Nested loops

We are sometimes required to perform a loop within another loop.

Solving array problems

We can use for of to perform a loop within another loop. This function counts each occurence of a letter in our array of comments.

js

const comments = [
  "Zoja: I can't wait for the book signing!",
  "Svetlana: Let's talk about your car's extended warranty.",
  'Rufus: Drinks tonight?',
  'Malak: I just bought tickets!'
]

function search(comments, letter) {
  let count = 0

  for (const message of comments) {
    // inner loop start:
    for (const character of message) {
      if (character.toLowerCase() === letter) {
        count += 1
      }
    }
    // inner loop end.
  }

  return count
}

console.log(search(comments, 'r'))

output

7

Further example

As another example, let's use a for of loop to make some Dewey Decimal codes.

js

for (let i = 1; i < 10; i++) {
  console.log('Next genre:')
  let genre = 100 * i

  // inner loop start:
  for (let j = 1; j <= i; j++) {
    let subgenre = j
    console.log(`${genre}.${subgenre}`)
  }
  // inner loop stop.
}

output

Next genre:
100.1
Next genre:
200.1
200.2
Next genre:
300.1
300.2
300.3
Next genre:
400.1
400.2
400.3
400.4
Next genre:
500.1
500.2
500.3
500.4
500.5
Next genre:
600.1
600.2
600.3
600.4
600.5
600.6
Next genre:
700.1
700.2
700.3
700.4
700.5
700.6
700.7
Next genre:
800.1
800.2
800.3
800.4
800.5
800.6
800.7
800.8
Next genre:
900.1
900.2
900.3
900.4
900.5
900.6
900.7
900.8
900.9