# Introduction
This tutorial offers an informal introduction to the fundamental concepts and
features of the language and system. While having an interpreter at hand can
enhance your hands-on experience, all examples provided are self-contained,
allowing you to read the tutorial offline as well. Rather than aiming for
comprehensiveness by covering every single feature or even every commonly used
one, this tutorial focuses on introducing many of the most notable features. It
will provide you with a solid understanding of the language's flavor and style.
## The interactive mode
When p2sh is executed without any arguments, it enters interactive mode,
allowing users to execute commands on the fly.
```
The p2sh Programming language v0.4.0
Ctrl+D to quit
>> puts("💖💖💖 Hello, World! 💖💖💖")
💖💖💖 Hello, World! 💖💖💖
>> len("💖");
4
```
## Using p2sh as a calculator
```
>> 2 + 2
4
>> (50 - 5 * 6) / 4
5
>> 10 / 3 # Integer division
3
>> 10 % 3 # Integer modulo
1
>> 10 / 3. # Floating point division
3.3333333333333335
>> 10. / 3 # Floating point division
3.3333333333333335
>> 10 / 0
[line 1] Runtime error: Division by zero.
```
## Defining variables
Variables need to be defined before use.
```
n = 1
[line 1] compile error: undefined identifier 'n'
```
### The let keyword
```
>> let n = 10;
10
>> n
10
>> let s = "Hello";
"Hello"
```
## Comments
```
>> # this is a comment
>> // this is also a comment
>> n // n is not part of the comment
10
>> n # n is not part of the comment
10
>>
```
## Arrays
```
>> let a = [1, 2, 3, 4, 5]
[1, 2, 3, 4, 5]
>> a[0]
1
```
### Out of bound array access
```
>> a[5]
[line 1] Runtime error: IndexError: array index out of range.
>> get(a, 5) # A null result is not echoed
>> get(a, 5) == null
true
>> get(a, 1)
2
```
## Maps
Note that maps are not ordered. Note the `map` keyword before the curly braces.
```
>> let m = map {"a": 1, "b": 2, "c": 3}
map {"b": 2, "a": 1, "c": 3}
>> map["b"]
1
```
### Access unknown keys
```
>> m["d"]
[line 1] Runtime error: KeyError: key not found.
>> get(m, "d")
>> get(m, "a")
1
```
## Some programming
```
let a = 0; let b = 1; let t = 0;
0
>> while a < 1000 { print("{},", a); t = a; a = b; b = t + b; } puts();
0,1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,
```