Feat: add functions section

Author: MahdiTa97

FUNCTIONS/Cargo.lock

@@ -0,0 +1,8 @@
+[package]
+name = "FUNCTIONS"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

FUNCTIONS/Cargo.toml

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+// Rust code uses snake case as the conventional style for function and variable names
+
+// fn main() {
+//     print_labeled_measurement(5, 'h');
+// }
+
+// fn print_labeled_measurement(value: i32, unit_label: char) {
+//     println!("The measurement is: {value}{unit_label}");
+// }
+
+// ============ Statements & Expressions ============
+// Statements are instructions that perform some action and do not return a value
+// Expressions evaluate to a resulting value
+
+// Calling a function is an expression.
+// Calling a macro is an expression.
+// Expressions do not include ending semicolons. If you add a semicolon to the end of an expression, you turn it into a statement, and it will then not return a value.
+
+// fn main() {
+//     let y = {
+//         let x = 3;
+//         x + 1
+//     };
+
+//     println!("The value of y is: {y}");
+// }
+
+// ============ Functions with Return Values ============
+// We don’t name return values, but we must declare their type after an arrow (->)
+// You can return early from a function by using the return keyword and specifying a value, but most functions return the last expression implicitly.
+
+// Example 1
+// fn five() -> i32 {
+//     5
+// }
+
+// fn main() {
+//     let x = five();
+
+//     println!("The value of x is: {x}");
+// }
+
+// Example 2
+fn main() {
+    let x = plus_one(5);
+
+    println!("The value of x is: {x}");
+}
+
+fn plus_one(x: i32) -> i32 {
+    x + 1
+}