Have you heard about Competitive programming ?

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Competitive Programming

If you are looking for job in Google,Amazon or facebook as programmer or developer/designer, definitely you will be familiar with the word "Competitive programming ".

I thought of sharing some of my thoughts on "competitive programming ".
As per wikipedia - Competitive programming is a mind sport usually held over the Internet or a local network. https://en.wikipedia.org/wiki/Competitive_programming

As per definition it’s a “sports”

I heard about hackerrank 3 or 4 years back. I got surprised seeing the success of hackerrank. Definitely its very useful website those who are trying to understand different algorithms/concepts/problems in computer science. After that there was a rain of similar websites … Codechef, CodeByte etc …etc … I don’t know which one of these came first.

But I would suggest nothing harm in login those websites and try to solve some of the problems . Don’t get disappointed /discouraged if you are not able to solve the questions. you don’t need to be a competitive programmer to be a programmer.

I am not huge fan of competitive programming. But it improves your programming skill in multiple perspective and definitely you might need to have this skill for getting into Product based Software companies like Google, Amazon and facebook etc . When I started trying to solve some of the problems in code force , I thought I would require PHD in mathematics to understand the problems . Most of them will be really tough mathematical problems. But if you start from easy , medium and then to difficult once, I hope you won’t need to run away from it :-)

Let me start with advantages of Competitive programming

• You are going to get good understanding on the Algorithms and data structures .

• You will get to know different areas in computer science - number theory,set theory,graph theory,string analysis etc. If you ask me , Are these mathematics subjects? I would need to say “YES”. But the fact is you need to have good understanding on these areas in order to develop something unique. ( This is debatable subject) , you can develop innovative or useful applications without much deep understanding on these areas.

• definitely it’s a plus point for you to get into Software companies.

• This gives you the ability to approach problems in different ways and helps to get the optimized one.

• Definitely you will come to time complexity O(n) and space complexity in computer programming :-)

• Some of the competitive programming sites gives you only limited time to solve the problem. When you start solving the problems in time bound question ,this helps a lot in interviews. You will be able to face the software companies interviews more efficiently.

I think I wrote some advantages , so let me mention some of the websites you can try this .

• Codechef
• Codecup
• Codeforces
• CodeGround
• CodeJam
• DoSelect for developers
• GeeksForGeeks
• GeekyPrep.com
• Hackerearth
• Hackerrank
• InterviewBit
• Kattis
• LeetCode
• Programming Hub
• Spoj
• TechGig
• Techie Delight
• Topcoder
• UVa Online
• FreeCodecamp

In my opinion you don’t need to be competitive programmer to become a programmer/developer.

Competitive programming Drawbacks:

• First of all , its not a technology or programming style or skill that every programmer is needed. Everybody know we need good algorithms to perform the program well. Reading,writing,debugging programs will makes you skillful programmer. Sometimes competitiveness makes to take decision immediately or jump into conclusion quickly.

• For getting into competitive programming there are some prerequisites - good mathematical knowledge, Computer languages that the websites are using, good knowledge in computer science concepts.

• If you are trying to learn a programming language from basics (eg: C programming ), I don’t think those sites are the starting point. Those who are good in C, C++,python etc languages , these websites will be a good place to play around.

• Competitive programming will not take you to most of the computer applications areas .
  eg: Those who are interested in embedded systems , will not get any knowledge on peripheral or architecture of the micro controller/processor from this. or those who are all interested website designing and development not going to get much from competitive programming apart from understanding on the algorithms or logical thinking.

• Nowadays Application building is like “building blocks” game. More you know about libraries and modules on the area, you will be succeeded in building some good applications.

• Software life cycle is totally different - As you know it start with requirement gathering,designing,coding,unit testing, functional testing and User acceptance testing and deployment etc. I don’t see competitive programming give much importance to designing and collaboration and testing.

• If you look at object oriented programming or functional programming , they are a programming style that every programmer should be aware of .

• You can build innovative applications without competitive programming skills.

Conclusion

Please don’t consider Competitive programming as waste of time. It will definitely improve knowledge on algorithms and computer theories. Keep improving your programming skill on competitive perspective as well. Also be ware of that there are lot of other areas in programming where we can focus on application building or we can playground. As I said earlier You don’t need to be competitive programmer to be developer/programmer.

