Loop patterns that solve real problems

Why learn patterns?

Beginners often stare at a blank editor. Experienced programmers recognise the shape of the problem first: "this is a count", "this is a search". Each shape has a standard loop. Here are the five you will use most.

1. The accumulator

An accumulator is a variable that collects a result as the loop goes: a running total or a count. Start it at the "empty" value (0 for sums, 0 for counts, '' for text) BEFORE the loop.

const bills = [450, 120, 980, 60];
let total = 0;
let bigBills = 0;
for (const b of bills) {
  total += b;
  if (b > 400) bigBills++;
}
console.log(total, bigBills); // 1610 2

2. The search

Look for the first item that matches, and stop as soon as you find it. Keep a result variable that starts as "not found" (null or -1).

const pnrs = ['4521', '8890', '1234'];
let foundAt = -1;
for (let i = 0; i < pnrs.length; i++) {
  if (pnrs[i] === '8890') {
    foundAt = i;
    break;   // no need to look further
  }
}
console.log(foundAt); // 1

3. Best so far (min and max)

Start with the first item as the best, then compare each next item and replace the best when you find a better one. Starting at 0 is a bug when all values are negative; start with the first item (or -Infinity / Infinity).

const temps = [31, 35, 29, 38, 33];
let hottest = temps[0];
for (const t of temps) {
  if (t > hottest) hottest = t;
}
console.log(hottest); // 38

4. The builder

Make a new array or string from an old one: start with [] or '' and add to it inside the loop. Later you will write the same thing with map and filter (module 4).

const names = ['asha', 'ravi', 'meena'];
const upper = [];
for (const n of names) upper.push(n.toUpperCase());
console.log(upper); // ['ASHA', 'RAVI', 'MEENA']

5. Nested loops

A loop inside a loop handles two dimensions: rows and seats, days and hours. The inner loop runs completely for every round of the outer loop, so 10 rows × 20 seats = 200 inner rounds.

for (let row = 1; row <= 3; row++) {
  let line = '';
  for (let col = 1; col <= row; col++) line += '*';
  console.log(line);
}

Off-by-one errors