I'm doing an algorithm course and here is the instructor's answer about how to reverse an array without using reverse js method:
function solution(arr) {
for(var i=0; i < arr.length/2; i++) {
var tempVar = arr[i]
arr[i] = arr[arr.length - 1 - i]
arr[arr.length - 1 - i] = tempVar
}
return arr
}
I did understand everything, EXCEPT this detail:
arr.length/2
In this line below:
for(var i=0; i < arr.length/2; i++) {
what does it mean? What its purpose?
To reverse a string, you have to swap characters of first half of the string with the last half.
let str = 'abcde';
You have to swap a with e, b with d.
ab is the first half of the string. So simply run loop over the first half of the string and swap ith character with arr.length - 1 - ith character as below
var tempVar = arr[i]
arr[i] = arr[arr.length - 1 - i]
arr[arr.length - 1 - i] = tempVar
For reverse a string, you have to swap first half of the string with the last half then for string abcdef your result will be cdeabc - which is not reversed string.For reverse a string, you have to swap characters of half of the string with the last half - also is not correct.The algorithm swaps two elements that are on equal distance from both ends of the array. The number of operations needed is number_of_operations = number_of_elements / number_elements_operated_on and since it's doing two elements at once, that's number_of_elements / 2. And hence the reason to use arr.length / 2 as the limit of the for loop. Here is a representation of what happens.
Given an array [1, 2, 3, 4, 5, 6] then array.length is 6 and the following operations are performed:
//loop i = 0, 0 < 3 == true, execute
[1, 2, 3, 4, 5, 6] -> [6, 2, 3, 4, 5, 1]
^--------------^ ^--------------^
//loop i = 1, 1 < 3 == true, execute
[6, 2, 3, 4, 5, 1] -> [6, 5, 3, 4, 2, 1]
^--------^ ^--------^
//loop i = 2, 2 < 3 == true, execute
[6, 5, 3, 4, 2, 1] -> [6, 5, 4, 3, 2, 1]
^--^ ^--^
//i = 3, 3 < 3 == false, loop stops
This works perfectly fine with odd number of elements, since there is going to just be one element in middle when you get to it.
Given an array [1, 2, 3, 4, 5] then array.length is 5 and the following operations are performed:
//loop i = 0, 0 < 2.5 == true, execute
[1, 2, 3, 4, 5] -> [5, 2, 3, 4, 1]
^-----------^ ^-----------^
//loop i = 1, 1 < 2.5 == true, execute
[5, 2, 3, 4, 1] -> [5, 4, 3, 2, 1]
^-----^ ^-----^
//loop i = 2, 2 < 2.5 == true, execute
[5, 2, 3, 4, 1] -> [5, 4, 3, 2, 1]
^ ^
//i = 3, 3 < 2.5 == false, loop stops
Algorithm start with first and last element and swap them. Next it take the second element from the begin and from the end and swap them. And etc it swap all the elements with same distance from center.
So algorithm go to the center of array from both sides. And it need only half of length of array from both side to proceed. So that statement arr.length/2 actually the expression which is a half of length.
Which is used as limit of the loop.
forloop stops. Are you familiar withforloops or is it just this algorithm which is not clear?