Please share your thoughts and comments. Thanks!

Rust Iterators

Iterators in Rust programming

Nowadays Iterators are every programmer’s tool in hand.When I program array of objects/strings in C programming , I always wish that I could have some tool/function/operator where we can iterate through these objects or strings(words). “for” loop or “while” loop are going through array index in C programming .

If you are storing words in Array it become 2 dimensional array. So iterating through “words” become even more difficult in C programming .But it will be convenient if you can iterate through the types values/objects in the array. eg: Array(“I”,“like”,“rust”,“programming”) …if you can just iterate through the words rather than reading through the letters, it will be easy to make the words manipulations.

It is Functional programming style where we can just tell the program to iterate through the specified type .
Another use case would be eg: In the given Array (2,4,6,8) of elements if we want to find cube for all the elements in the array and then filter if cube is even number.

For conventional Loop structure we will be iterating through each element and find cube then check and filter if it is even. How great it would be if we can have something like below give us the same output number/maths calculations

example: array.iter().filter(x/2==0).collect()

eg2: words or sentence manipulation

array.iter().filter( words_contains_vowels)

It improves the readability of the program a lot. This functional programming style improves the readability ,but at the same time we can’t compromise the performance . Since they are as fast as our native loops , they are “Zero cost abstraction” in Rust ## iter from standard library The standard library std::iter provides Trait is Iterator. You want to build iterator any Struct ,

you can implement Iterator on your collection with below trait

trait Iterator { type Item fn next(&mut self) ->Option(Self::Item); }

In the below example, suppose if I have an input stream coming from outside and I want to iterator over word by word

//the structure which store the words and index of the words
//note :Since we are using String slice reference , the life time needs to provided explicitly 

struct Mystream<'a>{
    words: Vec<&'a str>,
    index:usize
}

//implementing the structure with methods "new" and "next_word"
//"new" method create a structure object with words generated from  stream
//next_word method calls next method in the Iterator trait

impl <'a>Mystream<'a>{

//"new" method get the string and split them into words vector
//index will be zero for object, but we will increment it in each iteration and use it as indexing the vector
    fn new(stream:&'a str) -> Mystream{
        Mystream{
             words : stream.split(' ').collect(),
             index:0,
        }
    }

//next_word method calls the "next" method from the iterator trait    
    
    fn next_word(&mut self) ->Option<&'a str>{
            self.next()
    }

}

//standard implementation for Iterator for "Mystream" struct

impl <'a>Iterator for Mystream<'a>{
    type Item = &'a str;
    
    fn next(&mut self) -> Option<&'a str>{
            self.index +=1;
            match Some(self.words[self.index]){
                Some(word) => Some(word),
                None => None,
            }
    }
}



fn main(){
    
    let mut mystream = Mystream::new("I like to implement Rust Iterator for my stream");
    let mut len = mystream.words.len();
    println!("{:?},{}",mystream.words[0],len);
    len = len -1;
    while len > 0 {
    
         println!("{:?},{}",mystream.next_word().unwrap(),len);
         len = len -1;
    }
    
}

Not sure above example is tough one, but let me try to explain it… Let assume we have a stream of words coming from input port, our task is to store it in a struct as words .(ie: split the sentence into words). I tried to explain most of the things in the comment section. But still we will go through it … The implementation of Iterator for “Mystream” struct is important part in the code. For understanding Iterator implementation you should know what is trait and struct creation and implementation of struct in rust

If look at the Iterator trait documentation, they ask us to create an associated type Item (Associated types are a way of associating a type placeholder with a trait such that the trait method definitions can use these placeholder types in their signatures.) and required method “next”

The next method passing parameter and return parameters are mentioned in the example.

As a general rule , for any Trait Rust documentation you will get the required methods or associated types needs to implemented for using it for your struct. Also we need to implement that Trait for our struct.
This case,

impl <'a>Iterator for Mystream<'a>

Since we are using vector of string slice(a reference type), we need to provide life time for it and struct.

Here we have given the associated type Item as &'a str (string slice) which means we are planning to iterate through string slice when the next method calls.

Return type for next method is Option <Self:Item> which means we need use match keyword to return Some(word) if there is value and None if nothing.

