Accumulator, Counter, Best-So-Far

Most useful loops carry state: a running total, a count, a best-so-far. Three patterns cover the majority.

Accumulator — total or concatenate:

double total = 0;
for (double p : prices) {
    total += p;
}

Counter — count events, not elements:

int passing = 0;
for (int s : scores) {
    if (s >= 60) passing++;
}

Best-so-far (extremum) — track the max or min with its seed value:

int max = Integer.MIN_VALUE;   // seed: smaller than any possible value
for (int s : scores) {
    if (s > max) max = s;
}

Seeding matters: max = 0 is wrong when all scores are negative. The opposite seed (Integer.MAX_VALUE) finds the minimum.

Searching vs counting: a search stops early (break when found); a count or total must see everything. Choosing not to break a search is leaving performance on the table; breaking an accumulation is almost always a bug.

Common mistakes, all seen in real code review this month:

• updating the loop variable inside the body (i++ twice per pass);
• accumulating into a variable declared inside the loop (reset every pass);
• < vs <= off-by-one at the boundary;
• floating-point accumulation for money (0.1 + 0.2 != 0.3) — use long cents or BigDecimal for currency (a habit worth forming now).

Next: nested loops and the shapes they draw.