In Rust, lifetime is a unique language feature used to manage the lifetime of references at compile time. They allow the compiler to perform static analysis to ensure that there are no invalid references or dangling (references to data that no longer exists) in the code.

Lifetime are expressed using time-to-live parameters that indicate how long a reference is valid. The basic syntax is to place a name preceded by an apostrophe (‘) before the type it refers to. For example, ‘a could be a lifetime name.

Here is a simple example to illustrate how lifetimes are used in Rust:

fn main() {
    // Create two strings
    let string1 = String::from("Hello");
    let string2 = String::from("World");

    // Call the function that takes two references to strings
    let result = longer_string(string1.as_str(), string2.as_str());

    // Print the result
    println!("The longer string is: {}", result);
}

// The longer_string function takes two references to strings with different lifetimes
fn longer_string<'a, 'b>(s1: &'a str, s2: &'b str) -> &'a str {
    if s1.len() > s2.len() {
        s1
    } else {
        s2
    }
}

In this example, the longer_string function takes two references to strings (&’a str and ‘b str). The names ‘a and ‘b are just labels to represent the lifetimes of the references. The function returns a reference to the longer string, and the lifetime of that reference is tied to the lifetime of the shorter reference (in this case, ‘a). This ensures that the returned reference is valid as long as the shorter reference (s1) is still valid.

It’s important to note that lifetimes do not affect the runtime of the program; they are a static analysis tool that helps the compiler ensure the safety of references at compile time.

In Rust, the ownership system is a key feature that enables memory safety without the need for a garbage collector. This system is based on three fundamental concepts: ownership, borrowing, and lifetimes.

Let’s take a look at a simple example to illustrate how Rust manages memory without garbage collection:

fn main() {
    let mut str = String::from("Hello Rust!");

    str.push_str(" How are you?");
    println!("{}", str);

    let str2 = takes_and_gives_back(str);

    println!("{}", str2);
}


fn takes_and_gives_back(some_string: String) -> String {
    let result = some_string + " Have a great day!";

    result
}

In this example:

We create a String and manipulate it. The ownership of the String is transferred to the takes_and_gives_back function. Inside the function, we perform operations on the String, and then the ownership is transferred back to the calling function. The original String is now owned by the variable str2. Rust’s ownership system ensures that there is always one and only one owner for a piece of data. This guarantees memory safety without the need for garbage collection because the ownership system allows Rust to statically analyze the code and ensure that references to data are always valid. If you try to use a variable after it has been moved or deallocated, the Rust compiler will catch it at compile time, preventing common memory-related bugs.

Debugging select is easy because the only thing you need to know is whether the predicate was true for a particular element. For example, a select filter that only keeps even numbers (number % 2== 0) could be debugged like this:

[1,2,3,4,5].select do |number|
  predicate = number % 2== 0
  puts "The predicate is #{predicate} for number #{number}"
  predicate
end

The predicate is false for number 1 The predicate is true for number 2 The predicate is false for number 3 The predicate is true for number 4 The predicate is false for number 5 => [2, 4]

Here, you can see that only the numbers for which the predicate was true ended up in the answer ([2,4]).

FizzBuzz

This program is used as a programming question. Write a program to go through a list of integers 1..N. If the number is divisible by 2, print “fizz”; if it is divisible by 3, print “buzz”; if it is divisible by 2 and 3, print “fizzbuzz”; otherwise, print the number. The input can be modeled as a range: (1..15).

The output should look like this:

1
fizz
buzz
fizz
5
fizzbuzz
7
fizz
buzz
fizz
11
fizzbuzz
13
fizz
buzz

(1..15).map do |n|
if n % 2 == 0
   "fizz"
else
  n.to_s
  end
end

That gives us the following:

=> ["1", "fizz", "3", "fizz", "5", "fizz", "7", "fizz", "9",
"fizz", "11", ... \ and so on

That is close, but we didn’t address the buzz:

(1..15).map do |n|
if n % 2 == 0
   "fizz"
elsif n % 3 == 0
    "buzz"
else
   n.to_s
  end
end

That gives us the following:

=> ["1", "fizz", "buzz", "fizz", "5", "fizz", "7", "fizz",
"buzz", "fizz", "11",\
"fizz", "13", "fizz", "buzz"]

That’s close, but the fizzbuzz examples didn’t happen at 6 and 12. Let’s add in another clause. To be divisible by 2 and 3 implies that it must be divisible by 6:

fizzbuzz_list =
(1..15).map do |n|
   if n % 2 == 0
   "fizz"
elsif n % 3 ==0
   "buzz"
elsif n % 6 == 0
   "fizzbuzz"
else
   n.to_s
  end
end

This yielded the same result as the previous attempt. A closer inspection of the code suggests that if the number is divisible by 6, it will always be even and will be caught by the n%2==0 clause. However, if we catch the n%6==0 clause first, what happens?

fizzbuzz_list =
(1..15).map do |n|
  if n % 6 == 0
   "fizzbuzz"
elsif n % 3 ==0
   "buzz"
elsif n % 2 == 0
   "fizz"
else
   n.to_s
   end
end

That yields the following:

=> ["1", "fizz", "buzz", "fizz", "5", "fizzbuzz", "7", "fizz",
"buzz", "fizz", "\
11", "fizzbuzz", "13", "fizz", "buzz"]

To print it, just do the following:

fizzbuzz_list.each{|e| puts e}

Debugging reduce is a bit different because the memo is carried from element to element. Therefore, you want to print out the element as well as the memo.

final_answer =
[1,2,3,4,5].reduce(0) do |memo, number|
  memo = memo + number
  puts "After operating on number: #{number}, the memo is #{memo.inspect} "
  memo
end

After operating on number: 1, the memo is 1
After operating on number: 2, the memo is 3
After operating on number: 3, the memo is 6
After operating on number: 4, the memo is 10
After operating on number: 5, the memo is 15

Again, make sure that you save the memo before printing it, then return it at the end of the block. If you are using memo to build a different data structure as a part of the reduce block, inspecting the memo becomes extremely useful in converging on a good solution.

The thing to remember is that map operates on each element of the collection for each run of the block. When debugging, you want to see how the block operated on each element.

[1,2,3,4,5].map do |number|
  result = number* number
  puts "The result of the function on element: #{number} is #{result}"
  result
end

It is important to save the result in a local variable so that you can return it as the last statement of the block. If you don’t, the result of the puts statement will be returned (usually nil).