# For...Loop

There is probably a programming language out there that does not have some sort of implementation of the for loop, but they are few and far between. It is almost a requirement in order to have the computer do code repeatedly **a certain number** of times. 

The loop contains a counter or **index** that the programmer defines. The code will run and loop back to the top as long as the "**continue condition**" is true. Each loop iteration needs to modify the index somehow or else you have an infinite loop. Typically you increase the index by 1 but you can do any sort of mathematical operation on the index.

### Syntax

 The syntax is fairly simple:

<div align="center"><img src="/files/-LPm8HWLVOdtIUBDwh1v" alt=""></div>

```javascript
for (index start value; continue condition; index modifier) {
    // code
}
```

### Example 1 - [Hello World](https://tech.io/snippet/HU3d1Dk)

```javascript
// The following prints "Hello World!" 5 times, showing the line number
// Start at 1, end before 6 (so, 5), increase by 1 every interation
for ( let index = 1; index < 6; index++ ) {

    console.log(`${index}: Hello world!`);

}
```

### Example 2 - Animation

Here is an animation of a for loop in action. The code prints the powers of 2 from a power of zero to ten (inclusive). It also keeps track of a running total and prints that at the end.

![](/files/-LQRF9dT7G220-aFU38g)

{% hint style="info" %}
For a faster animation, [click here](https://drive.google.com/file/d/0BzyP6cTwQ57eaEFjX2lkdW9zWDQ/view?usp=sharing) (and for a really fast one, [click here](https://drive.google.com/file/d/0BzyP6cTwQ57eaFBIMXAtQ05nYjg/view?usp=sharing)).
{% endhint %}

### Example 3 - [Decrement](https://tech.io/snippet/Ra72fAC)

While most examples and definitions show `i++` to *increment* the index, it does not need to be counting *up.* You can *decrement* (subtract one), add 2, multiply by a value... it all depends on the needs of the programmer. Here is an example of a for loop that counts *down* by 1:

```javascript
// Print from 20 (inclusive) to -10 (exclusive)
// Start at 20, loop if greater than -10 (so stop after -9), decrease by 1 each loop
for (let i = 20; i > -10; i--) {
    console.log(i);
}
```

### Example 4 - [Nested Loops](https://tech.io/snippet/P1NODsX)

Because a for loop can contain any amount of code (scope block `{ }`), you can write *another* for loop inside (and another inside that, etc). This is called *nested looping*. An example would be printing a multiplication table or other 2-dimensional work:

```javascript
// Print the multiplication table from 1 to 10 (inclusive)
let output;

// Rows (1 top, 10 bottom)
for (let y = 1; y <= 10; y++) {
    output = ''; // Make sure it is a string
    
    // Columns (1 left, 10 right)
    for (let x = 1; x <= 10; x++) {
        // Build the output row
        output += x*y + "\t";
    }
    
    // Print the current row
    console.log(output);
}
```

### More Information

#### If you need more information, here are some other sources:

{% embed url="<https://www.khanacademy.org/computing/computer-programming/programming/looping/pt/for-loops-a-new-kind-of-loop>" %}

{% embed url="<https://youtu.be/BxFi7vVZx4s?t=86>" %}


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