Difference between then and then begin?

Question

I am pretty new at coding and have no idea the benefits/drawbacks/differences between then *insert code* and then begin *insert code*.

if ... then
...

if ... then
begin

`begin` starts a new statement block. If the condition evaluates to `true`, everything between `begin` and `end` will be executed. Without `begin` (and the later `end`), only the first statement after `then` will be executed. This is basic Delphi/Pascal BTW. It is great that you are trying things out, but if you don't mind my saying so, I think you would benefit from some formal instruction in the language. You may benefit from a beginner's book or course about the language. — S.L. Barth is on codidact.com, Mar 29 '19 at 10:19
[Declarations and Statements](http://docwiki.embarcadero.com/RADStudio/Rio/en/Declarations_and_Statements_(Delphi)) — J..., Mar 29 '19 at 11:08
You can always use the begin/end format, even if it contains just a single line, but for multiple lines begin/end is always required. It is a matter of style, but for me I *always* use begin/end even when it is not strictly necessary, both because I find it easier to read and because I find it easier to maintain when I am changing from a single line to multiple lines in later development. — Dsm, Mar 29 '19 at 11:33

Rudy Velthuis · Answer 1 · 2019-03-29T11:49:50.363

Delphi needs begin and end to form blocks. It does not keep blocks of code together by indentation, like e.g. Python does, it uses begin end to delineate these blocks. Languages like C, C++, C#, Java and JavaScript use { and } instead, for the same purpose.

In Pascal, the if-statement is as follows:

if <condition> then operation1

or, if there is an else-clause:

if <condition> then operation1 else operation2

Where <condition> can be any expression that has a boolean result

The operation can be a single statement, e.g.

Writeln('Hello')

or it can be a so called compound statement, which is a begin-end block with multiple statements inside (although zero or one statements are also allowed), e.g.:

begin
  Writeln('Hello');
  Writeln('World!')
end

So the difference is not between then and then begin, but between single statements and compound statements. An, admittedly rather simple, example:

if NeedOneLine then
  Writeln('Hello, World!') // single statement
else
begin                      // compound statement: multiple statements enclosed 
  Writeln('Hello,');       // in "begin" and "end"
  Writeln('World!');
end;

The above code either writes one line or two lines. The following code, that does not use begin end, looks similar, but has a totally different result:

if NeedOneLine then
  Writeln('Hello, World!') 
else
  Writeln('Hello,');
  Writeln('World!');

That will either write 'Hello, World!' or 'Hello,' and always be followed by 'World!', because it is exactly the same as:

if NeedOneLine then
  Writeln('Hello, World!') 
else
  Writeln('Hello,');
Writeln('World!');

In other words, the last line is not part of the if-statement anymore and will be executed unconditionally. That is why begin and end are important here.

Difference between then and then begin?

1 Answers1