added some PG (Programming) notes:

1st-semester/pg/conditional-statements.md

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+# Conditional statements
+
+You can control your code using conditional statements:
+
+- `if`
+- `else`
+- `else if`
+- `when`
+
+The if statement evaluates the test expression inside the parenthesis ().
+
+- If the test expression is evaluated to true, statements inside the body are executed.
+- If the test expression is evaluated to false, statements inside the body are not executed.
+
+You can read it almost as an english phrase:
+
+```kt
+if (something_is_true) {
+    // execute this
+} else {
+    // if something is not true, execute else 
+}
+```
+
+Example:
+
+```kt
+if (true) {
+    println("im inside") // will run
+}
+
+if (false) {
+    println("nope") // wont run
+}
+```
+
+Example using `if`, `else if`, `else` and `when`.
+
+```kt
+val age = 18
+
+if (age > 18) {
+    println("adult")
+} else if (age == 18) {
+    println("well I guess you're an adult")
+} else {
+    println("not an adult")
+}
+```
+
+This can be simplified to:
+```kt
+val age = 18
+
+when {
+    age > 18 -> println("adult")
+    age == 18 -> println("well I guess you're an adult")
+    else -> println("not an adult")
+}
+```

1st-semester/pg/data-storage.md

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+# Data storage
+
+Declaring variables in Kotlin:
+```kt
+val age = 18
+val name = "roby"
+val price = 33.5
+```
+
+What if you dont want to give it a initial value?
+You can do that, but:
+- you have to tell the compiler what type the variable will store
+- you can not use the variable until you assign a value
+
+```kt
+val x: Int  // variable declaration that will store an Integer
+println("hello")
+x = 3       // ok
+```
+
+```kt
+val x: Int  // variable declaration that will store an Integer
+println(x)  // error, x was not defined yet
+x = 3
+```
+
+When declaring a variable without writing its type, you need to give it a initial value, so the compiler can infer its type.
+
+```kt
+val z       // error, the compiler does not know what type z is
+
+val x: Int  // variable declaration that will store an Integer
+x = 3       // ok
+
+val y: Int  // variable declaration that will store an Integer
+y = "hi"    // error, "hi" is not an integer
+
+val name = "john" // the compiler analyzes "john" and uses its type (String)
+val name: String = "john" // same as above
+
+var age: Int = "john" // error, age's type is Integer, can't store a String
+```
+
+In Kotlin there is also mutable and immutable variables:
+
+- `var` - variable that you can change its values
+- `val` - value that can not be reassigned after it's initializzed.
+
+```kt
+val x = 5   // x will store 5 (an Integer)
+x = 10      // error, you can't reassign values
+
+var name = "apple"   // name will store "apple" (a String)
+name = "melon"  // ok, name will store "melon" now
+```

1st-semester/pg/functions.md

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+# Functions
+
+Kotlin has many functions that you can use, one example is `println`, which takes a `String` argument with text that you want to print.
+
+```kt
+println("test")
+```
+
+This is how you declare your own functions:
+
+```kt
+fun main() {
+    println("hi")
+}
+
+fun add(a: Int, b: Int): Int {
+    return a + b
+}
+```
+
+The main function does not do much, just prints "hi".
+Lets see the `add` function:
+
+- function name: `add`
+- parameters: `a` and `b` (both Integers)
+- return type: `Int` 
+    - this is the type of the "data" that the function will return, in this case we return `a+b`, when you add 2 `Int`'s, it produces another `Int`
+- function body: `return a + b`
+
+How to use the function?
+
+```kt
+fun add(a: Int, b: Int): Int {
+    return a + b
+}
+
+fun main() {
+    val x = 3
+    val y = 5
+    val result = add(x, y) 
+    // result will store the return value of add (a+b = x+y = 3+5)
+}
+```
+
+This calls the function and stores the result (`a+b`) in the value `result`.
+With Kotlin you can cool stuff, like this:
+```kt 
+fun add(a: Int, b: Int): Int {
+    return a + b
+}
+
+// is the same as 
+
+fun add(a: Int, b: Int): Int = a + b // single return expression
+
+// and the same as 
+
+fun add(a: Int, b: Int) = a + b // compiler infers the type automatically
+```
+
+Another example:
+
+```kt
+// This returns true if the parameter age is >= 18
+fun isAdult(age: Int): Boolean {
+    if (age >= 18) return true
+    else return false
+}
+
+// same as putting return out of if expressions
+
+fun isAdult(age: Int): Boolean {
+    return if (age >= 18) true
+    else false
+}
+
+// same as 
+
+fun isAdult(age: Int): Boolean = if (age >= 18) true else false
+
+// same as 
+
+fun isAdult(age: Int) = if (age >= 18) true else false
+```
+
+There is also "extension" functions, where you can write new functions for a class or an interface from a third-party library that you can't modify.
+
+To declare an extension function, prefix its name with a receiver type, which refers to the type being extended. The following adds a `isAdult` function to `Int`:
+
+```kt
+fun Int.isAdult() = if (this >= 18) true else false
+
+fun main() {
+    val x = 23.isAdult() // x is "true"
+
+    val age = 17
+    println(age.isAdult()) // will print false
+}
+```

1st-semester/pg/io.md

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+# Input & Output
+
+You can use `println()` and `print()` functions to send output to the standard output (screen):
+
+```kt
+fun main() {
+    println("hello") // prints hello when you run the program
+}
+```
+
+The difference between `println` and `print` is that `println` will insert a new line after.
+
+```kt
+fun main() {
+    println("hello 1")
+    println("hello 2")
+    print("hi")
+    print("hehe")
+}
+```
+
+Output: 
+```
+hello 1
+hello 2
+hihehe
+```
+
+You can print variables by using `$`:
+```kt
+val name = "john"
+println("My name is $name") // prints "My name is john"
+```
+
+In case you wanna print a result of a function, or access object's methods/members, you need to use braces.
+
+You can print variables by using `$` and `println`:
+```kt
+fun myName(name: String) = "My name is $name"
+
+println(" ${myName("john")} ") // prints "My name is john"
+println(myName("john")) // prints "My name is john"
+println(" $myName("john") ") // error
+```
+
+To read input, you use the `readln()` function:
+
+```kt
+fun main() {
+    print("write something: ")
+
+    val input = readln()
+    println("text entered: $input")
+}
+```