Pascal - if then else statement

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An if-then statement can be followed by an optional else statement, which executes when the Boolean expression is false.

Syntax

Syntax for the if-then-else statement is −

if condition then S1 else S2;

Where, S1 and S2 are different statements. Please note that the statement S1 is not followed by a semicolon. In the if-then-else statements, when the test condition is true, the statement S1 is executed and S2 is skipped; when the test condition is false, then S1 is bypassed and statement S2 is executed.

For example,

if color = red then
   writeln('You have chosen a red car')

else
   writeln('Please choose a color for your car');

If the boolean expression condition evaluates to true, then the if-then block of code will be executed, otherwise the else block of code will be executed.

Pascal assumes any non-zero and non-nil values as true, and if it is either zero or nil, then it is assumed as false value.

Flow Diagram

Example

Let us try a complete example that would illustrate the concept −

program ifelseChecking;
var
   { local variable definition }
   a : integer;

begin
   a := 100;
   (* check the boolean condition *)
   if( a < 20 ) then
      (* if condition is true then print the following *)
      writeln('a is less than 20' )
   
   else
      (* if condition is false then print the following *) 
      writeln('a is not less than 20' );
      writeln('value of a is : ', a);
end.

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

a is not less than 20
value of a is : 100

The if-then-else if-then-else Statement

An if-then statement can be followed by an optional else if-then-else statement, which is very useful to test various conditions using single if-then-else if statement.

When using if-then , else if-then , else statements there are few points to keep in mind.

• An if-then statement can have zero or one else's and it must come after any else if's.

• An if-then statement can have zero to many else if's and they must come before the else.

• Once an else if succeeds, none of the remaining else if's or else's will be tested.

• No semicolon (;) is given before the last else keyword, but all statements can be compound statements.

Syntax

The syntax of an if-then-else if-then-else statement in Pascal programming language is −

if(boolean_expression 1)then 
   S1 (* Executes when the boolean expression 1 is true *)

else if( boolean_expression 2) then 
   S2 (* Executes when the boolean expression 2 is true *)

else if( boolean_expression 3) then 
   S3 (* Executes when the boolean expression 3 is true *)

else 
   S4; ( * executes when the none of the above condition is true *)

Example

The following example illustrates the concept −

program ifelse_ifelseChecking;
var
   { local variable definition }
   a : integer;

begin
   a := 100;
   (* check the boolean condition *)
   if (a = 10)  then
      (* if condition is true then print the following *)
      writeln('Value of a is 10' )
   
   else if ( a = 20 ) then
      (* if else if condition is true *)
      writeln('Value of a is 20' )
   
   else if( a = 30 ) then 
      (* if else if condition is true  *)
      writeln('Value of a is 30' )
   
   else
      (* if none of the conditions is true *)
      writeln('None of the values is matching' );
      writeln('Exact value of a is: ', a );
end.

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

None of the values is matching
Exact value of a is: 100