Input & Output

Write outputs values; Writeln also outputs a line break.

8.1 Write and Writeln

Write outputs values; Writeln also outputs a line break. Both accept any number of arguments of simple types and strings:

Write('Name: ');
Writeln('Alice', ' is ', 30, ' years old');
Writeln;                       // empty line

8.2 Formatting with width and decimals

Value:Width right-aligns in a field; RealValue:Width:Decimals fixes the number of decimals:

Negative widths are not supported for left alignment; use Format instead.

program Formatting;
{$mode objfpc}{$H+}
var
  X: Double = 3.14159;
begin
  Writeln('[', 42:6, ']');          // [    42]
  Writeln('[', 'abc':6, ']');       // [   abc]
  Writeln('[', X:0:2, ']');         // [3.14]
  Writeln('[', X:10:3, ']');        // [     3.142]
  Writeln(X);                       // 3.1415899999999999E+000 (scientific by default)
  Writeln(True:6);                  // "  TRUE"
end.

8.3 Format — printf-style formatting

general (shortest) number with thousands separators money (currency format) left-aligned width 10

uses SysUtils;
...
Writeln(Format('%-10s|%5d|%8.2f', ['Apple', 12, 3.5]));
// "Apple     |   12|    3.50"
Writeln(Format('%x %.3d %s %%', [255, 7, 'ok']));   // "FF 007 ok %"
Writeln(Format('%1:s %0:s', ['World', 'Hello']));   // "Hello World" (argument index)

8.4 Read and Readln

Read reads values without consuming the end of line; Readln consumes the rest of the line.

Multiple values on one line: Readln(A, B, C); (separated by spaces).

Robust input: read a string, then convert with TryStrToInt.

program Greeting;
{$mode objfpc}{$H+}
var
  Name: string;
  Age: Integer;
begin
  Write('Your name: ');
  Readln(Name);
  Write('Your age: ');
  Readln(Age);              // run-time error 106 if the input is not a number
  Writeln('Hello, ', Name, '! Next year you will be ', Age + 1, '.');
end.

function ReadInteger(const Prompt: string): Integer;
var
  S: string;
begin
  repeat
    Write(Prompt);
    Readln(S);
  until TryStrToInt(Trim(S), Result);
end;

8.5 Standard files and EOF

Input and Output are predefined text files; ErrOutput (and StdErr in FPC) writes to standard error. Eof without arguments tests the standard input, which makes filters easy:

Run: linecount < data.txt.

program LineCount;
var
  Line: string;
  N: Integer = 0;
begin
  while not Eof do
  begin
    Readln(Line);
    Inc(N);
  end;
  Writeln(N, ' lines');
end.

8.6 Command-line arguments

For option parsing, see TCustomApplication (FPC CustApp unit) or FindCmdLineSwitch in SysUtils.

var
  I: Integer;
begin
  Writeln('Program: ', ParamStr(0));
  for I := 1 to ParamCount do
    Writeln('Arg ', I, ': ', ParamStr(I));
end.