Printing a Collection In Reverse

Problem Statement

Problem Statement: Write an algorithm to println a collection of elements in reverse using the "println" method for each element.

Given: Variable 'elements' that holds a collection of objects.

Writing Algorithm in Human Language

Write the algorithm in human language (English).

Human Language:

function printInReverse(list) {
	Let 'len' equal to length of 'elements'
	Let 'i' equal to len - 1
	While 'i' is equal or greater than 0
		Let 'nextElement' equal to the element in 'elements' at index 'i'
		println 'nextElement'
		Let 'i' equal to 'i' minus 1
}

Writing Algorithm in Pseudocode

Write the algorithm in pseudocode.

Pseudocode:

function printInReverse(elements) {
	let len = elements.length
	let i = len - 1;
	while (i >= 0) {
		let nextElem = elements[i]
		println(nextElem)
		i = i - 1
	}
}

//try-it
let list = [10, 20, 30, 40];
printInReverse(list);

Pseudocode Breakdown

let len = elements.length

let len = elements.length -- says to let variable "len" have the length of "elements"

let i = len - 1

let i = len - 1 -- says to let variable "i" have value "len - 1" -- this is the end of "elements", where we want the iteration to begin

while (i >= 0) {

while -- says to do a while (true) loop
(i >= 0) -- says to compare if "i" is >= "0" (the start of "elements"), the loop continues if true, or ends if false
We could think of this statement generally as "while (CONDITION)"

while (i < elements.length) {
	let nextElem = elements[i]
	println(nextElement.toString())
	i = i + 1
	}

{} -- The braces define the loop block. The block of code within the braces executes for every loop
let nextElem = elements[i] -- says to get the element at index "i" and assign to var "nextElem"
println(nextElement.toString()) -- says to print the resulting string
i = i + 1 -- says to increment var "i"

Notes:

A loop variable is the same as a local variable (the local context is the "loop block").
If the loop block contains only one statement, the braces "{}" are optional.

Given Data

Let us say that the provided array is a collection of student (objects) as follows:

elements = [Riya, Asha, Chin]

[Riya, Asha, Chin]
printInReverse(elements) {

Initialization

Before beginning the loop we initialize the length variable "len", and the loop variable.

     3
let len = elements.length
    2    3
let i = len - 1

Loop 1

The first loop.
Get the element at index "i" and assign to var "nextElem"
Print out nextElem
Lastly, decrement "i"

       2
while (i >= 0) {
          Chin              2
    let nextElem = elements[i]
                  Chin
    println(nextElement.toString())
    1  2
    i = i - 1
}

Loop 2

Second loop.
Same loop logic except loop var "i" is "1",
and "nextElem" (from elements) is thus "Asha"

       1
while (i >= 0) {
          Asha              1
    let nextElem = elements[i]
                  Asha
    println(nextElement.toString())
    0   1
    i = i - 1
}

Loop 3

Third loop.
Same loop logic except loop var "i" is "0",
and "nextElem" (from elements) is thus "Riya"

       0
while (i >= 0) {
          Riya              0
    let nextElem = elements[i]
                   Riya
    println(nextElement.toString())
   -1   0
    i = i - 1
}

LOOP ENDS

The condition "i >= 0" (-1 >= 0) is false, and thus the loop ends.

      -1
while (i >= 0) {