Example solution:
using System;
class Program
{
static void Main()
{
for (int i = 1; i <= 5; i++)
{
Console.WriteLine(i);
}
}
}Test yourself
-
How many lines does for (int i = 0; i < 3; i++) Console.WriteLine(i); print?
for (int i = 0; i < 3; i++) Console.WriteLine(i);- 2 lines: 0, 1
- 4 lines: 0, 1, 2, 3
- 3 lines: 0, 1, 2
- 1 line: 3
Show answer
3 lines: 0, 1, 2 — Correct. The condition holds for i = 0, 1, 2; it stops when i = 3.
-
Complete the for header
for (int i = 0; ____; i++) { Console.WriteLine(i); }Show answer
for (int i = 0; i < 5; i++) { Console.WriteLine(i); } -
Match each for part to its meaning
- int i = 0
- Init
- i < 5
- Condition
- i++
- Update
- { ... }
- Loop body