# How To Reduce Redundancy

Imagine someone telling you to pick up a few things at the grocery store:

"Go to the Safeway at 50th and Brooklyn in the U-District by the Jack in the Box and pick us up a gallon of milk. Oh, also, while you're at the Safeway at 50th and Brooklyn in the U-District by the Jack in the Box you should also buy a 12-pack of Mountain Dew. It's a fun errand. The cashier guy at that Safeway at 50th and Brooklyn in the U-District by the Jack in the Box is hilarious."

Wow. Why did they have to tell us exactly where it is so many times? We got the idea the first time they clarified it. Although it's easy to see that telling someone the exact location of a Safeway several times in conversation is unnecessary in a conversation, it's easy to make the same sort of mistake in programming.

## Learning Objectives

* define redundancy
* identify examples of redundancy
* see how to reduce redundancy
* practice reducing redundancy

## What is Redundany?

`Redundancy` is any code that repeats itself unnecessarily. Redundant code can take a long time to write. Redundant code exposes programs to silly errors, like increasing the likelihood of making a typo when you type something several times, or making a change in 9/10 places, but forgetting to make a change in the last tenth place.

Programmers like to reduce redundancy for several reasons:

* code becomes easier to read
* code becomes easier to maintain
*   code may become more efficient because a computer doesn't have to

    process identical operations over and over again.
* it's easier to make changes to
* changes made in just one place are guaranteed to be consistently the same.
*   sometimes programmers just like to write code because it's shorter and

    faster to write!

Here's a collection of several examples of redundant code and what you can do to prevent redundancy.

### Flipping Boolean Values

It's super common to flip a boolean value back and forth between true and false. Instead of using a four-line if/else structure, you can use the `not` operator to turn `true` into `false` and `false` into `true`. Here's your options:

```javascript
if (booleanValue === true) {
  booleanValue = false;
} else {
  booleanValue = true;
}
```

```javascript
booleanValue = !booleanValue;
```

### Iterating Through Numbers

Here we're trying to write code that steps us through the strings cell0, cell1, cell2 ... up to cell8. We don't need to write this all out manually.

It's possible to use a for loop to step through the sequence of numbers we need, and we can use string concatenation to build up the string we need.

```javascript
document.getElementById("cell0").textContent="";
document.getElementById("cell0").style.backgroundColor= "white";
document.getElementById("cell1").textContent="";
document.getElementById("cell1").style.backgroundColor= "white";
document.getElementById("cell2").textContent="";
document.getElementById("cell2").style.backgroundColor= "white";
document.getElementById("cell3").textContent="";
document.getElementById("cell3").style.backgroundColor= "white";
document.getElementById("cell4").textContent="";
document.getElementById("cell4").style.backgroundColor= "white";
document.getElementById("cell5").textContent="";
document.getElementById("cell5").style.backgroundColor= "white";
document.getElementById("cell6").textContent="";
document.getElementById("cell6").style.backgroundColor= "white";
document.getElementById("cell7").textContent="";
document.getElementById("cell7").style.backgroundColor= "white";
document.getElementById("cell8").textContent="";
document.getElementById("cell8").style.backgroundColor= "white";
```

```javascript
for (var i = 0; i < 9; i++) {
  document.getElementById("cell" + i).textContent="";
  document.getElementById("cell" + i).style.backgroundColor= "white";
}
```

### Factoring if/else Branches

Whenever a piece of code appears in every branch of an if/else if/else statement we can factor it out so it appears just once immediately after the entire if statement.

Think of this like reducing a common denominator from a fraction. If every branch of an if/else statement is "divisible" by a statement then the statement can be "factored out" and written just just before or after the entire if/else statement.

```
6/8 = (2 * 3)/(2 * 4) = 3/4
```

```javascript
if (whosTurn == "X"){
    whosTurn = "O";
    displayX(this);
} else {
    whosTurn = "X";
    displayO(this);
}
```

```javascript
if (whosTurn == "X"){
    whosTurn = "O";
} else {
    whosTurn = "X";
}
displayO(this);
```

### Encapsulating Redundancy With Functions

These two functions repeat code defining how to set a background image. We can make a third function that accepts a parameter representing The filename to be used as the background image, and each of the original functions can pass in their filename to the old function.

```javascript
function displayX(thing) {
    thing.style.background = "url(img/cross.jpg) 0% 0% / cover";
}

function displayO(thing) {
    thing.style.background = "url(img/circle.png) 0% 0% / cover";
}
```

```javascript
function displayX(thing) {
    displayImg(thing, "cross.jpg");
}

function displayO(thing) {
    displayImg(thing, "circle.png");
}

function displayImg(thing, filename) {
  thing.style.background = "url(img/" + filename + ") 0% 0% / cover";
}
```

