Decisions: if & case

The dangling else binds to the nearest if.

9.1 if-then-else

The dangling else binds to the nearest if. Use begin ... end to make intent clear:

if Score >= 90 then
  Grade := 'A'
else if Score >= 80 then
  Grade := 'B'
else if Score >= 70 then
  Grade := 'C'
else
  Grade := 'F';

if A > 0 then
begin
  if B > 0 then
    Writeln('both positive');
end
else
  Writeln('A not positive');

9.2 Boolean expressions directly

// Instead of:  if X > 0 then IsPositive := True else IsPositive := False;
IsPositive := X > 0;

// Instead of:  if Done = True then ...
if Done then ...

9.3 Conditional expressions

IfThen(Cond, A, B) from Math (numbers) or StrUtils (strings) — both arguments are always evaluated.

Delphi 13 Florence adds a true conditional expression: X := if A > B then A else B;. FPC does not support it.

9.4 case

case selects on an ordinal expression with constant labels, lists, and ranges:

Labels must be constants, unique, and non-overlapping.

else (or otherwise in FPC/ISO) handles unmatched values; without it, nothing happens (ISO Pascal treats an unmatched value as an error).

Each branch is a single statement — use begin ... end for several.

FPC additionally supports case on strings; Delphi does not.

case Ch of
  'a'..'z', 'A'..'Z': Writeln('letter');
  '0'..'9':           Writeln('digit');
  ' ', #9:            Writeln('whitespace');
else
  Writeln('other');
end;

case Month of
  1, 3, 5, 7, 8, 10, 12: Days := 31;
  4, 6, 9, 11:           Days := 30;
  2: if IsLeapYear(Year) then Days := 29 else Days := 28;
end;

{$mode objfpc}{$H+}
case LowerCase(Command) of
  'start': StartServer;
  'stop':  StopServer;
else
  Writeln('Unknown command');
end;