Basic Pascal Tutorial/Chapter 3/IF

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3Ca - IF (author: Tao Yue, state: changed)

The IF statement allows you to branch based on the result of a Boolean operation.

Format

IF expression THEN

statemrnt1

[ ELSE

statement2]

Where

expression is any comparison, constant or function which returns a boolen value, and

statement1 and statement2 are either a single statement, a Begin-end block, a repeat-until block, or the null statement.

The ELSE clause is optional.

There are two ways to use an IF statement, a one-way branch or a two-way branch/

One-way branch

The one-way branch format is:

if BooleanExpression then
  StatementIfTrue;

If the Boolean expression evaluates to true, the statement executes. Otherwise, it is skipped.

The IF statement accepts only one statement. If you would like to use a compound statement, you must use a begin-end frame to enclose the statements:

if BooleanExpression then
begin
  Statement1;
  Statement2;
end;

Two-way branch

There is also a two-way selection:

if BooleanExpression then
  StatementIfTrue
else
  StatementIfFalse;

Note there is never a semicolon ; immediately before the else.

if BooleanExpression then
begin
  Statement1;  // semicolon here is mandatory
  Statement2;  // semicolon here is optional
end    // semicolon here is forbidden
else
begin
  Statement3;
  Statement4;
end;

If the Boolean expression evaluates to FALSE, the statement following the else will be performed. Note that you may never use a semicolon after the statement preceding the else. That causes the computer to treat it as a one-way selection, leaving it to wonder where the else came from. And when a compiler wonders, it usually gets mad and throws a tantrum, or rather, it throws an error

If you need multi-way selection, simply nest if statements:

if Condition1 then
  Statement1
else
  if Condition2 then
    Statement2
  else
    Statement3;

Be careful with nesting. Sometimes the computer won't do what you want it to do:

if Condition1 then
  if Condition2 then
    Statement2
else
  Statement1;

The else is always matched with the most recent if, so the computer interprets the preceding block of code as:

if Condition1 then
  if Condition2 then
    Statement2
  else
    Statement1;

You can get by with a null statement:

if Condition1 then
  if Condition2 then
    Statement2
  else
else
  Statement1;

Or you could use a begin-end block.

The following proves a semicolon is absolutely forbidden before an else:

// Paul Robinson 2020-12-16

// Compiler test program  Err03.pas
// tests the proposition that ; is
// never legal before ELSE


program err03;
Var
    Test,test2: Boolean;


Begin

    Test := True;
    Test2 := true;

    if test then
       if test2 then
           Writeln('Reached Part 1');  // semi-colon here should be illegal
     else
        Writeln('Reached Part 2');

end.

But the best way to clean up the code would be to rewrite the condition.

if not Condition1 then
  Statement1
else
  if Condition2 then
    Statement2;

This example illustrates where the not operator comes in very handy. If Condition1 had been a Boolean like: (not(a < b) or (c + 3 > 6)) and g, reversing the expression would be more difficult than NOTting it.

Also notice how important indentation is to convey the logic of program code to a human, but the compiler ignores the indentation.