Pascal - Nested if-then Statements

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It is always legal in Pascal programming to nest if-else statements, which means you can use one if or else if statement inside another if or else if statement(s). Pascal allows nesting to any level, however, if depends on Pascal implementation on a particular system.

Syntax:

The syntax for a nested if statement is as follows:

if( boolean_expression 1) then
   if(boolean_expression 2)then S1
else
   S2;

You can nest else if-then-else in the similar way as you have nested if-then statement. Please note that, the nested if-then-else constructs gives rise to some ambiguity as to which else statement pairs with which if statement. The rule is that the else keyword matches the first if keyword (searching backwards) not already matched by an else keyword.

The above syntax is equivalent to

if( boolean_expression 1) then
begin
   if(boolean_expression 2)then
      S1
   else
      S2;
end;

It is not equivalent to

if ( boolean_expression 1) then  
begin  
   if exp2 then  
      S1  
end;  
   else  
      S2;

Therefore, if the situation demands the later construct, then you must put begin and end keywords at the right place.

Example:

program nested_ifelseChecking;
var
   { local variable definition }
   a, b : integer;
begin
   a := 100;
   b:= 200;
    (* check the boolean condition *)
   if (a = 100) then
      (* if condition is true then check the following *)
      if ( b = 200 ) then
         (* if nested if condition is true  then print the following *)
          writeln('Value of a is 100 and value of b is 200' );
   
   writeln('Exact value of a is: ', a );
   writeln('Exact value of b is: ', b );
end.

When the above code is compiled and executed, it produces the following result:

Value of a is 100 and b is 200
Exact value of a is : 100
Exact value of b is : 200


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