Discussion on: Creating 3 Stacks With 1 Array in JavaScript

[Konstantin Rouda]

Doesn't this code override the last element in the stack instead of adding it, as this.indexOfTop returns the index of the last element?

  this.values[this.indexOfTop(stackNumber)] = value;

[Dallas Reedy]

I’m thinking the same thing… if indexOfTop points at the top-most element, then setting that same index to the new value just keeps overwriting that top-most element, no?

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Edit: I actually tried it the way I thought it should be (adding 1 to the value of indexOfTop) and it definitely does not do the right thing (the first element of every stack is empty, so everything is off by 1). And when I removed that code, it worked perfectly.

So, now my only question is: How is it working!? How can indexOfTop be used to both show the top-most element and to insert a new element?

[Dallas Reedy]

Oh. I see it now 🤦🏻‍♂️!

It works because of the line immediately preceding that line:

this.sizes[stackNumber]++;

That seems tricky – difficult to come back to later on and immediately understand how those lines work in concert. Perhaps it would be more straight-forward to swap those two lines and add 1 to the indexOfTop instead:

// insert our new value to the top of the stack
this.values[this.indexOfTop(stackNumber) + 1] = value;
// increment our stack size by 1
this.sizes[stackNumber]++;

[Emma Bostian ✨]

It is a bit convoluted eh?

[Dallas Reedy]

Yeah, maybe a little. Not too bad. Once I actually ran the code, I began to understand the cleverness at work.