const and var parameters in Delphi

Tune reads a const station name and writes back through a var parameter.

program ParamsConstVar;

// const promises the callee will not change the argument.
// var passes the caller's variable itself, so writes stick.
procedure Tune(const Station: string; var Dial: Integer);
begin
  WriteLn('tuning to ', Station);
  Dial := Dial + 3;
end;

var
  Dial: Integer;
begin
  Dial := 88;
  WriteLn('before: ', Dial);
  Tune('Radio Orkney', Dial);
  WriteLn('after:  ', Dial);
end.

How it works

1. procedure Tune(const Station: string; var Dial: Integer); mixes both kinds.
2. Dial := Dial + 3; changes the caller's own variable rather than a copy.
3. Printing Dial before and after the call shows 88 becoming 91.

Keywords and builtins used here

• Integer
• Tune
• WriteLn
• begin
• const
• end
• procedure
• program
• string
• var

The run, in numbers

Lines

18

Characters to type

388

Tokens

78

Three-star pace

75 tpm

At the three-star pace of 75 tokens a minute, this run takes about 62 seconds.

Type this snippet

Step 3 of 3 in Procedures & functions, step 12 of 27 in Language basics.

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