Loops: for, while, do, foreach

The four shapes

for (int i = 0; i < 5; i++) { ... }        // counted: known number of passes
while (queue.Count > 0) { ... }            // pre-test: maybe zero passes
do { ... } while (again);                  // post-test: at least one pass
foreach (string name in names) { ... }     // walk a collection

• for owns its counter — best when the count is known or derivable.
• while tests first — zero iterations is normal.
• do/while runs the body first — menus and input loops ("ask at least once").
• foreach reads any collection element-by-element — no index, no off-by-one. (Use for when you must know where you are.)

Accumulation: the loop's inner state

Every statistics tool is a loop with state:

int total = 0;
for (int i = 1; i <= 100; i++)
{
    total += i;          // state updated once per pass
}
// total == 5050

Initialize the accumulator before the loop, update it inside, read it after. Declaring the accumulator inside the loop is the classic beginner bug — it resets every pass.

break and continue

break exits the loop now; continue skips to the next pass. Use both sparingly — a for with a clear condition plus an occasional break for "found it" reads fine; a maze of breaks signals the loop wants restructuring.

Off-by-one discipline

i < n visits n items (indices 0..n-1); i <= n visits n+1. Arrays and strings are 0-indexed: a 5-item array's last index is 4. When a loop crashes with IndexOutOfRangeException, look at the boundary first.