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Programming/Rust/Chapter 3 - Common Programming Concepts.md

@@ -91,7 +91,78 @@ escapes), hex values, octal values, binary values, or even as a byte(b'A').
 > but instead will wrap around to the first value in the possible range.
 
 # 3.3 Functions
+Rust functions have a couple of interesting properties:
+
+1. Rust functions don't care about order. They can be in any place in the code,
+as long as the scope of things is maintained relative to where calls happen.
+
+2. Functions start with `fn`, followed by a name in snake case, then inputs,
+then the function scope using {}.
+
+*functions* is an example of some basic functions
+
+## Parameters
+Parameters can be used to pass values into functions. When this is done, two
+things must happen: First, the value must be input into the function (duh!). 
+Second, the function must *match the type defined in the function*. 
+
+Arguments are a similar thing to parameters, but have a technical difference
+that arguments are concrete values. Like add(5), vs add(x).
+
+## Statements and Expressions
+A *statement* is  a line of code that does some computation or other calculation,
+and does NOT return a value.
+
+A *expression* evaluates to a resultant value.
+
+Rust does something interesting. When defining a variable, the output is a 
+*statement*, not an expression. Using `let` to define the variable does NOT 
+return the variable itself. Defining functions are also statements.
+
+Calling functions, macros, or using a scope block created with curly brackets 
+is an expression however. Here's a mind bending example:
+
+```Rust
+fn main() {
+    let y = {
+        let x = 3;
+        x + 1
+    };
+
+    println!("The value of y is: {y}");
+}
+```
+
+This is an statement (defining main), with a statement inside (defining y) which
+has an expression inside (the statement defining x, and then the expression adding x).
+
+>[!Important] Statements, Expressions, and Semicolons
+> Notice `x+1` does **not** end in a semicolon. If it did, it would be a statement.
+> Expressions do not end in semicolons.
+
+## Functions with Return Values
+Functions can have outputs, but they do not need to be defined with `return` keywords.
+Instead, the function will by default return the last expressions result. 
+
+Functions must declare the type of the output:
+
+```Rust
+fn five() -> i32{
+    5
+}
+```
+
 
 # 3.4 Comments
 
+This one is pretty simple.
+```Rust
+// This is a single line comment.
+let x = 22; // They can be after code
+
+/* Or, if I've really got something to say
+I can use this multiple line comment */
+
+```
+
 # 3.5 Control Flow