A function is what make a code reusable, you write a piece of code once and you use it as many times as you want. These function can either be built-in or custom constructed (user-defined) functions.

Programming languages (Python, Javascript and R in this case) have built in functions. These are functions that are already available by default within the language engine for you to use. There are many of these built-in functions, just to give an example of one lets look at a function that calculates the square root of a number.

Programming languages (Python, Javascript and R in this case) have built in functions. These are functions that are already available by default within the language engine for you to use. There are many of these built-in functions, just to give an example of one lets look at a function that calculates the square root of a number.

### Built-in square root function in Python

In python, this function is available in the built-in math module.```
import math
print(math.sqrt(25))
```

### User-defined/Constructed square root function in Python

```
def sqrt_num(x):
num_sqrt = x ** 0.5
print(num_sqrt)
sqrt_num(25)
```

### Built-in square root function in JavaScript

In JavaScript, this function is available in the built-in math library.

`Math.sqrt(25);`

### User-defined/Constructed square root function in JavaScript

```
function sqrt_num(x){
return num_sqrt = x ** 0.5;
}
```

### Built-in square root function in R

In R, this function is directly available as seen below.

`sqrt(25)`

### User-defined/Constructed square root function in R

```
sqrt_num <- function (x){
num_sqrt = x ** 0.5
print(num_sqrt)
}
```

That is it!

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