array idioms

[Jonathan M Davis]

On Friday, February 25, 2011 11:30:28 spir wrote:
> Hello,
> 
> What's the idiomatic way to:
> 
> * delete an element in an associative array

If you have the key, then just use remove. The online documentation for 
asseciative array discusses it. e.g.

b.remove("hello");

If you're looking to remove by value, however, you're going to have to figure out 
what its key is. And if you want to remove _all_ elements with the same value, 
then you're going to need to find _all_ of their keys. The best way to do that 
would probably just be to use a foreach loop:

foreach(k, v; aa)
{
    if(v == value)
        aa.remove(k);
}

I'm not sure if there are any problems with removing from an associative array 
while iterating over it though. I wouldn't think so, but I don't know so. Worst 
case though, you save the list of keys to remove and then remove them all once 
you have them all.

> * delete an element in a dyn array

I don't think that there is one right now. There should probably be a function 
in std.array which does what remove in std.container would do, but I don't 
believe that there's a function for it right now. So, the simplest way at the 
moment (albeit not the most efficient) would probably do to something like

auto found = findSplit(arr, value);
auto newArr = array(chain(found[0], found[2]));

The efficient way to do it, however, would involve shifting all of the elements in 
the array, which is more complicated and not the kind of code that you really 
want to be rewriting every time that you need to remove an element. But you _do_ 
need to remember that removing an arbitrary element from an array is _not_ 
cheap, because even in the most efficient case, that means moving a potentially 
large number of elements in the array - unlike a linked list where removal is 
cheap.

Of course, popFront and popBack would be the best way to remove from the ends of 
an array, and _that_ is efficient.

> * insert an element in a dyn array

Same as remove. There's no function for doing it at the moment. The best way at 
present would probably be to just concatenate the slices.

auto newArr = arr[0 .. i] ~ [value] ~ [i .. $];

But again, that's not particularly efficient. What you'd really want to do would 
be increase the size of the array by one, shift all of the elements after the 
insertion point over by one, and then set the element at the insertion point to 
the value that you're inserting. And of course, if appending to the array, ~= 
would be the best way to do it.

And again, inserting into arrays is _not_ efficient regardless, so if you're doing 
that sort of thing a lot, you probably should be using a different type of 
container.

- Jonathan M Davis