*mod*operator.

function digit_sum(number:longint):longint; var sum:longint; begin sum := 0; while (number > 0) do begin sum := sum + number mod 10; number := number div 10; end; digit_sum := sum; end;

To calculate the sum of the digits, we go through each digit by successively dividing the number by 10, and extracting each digit using the *mod* operator.

function digit_sum(number:longint):longint; var sum:longint; begin sum := 0; while (number > 0) do begin sum := sum + number mod 10; number := number div 10; end; digit_sum := sum; end;