### Encapsulating Redundancy With Functions (again)

Create a helper function to encapsulate logical redundancy:

```javascript
if(player === status[0] && player === status[1] && player === status[2]){
  return true;
} else if (player === status[0] && player === status[3] && player === status[6]){
  return true;
} else if (player === status[0] && player === status[4] && player === status[8]){
  return true;
} else if (player === status[1] && player === status[4] && player === status[7]){
  return true;
} else if (player === status[2] && player === status[4] && player === status[6]){
  return true;
} else if (player === status[2] && player === status[5] && player === status[8]){
  return true;
} else if (player === status[3] && player === status[4] && player === status[5]){
  return true;
} else if (player === status[6] && player === status[7] && player === status[8]){
  return true;
} else {
  return false;
}
```

```javascript
if (threeInARow(player, status, 0, 1, 2) ||
    threeInARow(player, status, 0, 3, 6) ||
    threeInARow(player, status, 0, 4, 8) ||
    threeInARow(player, status, 1, 4, 7) ||
    threeInARow(player, status, 2, 4, 6) ||
    threeInARow(player, status, 2, 5, 8) ||
    threeInARow(player, status, 3, 4, 5) ||
    threeInARow(player, status, 6, 7, 8)) {
  return true;
} else {
  return false;
}

function threeInARow(player, status, i0, i1, i2) {
  if (player === status[i0] && player === status[i1] && player === status[i2]) {
    return true
  }
  return false;
}
```

### Using Variables to Reduce Redundancy

If you're copying and pasting strings, you're probably writing redundant code. In this example we can save ourselves from writing a string twice. Instead of writing on both sides of an if/else statement we write it once at the end. The two parts of it that changes are saved as values in variables.

```javascript
if (turn % 2 === 0) {
  elem.innerHTML = '<img src="img/xmark.png">';
  elem.dataset.content = "X";
} else {
  elem.innerHTML = '<img src="img/omark.png">';
  elem.dataset.content = "O";
}
```

```javascript
var filename, content;

if (turn % 2 === 0) {
  filename = "xmark.png";
  content = "X";
} else {
  filename = "omark.png";
  content = "O";
}

elem.innerHTML = '<img src="img/' + filename + '">';
elem.dataset.content = content;
```

You can also write this to assume the variables have default values for the X turn and only update them to O values if it's O's turn.

```javascript
var filename = "xmark.png";
var content = "X";

if (turn % 2 !== 0) {
  filename = "omark.png";
  content = "O";
}

elem.innerHTML = '<img src="img/' + filename + '">';
elem.dataset.content = content;
```

### Exercise 1

Identify the redundancy in this code and get rid of it by using variables.

```javascript
if (state % 4 === 0) {
  document.getElementById(this.id).textContent="X";
  this.style.backgroundColor = "red";
} else if (state % 4 === 1){
  document.getElementById(this.id).textContent="O";
  this.style.backgroundColor = "blue";
} else if (state % 4 === 2){
  document.getElementById(this.id).textContent="Z";
  this.style.backgroundColor = "yellow";
} else if (state % 4 === 3){
  document.getElementById(this.id).textContent="M";
  this.style.backgroundColor = "green";
}
```

Click to view answer ![](https://1927212806-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MXTR9vKe-DgMk10eUP8%2Fsync%2F2193f658c12616af512e9aa5791a5e01828201e5.png?generation=1617573861176730\&alt=media)

### Exercise 2

Create a function that creates an image tag and appends it to an element. The function should be parameterized so it can accept an id of the element to attach the image to, and second parameter for the filename of the iamge.

```javascript
if (xTurn == true) {
    var xImg = document.createElement("img");
    xImg.src = "img/bacon-x.png";
    document.getElementById(this.id).append(xImg);
    xTurn = false;
} else {
    var xImg = document.createElement("img");
    xImg.src = "img/egg-o.png";
    document.getElementById(this.id).append(xImg);
    xTurn = true;
}
```

Click to view answer ![](https://1927212806-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MXTR9vKe-DgMk10eUP8%2Fsync%2Ff46729af68e793043b4e6fffe36140d35488f170.png?generation=1617573861413212\&alt=media)

### Exercise 3

There's some long lines that repeat themselves several times here. Create a function to reduce some redundancy here.

```javascript
for(i=0; i<3; i++){
  if(tileStatus[i]==player & tileStatus[i+3]==player & tileStatus[i+6]==player){
    console.log(player+" matches column!");
    document.getElementById("tile"+i).className = document.getElementById("tile"+i).className + " win";
    document.getElementById("tile"+(i+3)).className = document.getElementById("tile"+(i+3)).className + " win";
    document.getElementById("tile"+(i+6)).className = document.getElementById("tile"+(i+6)).className + " win";
    return true;
  }
}
```

Click to reveal answer ![](https://1927212806-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MXTR9vKe-DgMk10eUP8%2Fsync%2Fd14283f0f2adb802580f5721a85148f834d7f1a4.png?generation=1617573861313240\&alt=media)