In the case Collection types: we can use the
Array,Vec, Hash etc with iter metod.

use std::collections::HashMap;

fn main(){
    let arr = [1,2,3];
    let v = vec![4,5,6];
    let mut h = HashMap::new();
    h.insert(1,"naveen");
    h.insert(2,"davis");
    h.insert(3,"tom");
    println!("{:?}",arr.iter());
    
    for i in arr.iter(){
        println!("{}",i);
    }
    
    for i in v.iter(){
        println!("{}",i);
    }
    
    for (i,name) in h.iter(){
        println!("{:?}",name);
    }
    
    println!("{:?}",arr.iter().next()); //this is going print only first element
    println!("{:?}",arr.iter().next()); //this is going print only first element
    println!("{:?}",arr.iter().next()); //this is going print only first element
    println!("{:?}",arr.iter().next()); //this is going print only first element
    
    println!("{:?}",h.iter().next());  //this is going print only first element
    println!("{:?}",h.iter().next());  //this is going print only first element
    println!("{:?}",h.iter().next());  //this is going print only first element
    println!("{:?}",h.iter().next());  //this is going print only first element
}

Iter

We can iterate through collection in 3 ways

• iter(), which iterates over &T.
• iter_mut(), which iterates over &mut T.
• into_iter(), which iterates over T.

iterate through immutable reference , in this case we are not supposed to change the value and but we using only reference to value

Rule one: any borrow must last for a scope no greater than that of the owner
Rule two: you may have EITHER 1+ immutable borrows OR EXACTLY 1 mutable borrow

What happens when we compile this code ?
If you have already gone through some Rust program before, you will shout immediately  "compile time" error. As the vector moved in the first for loop and ownership has transfered, so when you try to use it again in the main the value was already dropped. We can use reference to the values to avoid this problem.

True . Rust won't allow to 
fn main() {
   
   let v = vec![3,4,5,7];
   
   for i in v {
      
            println!("{:?}",i);
   }
   
   for i in v {
       println!("{}",i);
   }
   
}

```rust_errors
use of moved value: `v`
  --> src/main.rs:10:13
   |
5  |    for i in v {
   |             - value moved here
...
10 |    for i in v {
   |             ^ value used here after move

In the below example , we solve above problem passing reference to for loop,
You can either use v.iter() or &v

fn main() {
   
   let v = vec![3,4,5,7];
   
   for i in &v {
      
            println!("{:?}",i);
   }
   
   for i in v.iter() {
       println!("{}",i);
   }
   
   println!("{:?}",v);
   
}

If you really want to pass the ownership to the for loop,
use either v or v.into_iter()

fn main() {
   
   let v = vec![3,4,5,7];

   for i in v.into_iter() {
       println!("{}",i);
   }
   
   println!("{:?}",v);
   
}
```rust_errors
 for i in v.into_iter() {
   |             - value moved here
...
11 |    println!("{:?}",v);
   |                    ^ value borrowed here after move
   |
   = note: move occurs because `v` has type `std::vec::Vec<i32>`

Another scenario is you want to update or change the value inside the for loop
you can either pass &mut v or you can use v.iter_mut()

fn main() {
   
   let mut v = vec![3,4,5,7];
   
   for i in &mut v {
      
        *i +=1;
       println!("{:?}",i);
   }
   
   println!("\nsecond iteration to change values\n");
   for i in v.iter_mut() {
      
        *i +=1;
       println!("{:?}",i);
   }
   
   println!("{:?}",v);
   
}

IntoIterator

If you want to use ’ for’ loop in your type, you will need to build the trait IntoIterator for the type.

We will see how to implement an IntoIterator for our type.

Let build a ‘for’ loop for your Struct

How nice it would be if you can do a for loop which iterate through

[derive(Debug)]
struct Sentence{
    words: Vec<String>,
}

impl IntoIterator for Sentence {
    type Item = String;
    type IntoIter = std::vec::IntoIter<std::string::String>;
    fn into_iter(self) -> Self::IntoIter {
        self.words.into_iter()
    }
}


fn main() {

    let sentence = Sentence { 
        words: vec!["one".to_string(), "two".to_string()]
    };
    
    for word in sentence{
        println!("{}",word.contains("w"))
    }
   
}

Standard Output
false
true

In the above example we are creating Sentence Struct which our type. In that we are storing words as Vector elements.

