Reversing a multidimensional array [duplicate]

Question

I want to reverse a multidimensional array and have the code below so far:

currentPositionPieceValues: [[], [], [], [], [], [], [], []]

gameDetails.currentPositionPieceValues
          .map(row => row.reverse())
          .reverse();

This seems to flip it horizontally, but not vertically, if you see what I mean.

How can I get it to do both?

Are you assigning the result to a variable? because .map is going to create a new array, not mutate the existing one (reverse, on the other hand, does mutate its array) — Nicholas Tower
– Nicholas Tower, Commented Aug 31, 2021 at 12:16
could you please share an example with sample data with it's expected output? Your code seems working fine for me. — Abhilash Augustine
– Abhilash Augustine, Commented Aug 31, 2021 at 12:17
@ChrisG That dupe target has a similar desired result, but not really the same problem. Nothing in it would explain to the asker why their very similar solution isn't working (E.g. it doesn't make any reference to the in-place modification of .reverse(), or the new array created by .map()) — DBS
– DBS, Commented Aug 31, 2021 at 12:26
@ChrisG - That's not a dupe. That's reversing the inner arrays only. This question wants both reversed. — I wrestled a bear once.
– I wrestled a bear once., Commented Aug 31, 2021 at 12:26

I wrestled a bear once. · Accepted Answer · 2021-08-31 12:19:25Z

1

You need to do:

gameDetails.currentPositionPieceValues = gameDetails.currentPositionPieceValues
          .map(row => row.reverse())
          .reverse();

This is because map creates a new array, so the outer array will get not get reversed unless you reassign it.

The inner arrays are being reversed because objects are always passed by reference in javascript.

answered Aug 31, 2021 at 12:19

I wrestled a bear once.

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6 Comments

user5734311 Over a year ago

This is 100% a dupe

epascarello Over a year ago

@ChrisG feel free to find it and mark it

I wrestled a bear once. Over a year ago

@ChrisG - Why are telling me that instead of voting to close? There are no votes to close as of the time of writing this.

epascarello Over a year ago

@ChrisG I have 5,514 close votes.... so sorry I spent 30 seconds and wrote up an answer instead of closing.... lol

user5734311 Over a year ago

It's not a 100% dupe, but it doesn't have to be. It shows the general way to do this and points out that you need to use the return value of map() which is a crucial piece of the answer. The other relevant dupe is stackoverflow.com/questions/42242257/…

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epascarello · Accepted Answer · 2021-08-31 12:19:58Z

0

Map returns a new array. So when you do the final reverse you are changing the new array, not currentPositionPieceValues.

const arr = gameDetails.currentPositionPieceValues;
arr.forEach(row => row.reverse());
arr.reverse();

answered Aug 31, 2021 at 12:19

epascarello

208k20 gold badges206 silver badges246 bronze badges

3 Comments

user5734311 Over a year ago

This is 100% a dupe...

user5734311 Over a year ago

This isn't about your record, it's about not teaching people to post instead of google, i.e. clutter up the site with duplicate content. You're not only encouraging this, you're contributing. If this is how SO works now, fine. But I don't think it does, or should.

I wrestled a bear once. Over a year ago

@ChrisG - That doesn't change the fact that the question you listed as a dupe of this one, was in fact not a dupe.

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