reading-notes

Class 8 - Operators and Loops

(Expressions & Operators can be found in the Class-7 notes)

Loops and Iteration

• Loops offer a quick and easy way to do something repeatedly.
• There are many different kinds of loops, but they all essentially do the same thing: they repeat an action some number of times.
• The various loop mechanisms offer different ways to determine the start and end points of the loop.

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for Statement

A for loop repeats until a specified condition evaluates to false and looks like the following:

for ([initialExpression]; [conditionExpression]; [incrementExpression])
  statement

When a for loop executes, the following occurs:

1. The initializing expression initialExpression, if any, is executed. This expression usually initializes one or more loop counters, but the syntax allows an expression of any degree of complexity. This expression can also declare variables.
2. The conditionExpression expression is evaluated. If the value of conditionExpression is true, the loop statements execute. Otherwise, the for loop terminates. (If the conditionExpression expression is omitted entirely, the condition is assumed to be true.)
3. The statement executes. To execute multiple statements, use a block statement ({ }) to group those statements.
4. If present, the update expression incrementExpression is executed.
5. Control returns to Step 2.

function howMany(selectObject) {
  let numberSelected = 0;
  for (let i = 0; i < selectObject.options.length; i++) {
    if (selectObject.options[i].selected) {
      numberSelected++;
    }
  }
  return numberSelected;
}

const btn = document.getElementById('btn');

btn.addEventListener('click', () => {
  const musicTypes = document.selectForm.musicTypes;
  console.log(`You have selected ${howMany(musicTypes)} option(s).`);
});

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while Statement

• A while statement executes its statements as long as a specified condition evaluates to true. A while statement looks as follows:

while (condition)
  statement

• If the condition becomes false, statement within the loop stops executing and control passes to the statement following the loop.
• The condition test occurs before statement in the loop is executed. If the condition returns true, statement is executed and the condition is tested again. If the condition returns false, execution stops, and control is passed to the statement following while.
• To execute multiple statements, use a block statement ({ }) to group those statements.

let n = 0;
let x = 0;
while (n < 3) {
  n++;
  x += n;
}

Example 1

With each iteration, the loop increments n and adds that value to x. Therefore, x and n take on the following values:

• After the first pass: n = 1 and x = 1
• After the second pass: n = 2 and x = 3
• After the third pass: n = 3 and x = 6

After completing the third pass, the condition n < 3 is no longer true, so the loop terminates.

Example 2

Avoid infinite loops. Make sure the condition in a loop eventually becomes false—otherwise, the loop will never terminate! The statements in the following while loop execute forever because the condition never becomes false:

// Infinite loops are bad!
while (true) {
  console.log('Hello, world!');
}