## 3.1. The alternative 

### The if structure
```Ocaml
if cond then expr1 else expr2
```
> ⚠️ `expr1` and `expr2` have to be the same type. `cond` is a `bool`

**For exemple**
```Ocaml
# if 1<2 then "higher" else "lower" ;;
-: string = "higher"
```

**Exercise : absolute program**
```Ocaml
# let abs(x) = 
	if x>0 then x
	else -x ;;;
val abs : int -> int <fun>
```

## 3.2. Exceptions
### Division by 0:
```Ocaml
# 1/0
Exception : Division by zero
```

### Failwith:
Failwith is a function that take a string argument (the reason) and return the error. That way, we can raise an error into our code
```Ocaml
# failwith;;
-: string -> a = <fun>

# failwith "oops";;
Exception : Failwith "oops"
```

### invalid_arg:
Invalid_arg is a function that take a string argument (the reason) and return the error. That way, we can raison an error about an argument
```Ocaml
# invalid_arg "oops";
Exception: Invalid_argument "oops"

(*example*)
# let div a b = 
	if b = 0 then invalid_arg("div: b = 0")
	else a/b ;;;
val div int -> int -> int = <fun>
```

## 3.3. Filtering
### Explicit filtering

> Explicit means that you matches the value


**General syntax**

```Ocaml
match expr with pattern1 -> result1 | pattern2 -> result2 | ... | patternn -> resultn 
```
> All expressions and pattern must avec the same type

**Example**
```Ocaml
# let f x = match x with 
	0 -> 18
	| 1 -> 24
	| y ->  y + y;;

val f : int -> int = <fun>

# f 1;;
- : int = 24