Ask HN: Mnemonic for shift() and unshift()?

Learn shell programming (https://tldp.org/LDP/Bash-Beginners-Guide/html/sect_09_07.ht...) or MS-DOS (https://learn.microsoft.com/en-us/windows-server/administrat...), and it won’t be a problem remembering what shift does.

Neither of those has unshift, but you can infer its meaning from knowing about shift.

I also think push and pop aren’t good names. append, respectively removelast are way better.

lifo: last in, first out (cafeteria food line) -- taking from front/top, adding to back/bottom -- enought shifts will push element out of array queue without need to 'unshift'

fifo : first in, first out (stack of plates) -- taking from front/top, adding to front/top. -- shift on "1"/push; unshift off "0"/pop. start with at least one shift/push. ideally equal number of unshifts/pops by end of task to clear the stack array.

You have these the wrong way around. LIFO is a stack, FIFO is a queue.

"append" means adding something to the back end, e.g. appendices in a book are encountered later when sequentially processing the book's pages.

"prepend" is used to describe adding something at the beginning of a sequence.

Good old "cons" has the specific technical meaning of placing data at the lowest address of a sequential memory structure...and it is shorter than "append" and "prepend."

'idice' is the specific location context to put at end.

Yes append() and prepend() would be much better names than push() and unshift().

IF JavaScript had them as built-ins it would be great, then I wouldn't need to look for mnemonics for shift() and unshift() :-)

But, I googled "prepend" and guess what: google replied

"Although it sounds correct, prepend is not an English word. It was created to sound like the opposite of "append," which means to add to the end. The correct English word is "prefix".

A slight problem with that is that "prefix" is more often used as a noun than as a verb, I think.

"Prepend" might not be queen's English. I'm American so so what.

Anyway, "cons" is a better term in technical contexts.

I agree that append() and removeLast() would be better names. But they don't exist as built-in functions in JavaScript. Therefore I need a mnemonic that makes it easy for me to remember what shift() and unshift() do.

Try

  Array.prototype.append = Array.prototype.push;

Etc

That is tempting, and I've dome similar things earlier.

But I don't want to do that. Why because: It is not a good practice to modify JavaScript built-in prototypes. If that was the recommended practice then someone else's code might do a similar thing, but assign some other function to it. Whoever writes it last overrides any previous definition.

> someone else's code

I don't use someone else's code. I use my code. But I may be the minority.

Use a namespace, then.

  Array.prototype.glAppend = Array.prototype.push;

Are you familiar with bit shifts? When you shift a 32-bit integer by one, for example, one bit will get shifted “off” the integer. In other words, the integer will have lost that bit.

Shift on an array similarly shifts an element off the array. Unshift puts an element back on the array.

Maybe more graphically: I you shift books on a shelf, the outermost book may fall off the shelf. To put it back on, you first need to “unshift” the books.

How I actually remember it is from Perl, where shift is commonly used to process the next argument from the argument array. And you rarely unshift.

The problem with a word like "shift" is that it doesn't say anything about which direction you are "shifting" the books on your shelf (or bits in your byte.) Same is true for4 unshift.

Also "unshift" does not imply in any way that you are adding a new book to your shelf, and to which side of it.

I'm not sure if you just want to complain about the naming, or are still genuinely looking for mnemonics. They may not be perfect, but that doesn’t mean they aren't helpful.

The direction is simple to remember: It’s the opposite from push/pop. I thought this wasn’t your issue. But there is another way to remember, in case you forget what direction push/pop is: Removing an item from the end of an array doesn’t require shifting (moving) the remaining elements (their indices remain the same). Removing an element from the front, however, requires shifting the elements (their indices do change). Therefore shift/unshift are the operations at the front of the array, and push/pop are the ones at the back.

The bookshelf analogy relies on "shift" being more common than "unshift" (otherwise the operations would have been named the other way around), and that shifting your books on a shelf is more common than the case where one book falls off the shelf and you have to put it back up ("unshift").

The reason using both shift and unshift is less common, as opposed to push and pop, is again due to efficiency considerations, since the latter don’t require shifting all elements. If you want to use the array as a LIFO stack, you use push and pop. If you want to use it as a FIFO queue, you use push and shift. Unshift is usually only used in more exceptional cases.

But you can do left shifts and right shifts. Your analogy works in comparison to right shifts, but it's backwards if compared to left shifts.

For the question of whether “shift” removes and “unshift” adds or vice versa, it doesn’t matter if it’s left or right shift. Yes, the bookshelf analogy doesn’t tell you at which end the removal or addition occurs. But as I said, I didn’t think this was the question being asked, and if it is being asked, then it can be answered with the index shifting and the comparison to push/pop.

Unshift, like push, adds an element to the array.

“Unshift”, like “push”, contains the letter “u”.

That's a great mnemonic, I'll "push" it to my brain :-)

I've never had a good one to tell my jr devs. Thank you for this!!!!

Certainly you will remember the difference now, after writing this.

I remember the same way I remember anything else — by memorizing.

In British English “shift” means “move,” so shifting something off an array means moving it (into a variable). Un- as a prefix indicates negation or reversing in English. So you just need to remember what shift does, and unshift does the opposite, as indicated by the word.

I think that might be the cause of some of the confusion around these terms. They're terms that use a context of those words only native English speakers would be familiar with. If you're not a native English speaker or even maybe just younger and not familiar that specific context of those terms I can see how the terminology may be confusing and counterintuitive at first.

> Certainly you will remember the difference now, after writing this.

Indeed, part of my motivation for writing this was to remember it. I have spent too much time thinking about what shift() vs. unshift() do.

The other part was that I wanted to know how others figure out when to use shift() and when to use unshift(). Thanks for everybody for your suggestions.

> Word "shift" is shorter than the word "unshift". Therefore shift() makes the array shorter.

> Word "unshift" is longer than the word "shift". Therefore unshift() makes the array longer.

This is quite useful as I too couldn't remember which is which. Thanks for sharing!

Unshift is a weird word for the weird method of the four. You’ve got a list. To use it as a stack you push and pop. To use it as a queue you push and shift. Unshift is the weird one.

Mix html with bit twiddle logic!

html < is open block, html > is close block.

< and << are logical shift left for array of bits.

> and >> are logical shift right for array of bits.

Looking at only 'logical value of 1'. Have to open/shift left before unopen/shift right to make thing correct. aka init variable to 0; shift left/push gives 1; shift right/removes aka pops the 1.

html < pushes/shifts all the commands onto stack.

html > pops all the commands todo off of the stack until reach < aka end of stack marker.

The easiest way is learning what shift does in Bash, like another comment said, but if you're not going to do that:

- C also has functions called getc() and ungetc(), and they're analogous.

- "Doing" and "undoing" are actually the same in some sense: doing causes information loss, undoing causes information gain.

as a 6y js/ts dev i never really bothered, i just end up coming back to mdn docs whenever im about to use those.

especially the internals of array sort method.

I have this problem with splice and slice too

Oof same, I'm always going back to the docs for these

I try to get by with slice() since it is simpler to understand. Semantics is "taking a slice". splice() has a meaning of not only splitting but also putting slices back together, and is used less often. I too always have to look up its definitions from docs, partly because I use it so infrequently.

I somehow remembered that with : splice is more „evil“ because it changes the original. I know, it makes no sense ..

bash is how i remember. shift consumes an argument starting at $1.

this is how it works in all languages

same concept as << (bit wise left shift)