Using a Set of Char

function IsPangram(const sentence : string) : boolean;
var
  found : set of char;
  i : integer;
begin
  found := [];
  for i := low(sentence) to high(sentence) do
    Include(found, LowerCase(sentence[i]));
  result := ['a'..'z'] <= found;
end;

Pascal's set of type is a built-in ordered collection of values from an ordinal type. set of char can hold any subset of the 256 possible char values, stored internally as a bit array.

Include adds an element to a set in-place — equivalent to found := found + [c] but potentially faster.

LowerCase converts a character to lowercase.

When working with arrays and strings, it is easy to make off-by-one errors. Indices for an array range from 0 to length(arr) - 1 inclusive. Indices for a string range from 1 to length(str) inclusive.

A convenient way to iterate over all valid indices of an array or string is to use low() to high(), which automatically gives the correct bounds.

The expression ['a'..'z'] <= found uses the subset operator <=. A set A is a subset of set B if every element of A is also in B. When found contains all 26 letters, this returns true.