How to check if a dynamic array is empty

Question

I have a procedure declared like so:

procedure MyProc(List: Array of string);

I want to know how to check if the List parameter is empty.

For example:

procedure MyProc(List: Array of string);
begin
  if List=[] then // here I want to check if the List array is empty
  //do something
  else 
  //do something else
end;

How I can do this?

Answer 1

you can use the Length function

procedure MyProc(List: Array of string);
begin
  if Length(List)=0 then // is empty ?
  //do something
  else 
  // do something else
end;

Answer 2

Empty arrays are equal to nil:

if List = nil then // it's empty

(That also means SetLength(List, 0) and List := nil are equivalent commands.)

Empty arrays have a last index that's less than the first index, which for the open array in your example means having a negative last index:

if High(List) < 0 then // it's empty

That means that if you want to avoid running a loop on an empty array, you don't have to do anything special. Just write the loop as you normally would:

for i := Low(List) to High(List) do // won't run if List is empty

Answer 3

Personally I always write

if Assigned(List) then

rather than

if List<>nil then

because I believe it reads better, and not just for dynamic arrays.

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That answers the question for dynamic arrays, but your example is an open array, so there are two possible questions here.

For open arrays I would use Length() or high() to take decision based on the size of the array. I would not be seduced by arguments that Pointer(List)<>nil is quicker than Length(List)<>nil. The difference in speed between these options will not be discernible and so you should use the most clear and readable option.