Loops

Computers are good at doing things over and over again.

A LOOP is when a program does something repeatedly, until some CONDITION is met.

While Loop

The simplest kind of loop is the while loop.

The loop below counts from one to five, followed by Ha ha ha! each time.

let count = 1;
while (count <= 5) {
  // the code below runs once per loop cycle
  console.log('' + count + ', Ha ha ha!');
  count = count + 1;
}

In English this reads, "While some condition is true, do some action".

While True

To loop forever, set the CONDITION to true:

while (true) {
  // will loop forever
  // because true is always true
  console.log('Hello');
}

To stop the loop, hold down the CONTROL+C keys, CRTL+C.

One Infinite Loop

Fun Fact: The address of Apple Headquarters is 1 Infinite Loop

Wikimedia Commons, image licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license. Attribution: Joe Ravi

Counting Forever

Write the following code in a file called, count.js.

Then run the program using node count.js.

let count = 1;
while (true) {
  console.log(count);
  count = count + 1;
}

Remember: CRTL+C will stop the program

LAB: Count to One Hundred

Update count.js so that it counts from 1 to 100, inclusive.

let count = 1;
while (/* count is less than or equal to 100 */) {
  console.log(count);
  count = count + 1;
}

Solution: Count to 100

while(/*this expression is true...*/) {
  // execute this code
}

let count = 1;
while (count <= 100) {
  console.log(count);
  count = count + 1;
}

Did your program stop at 100, 101, or 99?

While Loops Breakdown

• A while statement checks the condition every cycle
  • IF condition evaluates to true, THEN then loop again
  • IF condition evaluates to false, THEN stop looping and proceed to the code after the loop

let count = 1;
while (count <= 100) {
  console.log(count);
  count = count + 1;
}

While Breakdown

1. Create a variable named count and set the value to 1
2. Begin loop if count is less than 100
3. 1 is less than 100, so run the body of the loop
4. Repeat, starting from the condition at top of the loop

let count = 1;
while (count <= 100) {
  console.log(count);
  count = count + 1;
}

While Breakdown Cont

• Step 1

let count = 1

• Step 2

while (count <= 100) { ... }

• Step 3

console.log(count);
count = count + 1;

• Step 4

while (count <= 100) { ... }

Break in a While Loop

If the program encounters the keyword break the loops stops.

Alternative code counting from 1 to 100 using break is below

let count = 0;
while (true) {
  if (count > 100) {
    break;
  }

  console.log(count);
  count = count + 1;
}

For Loops

A while loop is simple, but requires a variable in the condition or the break keyword.

There is another kind of loop called a for loop.

for (let count = 1; count <= 100; count++) {
  console.log(count);
}

For Loop Breakdown

The for (...) part of a for loop is where the loop:

1. Initializes a variable to track
2. Checks whether to run again
3. Updates the variable after the loop body runs.

for (let count = 1; count <= 100; count++) {...}

For Loop Breakdown Cont

Think about the three parts as three separate lines of code.

1. let count = 0; <- Initialize a variable to track

2. count <= 100; <- Determine whether to run again

3. line++; <- Update the variable after the loop body runs

For Loop Body

The body of the loop is the code that runs on each time through the loop

for (let count = 0; count <= 100; count++) {
  // this code will run each time we loop
  // since `count` changes each time,
  // a different number printed each cycle
  console.log(count);
}

Everything inside the curly braces is the body

LAB: One Potato

Apply what you learned about while and for loops to print a poem.

Create a file called potato.js that prints the following when executed by node.

1 potato,
2 potato,
3 potato,
4!
5 potato,
6 potato,
7 potato,
More!

Make sure to use a while or for loop in your solution

Counting Potatoes Solution (pt.1)

Separate the solution into two parts.

Create a function poemLine does only one thing; create the correct output for a given line number.

function poemLine(lineNumber) {
  if (lineNumber === 8) {
      return "More!"
  } else if (lineNumber === 4) {
      return "4!"
  } else {
      return lineNumber + " potato,"
  }
}

Counting Potatoes Solution (pt.2)

The second part of the solution only loops, based on a counter lineNumber.

function poemLine(lineNumber) {
  if (lineNumber === 8) {
      return "More!"
  } else if (lineNumber === 4) {
      return "4!"
  } else {
      return lineNumber + " potato,"
  }
}

// New Code Below

let lineNumber = 1;

while (lineNumber <= 8) {
  console.log(poemLine(lineNumber));
  lineNumber = lineNumber + 1;
}

Counting Potatoes, alternative with break

Another solution using the break to stop looping after eight cycles.

function poemLine(lineNumber) {
  if (lineNumber === 8) {
      return "More!"
  } else if (lineNumber === 4) {
      return "4!"
  } else {
      return lineNumber + " potato,"
  }
}

// New Code Below

let lineNumber = 1;

while (true) {
  console.log(poemLine(lineNumber));
  lineNumber = lineNumber + 1;
  if (lineNumber > 8) {
      break;
  }
}