For/While Loop Concepts

For / While Loops

A loop is a sequence of statements which is specified once but which may be carried out several times in succession. The code "inside" the loop is obeyed a specified number of times, or once for each of a collection of items, or until some condition is met or indefinitely.

There are some different types of loops, These are:

For Loops (Count-controlled)
While Loops (Condition-controlled)
Foreach Loops (Collection-controlled)
Loop Essentials

For Loops

The for statement creates a loop that consists of three optional expressions, enclosed in parenthesis and separated by semicolons, followed by a statement (usually a block statement) to be executed in the loop.

An Example of how for loops work in JavaScript:


							var result = "";
for(let i=0; i<6; i++){
	
	result += i;

}

console.log(result);

// output should be "012345"

Basically, you want to count up from 0 to 5 and glue the results together. This is achieved by setting the conditions:


							Declaring a new local variable, i, as the "looping" variable.

Initializing the local variable (in this case, i=0)

The end condition (optional - in this case i<6), so we know that i will start at 0 and end at 6

Incrementing / Deincrementing the variable - (in this case its i++ ie i+=1)

So in the above statement i equal 0,1,2,3,4,5 in that order, and the statement will end after i=5

This statement helps to condense lines of code a lot more as, rather tha writing the same lines to add to "result" 5 times you do do it in one for statement.

While Loops

A While Loops statement repeatedly executes a target statement as long as a given condition is true. Here, statement(s) may be a single statement or a block of statements. The condition may be any expression, and true is any non-zero value. The loop iterates while the condition is true. When the condition becomes false, the program control passes to the line immediately following the loop.

This is an example of the while loop in Python:


							counter = 10;
while (counter > 0):
	print 'The counter is at: ', counter
	counter = counter - 1

print "Bye bye"

// When the code is executed, the output is:

// The counter is at: 10
// The counter is at: 9
// The counter is at: 8
// The counter is at: 7
// The counter is at: 6
// The counter is at: 5
// The counter is at: 4
// The counter is at: 3
// The counter is at: 2
// The counter is at: 1
// "Bye bye"

There is also a variant of the while loop, which is called the "Do/While" loop. This loop will always execute at least once. This is an example in JavaScript:


							var text = "";
var i=0;
do {
	text += "The number is " + i;
	i++;
} while (i < 0);

// Statement will STILL execute at least once.

The above statement will execute at least once even though i is equal to 0 and i has to be less than 0 to break the statement.

While loops are used mainly used for condition controls, like a repeating if statement.

Foreach Loops

The Foreach loop is used to iterate over the over the elements of the collection. The collection may be an array or a list. It executes for each element present in the array.

An Example of how Foreach Loops work in C# - Here is the given code:


							using System;

class myClass{
	
  static public void Main(){

  	Console.WriteLine("Print me:");

  	int[] myArray = new int[]{1,2,3,4,5,6,7};

  	foreach(int items in myArray){

  	  Console.WriteLine(items);

    }

  } 

}

// The output will be:

// Print me:
// 1
// 2
// 3
// 4
// 5
// 6
// 7

As you can see above, the foreach loop is similar to the earlier for loops. JavaScript uses foreach functions and for-in loops. For-in loops can be expressed as:


							var myArray = [1,2,3,4,5,6,7];

Console.log("Print me:");

var items;
for(items in myArray){
	
	Console.log(items);

}

// output is the same

The foreach and for/in loops are useful for looping through collections, and are especially effective with looping in objects and other complex data structures which have lots of collections. However, the statement's execution time is noticeably slow compared to the other statements.

Loop Essentials

There are also other keywords that can be used inside loops to make them ore dynamic and useful, and to control the flow of data.

An Example of how the break statement works in JavaScript - Here is the given code:


							/*
	The break statement can be used to break out of a loop

*/

for(let i=0; i<5; i++){

	console.log("hi"+i);
	break;

}

// Result:

// hi0

After the for loop commences, right after the console.log() line, the loop will end via the break statement. There are also continue statements, these break one iteration in a loop. This can be expressed as:


							/*
	The continue statement can be used to jump an iteration of a loop
*/

for(let i=0; i<5; i++){
	
	console.log("hello");
	continue;
	console.log("hi"+i);

}

// Result:

// hello
// hello
// hello
// hello
// hello

When the loop is executed, the first console.log() line is printed, but then the next line is the continue statement which then jumps the rest of the iteration, and begins the new iteration.

The break and continue statements, if placed very well in loops, make very good algorithms.

These are the main concepts of loops.