** Explanation Needed ** Reverse a String

JavaScript
1

Link to challenge:

Solution:

function reverseString(str) {
  for (var reversedStr = "", i = str.length - 1; i >= 0; i--) {
    reversedStr += str[i];
  }
  return reversedStr;
}

Could someone please explain to me this line of code:

for (var reversedStr = "", i = str.length - 1; i >= 0; i--)

The explanation on the guide says:

  • Starting at the last character of the string passed to the function, you build a new string reversedStr from str .
  1. Why is var reversedStr = " "?
  2. How does it start at the last character of the string?
  3. What is the role of i-- here?

Thanks in advance!

2

ooh, that’s a bad solution.

This is a clearer version:

function reverseString(str) {
  let reversedStr = ""
  for (let i = str.length - 1; i >= 0; i--) {
    reversedStr += str[i];
  }
  return reversedStr;
}
1 Like
3

Side note - you were just on the previous algorithm challenge. I wouldn’t jump right to the answers before figuring out how to do the algorithm challenges for yourself. This section is really where you start to learn how to become a programmer.

2 Likes
4

I still don’t understand the following:

for (let i = str.length - 1; i >= 0; i--)

From my understanding, the first condition of for loop is the starting point, the second condition is the ending point, and the third condition is increment/decrement.

I understand that i-- indicates that the loop is decrementing by 1.

But how does i = str.length - 1; i >= 0; reverses the string?

5

What is str[str.length - 1]?

6

It’s best to think of the clauses in a for loop as loop (index) initializtion; loop (index) test; index adjustment rather than as starting and ending points. Each clause can be arbitrarily complex (but you should keep them as simple as possible).

In fact, the code blocks

for(let i=0; i<some_limit; i++) {
// do stuff
}

and

let i=0;
while(i<some_limit) {
// do stuff
i++;
}

are functionally equivalent.

With that out of the way, to directly answer your confusion, the for loop in the posted solution doesn’t reverse a string. So what does it do? If you can answer that, you’ll be well on your way to understanding.

jrm

7

It takes the index of the final character in the string and iterates the string till it reaches index 0?

8

Yes, looks like you got it! :slight_smile:

1 Like
9

This topic was automatically closed 182 days after the last reply. New replies are no longer allowed.