As we discussed in IntoIterator trait implementation, we need to implement the trait for our type Sentence

So two associated types are required ,

one is Item which is the item we are iterating through.
second is IntoIter ie in which type we are iterating through. here it is std::vec::IntoIterstd::string::String

Note: if it were a vector of integers , it would be std::vec::IntoIter

The required method for IntoIterator trait is into_iter which is also implemented with Rust document mentioned parameter and return type.

Trait in rust

Trait in Rust programming

I believe people those who are from object oriented programming languages understand the usage of Trait easily . I have seen people saying Trait are similar to Interfaces in Java.
But we will start with zero perquisites or background knowledge on those. Only prerequisite would be to basic understanding on how “Struct” or user defined data types are created in Rust.

Trait - Collection of methods defined for unknown types.

The definition for Trait is an abstract over behavior that types can have in common.
don’t worry , if we are not able to digest the definition fully.
You might have noticed that in the real world lot of objects shares same behavior and they give the output to the behavior differently.
eg: Car and Bus can provide “ride”
Cat and Dog can have “talk” method
in a Game different objects can have “bonus”
We know these methods/functions are needed for these objects ,but they should produce different output for them differently for different types of objects.
eg : when Cat call “talk” method output would be “Meow Meow”
when Dog call talk method output would be “woof woof”
Also we don’t want other types of objects calling the method if they are not implemented
eg: in my case Cow can’t talk
It would be great if I get noticed in the compile time itself that there is no implementation for Cow for “talk” . rather than future at some point of time in the future the code goes through that piece code and fails badly.
So we can identify the shared behavior of objects in the design phase of your project, the ideal way of defining these common behavior will be through “Trait”
Now you have understood the use case of Trait .
I think we can jump into an example
example creates 3 structs for Cat, Dog and Cow

/*We create/define a trait ( unknown type) using keyword "trait" called Speaker and define the behavior of the trait.
in this example trait accept the unknown type which implements the trait and returns nothing.
Trait name is Speaker and method defined is speak. So we are going to call this "speak" method for  the struct object implements this.
*/

pub trait Speaker {
     fn speak(&self) -> ();
 }

/*Next step would be implementing the trait for whichever types have the common  behavior.
This case our Cat and Dog speaks. 
so implemented them.*/

impl Speaker for Cat { 
 fn speak(&self) { println!("meow!"); }
 }

/* Now we can call them from "main" function as it is in scope.First we need to create struct object and then call the trait defined method (speak) with dot(.) operator. */

cat1.speak();

pub struct Cat{
    color:(u8,u8,u8),
    
}
pub struct Dog{
    color:(u8,u8,u8),
}

pub struct Cow{
    color:(u8,u8,u8),
}


pub trait Speaker {
     fn speak(&self) -> ();
 }



 impl Speaker for Cat {
     fn speak(&self) {
           println!("meow!");
     }
 }

 impl Speaker for Dog {
     fn speak(&self) {
           println!("woof!");
     }
 }
 
fn main(){
    let cat1 = Cat{color:(30,30,30)};
    let dog1 = Dog{color:(30,30,30)};
    let cow1 = Cow{color:(30,30,30)};
    cat1.speak();
    dog1.speak();
    //cow1.speak(); fails in compile time saying "no implementaton of the trait"
}

Trait Bounds
Trait bounds restricts the function to have the types which implements the trait.So the function can not be called with any other types.Code that calls the function with any other type, like a String or an i32, won’t compile, because those types don’t implement "Speaker"

pub struct Cat{
    color:(u8,u8,u8),
    
}
pub struct Dog{
    color:(u8,u8,u8),
}

pub struct Cow{
    color:(u8,u8,u8),
}


pub trait Speaker {
     fn speak(&self) -> ();
 }



 impl Speaker for Cat {
     fn speak(&self) {
           println!("meow!");
     }
 }

 impl Speaker for Dog {
     fn speak(&self) {
           println!("woof!");
     }
 }

pub  fn notify(speaker: S) {
        speaker.speak();
}

fn main(){
    let cat1 = Cat{color:(30,30,30)};
    let dog1 = Dog{color:(30,30,30)};
    let cow1 = Cow{color:(30,30,30)};
    notify(cat1); 
//    notify(cow1); uncomment this line to see the compile time error.
}

In Rust a trait must be in scope for you to be able to call its methods.
Reference
https://doc.rust-lang.org/book/ch10-02-traits.html